text: import lie from 'lie';
  import getArguments from 'argsarray';
  import debug from 'debug';
  import events, { EventEmitter } from 'events';
  import inherits from 'inherits';
  import vm from 'vm';
  import levelup from 'levelup';
  import ltgt from 'ltgt';
  import Codec from 'level-codec';
  import ReadableStreamCore from 'readable-stream';
  import { obj } from 'through2';
  import Deque from 'double-ended-queue';
  import crypto from 'crypto';
  import vuvuzela from 'vuvuzela';
  import fs from 'fs';
  import path from 'path';
  import LevelWriteStream from 'level-write-stream';
  import PromisePool from 'es6-promise-pool';
  import scopedEval from 'scope-eval';
  
  /* istanbul ignore next */
  var PouchPromise = typeof Promise === 'function' ? Promise : lie;
  
  function isBinaryObject(object) {
    return object instanceof Buffer;
  }
  
  function cloneBinaryObject(object) {
    var copy = new Buffer(object.length);
    object.copy(copy);
    return copy;
  }
  
  // most of this is borrowed from lodash.isPlainObject:
  // https://github.com/fis-components/lodash.isplainobject/
  // blob/29c358140a74f252aeb08c9eb28bef86f2217d4a/index.js
  
  var funcToString = Function.prototype.toString;
  var objectCtorString = funcToString.call(Object);
  
  function isPlainObject(value) {
    var proto = Object.getPrototypeOf(value);
    /* istanbul ignore if */
    if (proto === null) { // not sure when this happens, but I guess it can
      return true;
    }
    var Ctor = proto.constructor;
    return (typeof Ctor == 'function' &&
      Ctor instanceof Ctor && funcToString.call(Ctor) == objectCtorString);
  }
  
  function clone(object) {
    var newObject;
    var i;
    var len;
  
    if (!object || typeof object !== 'object') {
      return object;
    }
  
    if (Array.isArray(object)) {
      newObject = [];
      for (i = 0, len = object.length; i < len; i++) {
        newObject[i] = clone(object[i]);
      }
      return newObject;
    }
  
    // special case: to avoid inconsistencies between IndexedDB
    // and other backends, we automatically stringify Dates
    if (object instanceof Date) {
      return object.toISOString();
    }
  
    if (isBinaryObject(object)) {
      return cloneBinaryObject(object);
    }
  
    if (!isPlainObject(object)) {
      return object; // don't clone objects like Workers
    }
  
    newObject = {};
    for (i in object) {
      /* istanbul ignore else */
      if (Object.prototype.hasOwnProperty.call(object, i)) {
        var value = clone(object[i]);
        if (typeof value !== 'undefined') {
          newObject[i] = value;
        }
      }
    }
    return newObject;
  }
  
  function once(fun) {
    var called = false;
    return getArguments(function (args) {
      /* istanbul ignore if */
      if (called) {
        // this is a smoke test and should never actually happen
        throw new Error('once called more than once');
      } else {
        called = true;
        fun.apply(this, args);
      }
    });
  }
  
  function toPromise(func) {
    //create the function we will be returning
    return getArguments(function (args) {
      // Clone arguments
      args = clone(args);
      var self = this;
      var tempCB =
        (typeof args[args.length - 1] === 'function') ? args.pop() : false;
      // if the last argument is a function, assume its a callback
      var usedCB;
      if (tempCB) {
        // if it was a callback, create a new callback which calls it,
        // but do so async so we don't trap any errors
        usedCB = function (err, resp) {
          process.nextTick(function () {
            tempCB(err, resp);
          });
        };
      }
      var promise = new PouchPromise(function (fulfill, reject) {
        var resp;
        try {
          var callback = once(function (err, mesg) {
            if (err) {
              reject(err);
            } else {
              fulfill(mesg);
            }
          });
          // create a callback for this invocation
          // apply the function in the orig context
          args.push(callback);
          resp = func.apply(self, args);
          if (resp && typeof resp.then === 'function') {
            fulfill(resp);
          }
        } catch (e) {
          reject(e);
        }
      });
      // if there is a callback, call it back
      if (usedCB) {
        promise.then(function (result) {
          usedCB(null, result);
        }, usedCB);
      }
      return promise;
    });
  }
  
  var log = debug('pouchdb:api');
  
  function adapterFun(name, callback) {
    function logApiCall(self, name, args) {
      /* istanbul ignore if */
      if (log.enabled) {
        var logArgs = [self.name, name];
        for (var i = 0; i < args.length - 1; i++) {
          logArgs.push(args[i]);
        }
        log.apply(null, logArgs);
  
        // override the callback itself to log the response
        var origCallback = args[args.length - 1];
        args[args.length - 1] = function (err, res) {
          var responseArgs = [self.name, name];
          responseArgs = responseArgs.concat(
            err ? ['error', err] : ['success', res]
          );
          log.apply(null, responseArgs);
          origCallback(err, res);
        };
      }
    }
  
    return toPromise(getArguments(function (args) {
      if (this._closed) {
        return PouchPromise.reject(new Error('database is closed'));
      }
      if (this._destroyed) {
        return PouchPromise.reject(new Error('database is destroyed'));
      }
      var self = this;
      logApiCall(self, name, args);
      if (!this.taskqueue.isReady) {
        return new PouchPromise(function (fulfill, reject) {
          self.taskqueue.addTask(function (failed) {
            if (failed) {
              reject(failed);
            } else {
              fulfill(self[name].apply(self, args));
            }
          });
        });
      }
      return callback.apply(this, args);
    }));
  }
  
  // like underscore/lodash _.pick()
  function pick(obj, arr) {
    var res = {};
    for (var i = 0, len = arr.length; i < len; i++) {
      var prop = arr[i];
      if (prop in obj) {
        res[prop] = obj[prop];
      }
    }
    return res;
  }
  
  // Most browsers throttle concurrent requests at 6, so it's silly
  // to shim _bulk_get by trying to launch potentially hundreds of requests
  // and then letting the majority time out. We can handle this ourselves.
  var MAX_NUM_CONCURRENT_REQUESTS = 6;
  
  function identityFunction(x) {
    return x;
  }
  
  function formatResultForOpenRevsGet(result) {
    return [{
      ok: result
    }];
  }
  
  // shim for P/CouchDB adapters that don't directly implement _bulk_get
  function bulkGet(db, opts, callback) {
    var requests = opts.docs;
  
    // consolidate into one request per doc if possible
    var requestsById = {};
    requests.forEach(function (request) {
      if (request.id in requestsById) {
        requestsById[request.id].push(request);
      } else {
        requestsById[request.id] = [request];
      }
    });
  
    var numDocs = Object.keys(requestsById).length;
    var numDone = 0;
    var perDocResults = new Array(numDocs);
  
    function collapseResultsAndFinish() {
      var results = [];
      perDocResults.forEach(function (res) {
        res.docs.forEach(function (info) {
          results.push({
            id: res.id,
            docs: [info]
          });
        });
      });
      callback(null, {results: results});
    }
  
    function checkDone() {
      if (++numDone === numDocs) {
        collapseResultsAndFinish();
      }
    }
  
    function gotResult(docIndex, id, docs) {
      perDocResults[docIndex] = {id: id, docs: docs};
      checkDone();
    }
  
    var allRequests = Object.keys(requestsById);
  
    var i = 0;
  
    function nextBatch() {
  
      if (i >= allRequests.length) {
        return;
      }
  
      var upTo = Math.min(i + MAX_NUM_CONCURRENT_REQUESTS, allRequests.length);
      var batch = allRequests.slice(i, upTo);
      processBatch(batch, i);
      i += batch.length;
    }
  
    function processBatch(batch, offset) {
      batch.forEach(function (docId, j) {
        var docIdx = offset + j;
        var docRequests = requestsById[docId];
  
        // just use the first request as the "template"
        // TODO: The _bulk_get API allows for more subtle use cases than this,
        // but for now it is unlikely that there will be a mix of different
        // "atts_since" or "attachments" in the same request, since it's just
        // replicate.js that is using this for the moment.
        // Also, atts_since is aspirational, since we don't support it yet.
        var docOpts = pick(docRequests[0], ['atts_since', 'attachments']);
        docOpts.open_revs = docRequests.map(function (request) {
          // rev is optional, open_revs disallowed
          return request.rev;
        });
  
        // remove falsey / undefined revisions
        docOpts.open_revs = docOpts.open_revs.filter(identityFunction);
  
        var formatResult = identityFunction;
  
        if (docOpts.open_revs.length === 0) {
          delete docOpts.open_revs;
  
          // when fetching only the "winning" leaf,
          // transform the result so it looks like an open_revs
          // request
          formatResult = formatResultForOpenRevsGet;
        }
  
        // globally-supplied options
        ['revs', 'attachments', 'binary', 'ajax'].forEach(function (param) {
          if (param in opts) {
            docOpts[param] = opts[param];
          }
        });
        db.get(docId, docOpts, function (err, res) {
          var result;
          /* istanbul ignore if */
          if (err) {
            result = [{error: err}];
          } else {
            result = formatResult(res);
          }
          gotResult(docIdx, docId, result);
          nextBatch();
        });
      });
    }
  
    nextBatch();
  
  }
  
  // in Node of course this is false
  function isChromeApp() {
    return false;
  }
  
  // in Node of course this is false
  function hasLocalStorage() {
    return false;
  }
  
  inherits(Changes, EventEmitter);
  
  /* istanbul ignore next */
  function attachBrowserEvents(self) {
    if (isChromeApp()) {
      chrome.storage.onChanged.addListener(function (e) {
        // make sure it's event addressed to us
        if (e.db_name != null) {
          //object only has oldValue, newValue members
          self.emit(e.dbName.newValue);
        }
      });
    } else if (hasLocalStorage()) {
      if (typeof addEventListener !== 'undefined') {
        addEventListener("storage", function (e) {
          self.emit(e.key);
        });
      } else { // old IE
        window.attachEvent("storage", function (e) {
          self.emit(e.key);
        });
      }
    }
  }
  
  function Changes() {
    EventEmitter.call(this);
    this._listeners = {};
  
    attachBrowserEvents(this);
  }
  Changes.prototype.addListener = function (dbName, id, db, opts) {
    /* istanbul ignore if */
    if (this._listeners[id]) {
      return;
    }
    var self = this;
    var inprogress = false;
    function eventFunction() {
      /* istanbul ignore if */
      if (!self._listeners[id]) {
        return;
      }
      if (inprogress) {
        inprogress = 'waiting';
        return;
      }
      inprogress = true;
      var changesOpts = pick(opts, [
        'style', 'include_docs', 'attachments', 'conflicts', 'filter',
        'doc_ids', 'view', 'since', 'query_params', 'binary'
      ]);
  
      /* istanbul ignore next */
      function onError() {
        inprogress = false;
      }
  
      db.changes(changesOpts).on('change', function (c) {
        if (c.seq > opts.since && !opts.cancelled) {
          opts.since = c.seq;
          opts.onChange(c);
        }
      }).on('complete', function () {
        if (inprogress === 'waiting') {
          setTimeout(function (){
            eventFunction();
          },0);
        }
        inprogress = false;
      }).on('error', onError);
    }
    this._listeners[id] = eventFunction;
    this.on(dbName, eventFunction);
  };
  
  Changes.prototype.removeListener = function (dbName, id) {
    /* istanbul ignore if */
    if (!(id in this._listeners)) {
      return;
    }
    EventEmitter.prototype.removeListener.call(this, dbName,
      this._listeners[id]);
    delete this._listeners[id];
  };
  
  
  /* istanbul ignore next */
  Changes.prototype.notifyLocalWindows = function (dbName) {
    //do a useless change on a storage thing
    //in order to get other windows's listeners to activate
    if (isChromeApp()) {
      chrome.storage.local.set({dbName: dbName});
    } else if (hasLocalStorage()) {
      localStorage[dbName] = (localStorage[dbName] === "a") ? "b" : "a";
    }
  };
  
  Changes.prototype.notify = function (dbName) {
    this.emit(dbName);
    this.notifyLocalWindows(dbName);
  };
  
  function guardedConsole(method) {
    /* istanbul ignore else */
    if (console !== 'undefined' && method in console) {
      var args = Array.prototype.slice.call(arguments, 1);
      console[method].apply(console, args);
    }
  }
  
  function randomNumber(min, max) {
    var maxTimeout = 600000; // Hard-coded default of 10 minutes
    min = parseInt(min, 10) || 0;
    max = parseInt(max, 10);
    if (max !== max || max <= min) {
      max = (min || 1) << 1; //doubling
    } else {
      max = max + 1;
    }
    // In order to not exceed maxTimeout, pick a random value between half of maxTimeout and maxTimeout
    if(max > maxTimeout) {
      min = maxTimeout >> 1; // divide by two
      max = maxTimeout;
    }
    var ratio = Math.random();
    var range = max - min;
  
    return ~~(range * ratio + min); // ~~ coerces to an int, but fast.
  }
  
  function defaultBackOff(min) {
    var max = 0;
    if (!min) {
      max = 2000;
    }
    return randomNumber(min, max);
  }
  
  // We assume Node users don't need to see this warning
  var res = function () {};
  
  // forked from
  // https://github.com/vmattos/js-extend/blob/7023fd69a9e9552688086b8b8006b1fcf916a306/extend.js
  // TODO: I don't know why we have two different extend() functions in PouchDB
  
  var slice = Array.prototype.slice;
  var each = Array.prototype.forEach;
  
  function extend$1(obj) {
    if (typeof obj !== 'object') {
      throw obj + ' is not an object' ;
    }
  
    var sources = slice.call(arguments, 1);
  
    each.call(sources, function (source) {
      if (source) {
        for (var prop in source) {
          if (typeof source[prop] === 'object' && obj[prop]) {
            extend$1.call(obj, obj[prop], source[prop]);
          } else {
            obj[prop] = source[prop];
          }
        }
      }
    });
  
    return obj;
  }
  
  inherits(PouchError, Error);
  
  function PouchError(opts) {
    Error.call(this, opts.reason);
    this.status = opts.status;
    this.name = opts.error;
    this.message = opts.reason;
    this.error = true;
  }
  
  PouchError.prototype.toString = function () {
    return JSON.stringify({
      status: this.status,
      name: this.name,
      message: this.message,
      reason: this.reason
    });
  };
  
  var UNAUTHORIZED = new PouchError({
    status: 401,
    error: 'unauthorized',
    reason: "Name or password is incorrect."
  });
  
  var MISSING_BULK_DOCS = new PouchError({
    status: 400,
    error: 'bad_request',
    reason: "Missing JSON list of 'docs'"
  });
  
  var MISSING_DOC = new PouchError({
    status: 404,
    error: 'not_found',
    reason: 'missing'
  });
  
  var REV_CONFLICT = new PouchError({
    status: 409,
    error: 'conflict',
    reason: 'Document update conflict'
  });
  
  var INVALID_ID = new PouchError({
    status: 400,
    error: 'bad_request',
    reason: '_id field must contain a string'
  });
  
  var MISSING_ID = new PouchError({
    status: 412,
    error: 'missing_id',
    reason: '_id is required for puts'
  });
  
  var RESERVED_ID = new PouchError({
    status: 400,
    error: 'bad_request',
    reason: 'Only reserved document ids may start with underscore.'
  });
  
  var NOT_OPEN = new PouchError({
    status: 412,
    error: 'precondition_failed',
    reason: 'Database not open'
  });
  
  var UNKNOWN_ERROR = new PouchError({
    status: 500,
    error: 'unknown_error',
    reason: 'Database encountered an unknown error'
  });
  
  var BAD_ARG = new PouchError({
    status: 500,
    error: 'badarg',
    reason: 'Some query argument is invalid'
  });
  
  var INVALID_REQUEST = new PouchError({
    status: 400,
    error: 'invalid_request',
    reason: 'Request was invalid'
  });
  
  var QUERY_PARSE_ERROR = new PouchError({
    status: 400,
    error: 'query_parse_error',
    reason: 'Some query parameter is invalid'
  });
  
  var DOC_VALIDATION = new PouchError({
    status: 500,
    error: 'doc_validation',
    reason: 'Bad special document member'
  });
  
  var BAD_REQUEST = new PouchError({
    status: 400,
    error: 'bad_request',
    reason: 'Something wrong with the request'
  });
  
  var NOT_AN_OBJECT = new PouchError({
    status: 400,
    error: 'bad_request',
    reason: 'Document must be a JSON object'
  });
  
  var DB_MISSING = new PouchError({
    status: 404,
    error: 'not_found',
    reason: 'Database not found'
  });
  
  var IDB_ERROR = new PouchError({
    status: 500,
    error: 'indexed_db_went_bad',
    reason: 'unknown'
  });
  
  var WSQ_ERROR = new PouchError({
    status: 500,
    error: 'web_sql_went_bad',
    reason: 'unknown'
  });
  
  var LDB_ERROR = new PouchError({
    status: 500,
    error: 'levelDB_went_went_bad',
    reason: 'unknown'
  });
  
  var FORBIDDEN = new PouchError({
    status: 403,
    error: 'forbidden',
    reason: 'Forbidden by design doc validate_doc_update function'
  });
  
  var INVALID_REV = new PouchError({
    status: 400,
    error: 'bad_request',
    reason: 'Invalid rev format'
  });
  
  var FILE_EXISTS = new PouchError({
    status: 412,
    error: 'file_exists',
    reason: 'The database could not be created, the file already exists.'
  });
  
  var MISSING_STUB = new PouchError({
    status: 412,
    error: 'missing_stub'
  });
  
  var INVALID_URL = new PouchError({
    status: 413,
    error: 'invalid_url',
    reason: 'Provided URL is invalid'
  });
  
  function createError(error, reason) {
    function CustomPouchError(reason) {
      // inherit error properties from our parent error manually
      // so as to allow proper JSON parsing.
      /* jshint ignore:start */
      for (var p in error) {
        if (typeof error[p] !== 'function') {
          this[p] = error[p];
        }
      }
      /* jshint ignore:end */
      if (reason !== undefined) {
        this.reason = reason;
      }
    }
    CustomPouchError.prototype = PouchError.prototype;
    return new CustomPouchError(reason);
  }
  
  function generateErrorFromResponse(err) {
  
    if (typeof err !== 'object') {
      var data = err;
      err = UNKNOWN_ERROR;
      err.data = data;
    }
  
    if ('error' in err && err.error === 'conflict') {
      err.name = 'conflict';
      err.status = 409;
    }
  
    if (!('name' in err)) {
      err.name = err.error || 'unknown';
    }
  
    if (!('status' in err)) {
      err.status = 500;
    }
  
    if (!('message' in err)) {
      err.message = err.message || err.reason;
    }
  
    return err;
  }
  
  function tryFilter(filter, doc, req) {
    try {
      return !filter(doc, req);
    } catch (err) {
      var msg = 'Filter function threw: ' + err.toString();
      return createError(BAD_REQUEST, msg);
    }
  }
  
  function filterChange(opts) {
    var req = {};
    var hasFilter = opts.filter && typeof opts.filter === 'function';
    req.query = opts.query_params;
  
    return function filter(change) {
      if (!change.doc) {
        // CSG sends events on the changes feed that don't have documents,
        // this hack makes a whole lot of existing code robust.
        change.doc = {};
      }
  
      var filterReturn = hasFilter && tryFilter(opts.filter, change.doc, req);
  
      if (typeof filterReturn === 'object') {
        return filterReturn;
      }
  
      if (filterReturn) {
        return false;
      }
  
      if (!opts.include_docs) {
        delete change.doc;
      } else if (!opts.attachments) {
        for (var att in change.doc._attachments) {
          /* istanbul ignore else */
          if (change.doc._attachments.hasOwnProperty(att)) {
            change.doc._attachments[att].stub = true;
          }
        }
      }
      return true;
    };
  }
  
  function flatten(arrs) {
    var res = [];
    for (var i = 0, len = arrs.length; i < len; i++) {
      res = res.concat(arrs[i]);
    }
    return res;
  }
  
  // shim for Function.prototype.name,
  // for browsers that don't support it like IE
  
  /* istanbul ignore next */
  function f() {}
  
  var hasName = f.name;
  var res$1;
  
  // We dont run coverage in IE
  /* istanbul ignore else */
  if (hasName) {
    res$1 = function (fun) {
      return fun.name;
    };
  } else {
    res$1 = function (fun) {
      return fun.toString().match(/^\s*function\s*(\S*)\s*\(/)[1];
    };
  }
  
  var functionName = res$1;
  
  // Determine id an ID is valid
  //   - invalid IDs begin with an underescore that does not begin '_design' or
  //     '_local'
  //   - any other string value is a valid id
  // Returns the specific error object for each case
  function invalidIdError(id) {
    var err;
    if (!id) {
      err = createError(MISSING_ID);
    } else if (typeof id !== 'string') {
      err = createError(INVALID_ID);
    } else if (/^_/.test(id) && !(/^_(design|local)/).test(id)) {
      err = createError(RESERVED_ID);
    }
    if (err) {
      throw err;
    }
  }
  
  function listenerCount(ee, type) {
    return 'listenerCount' in ee ? ee.listenerCount(type) :
                                   EventEmitter.listenerCount(ee, type);
  }
  
  function parseDesignDocFunctionName(s) {
    if (!s) {
      return null;
    }
    var parts = s.split('/');
    if (parts.length === 2) {
      return parts;
    }
    if (parts.length === 1) {
      return [s, s];
    }
    return null;
  }
  
  function normalizeDesignDocFunctionName(s) {
    var normalized = parseDesignDocFunctionName(s);
    return normalized ? normalized.join('/') : null;
  }
  
  // originally parseUri 1.2.2, now patched by us
  // (c) Steven Levithan <stevenlevithan.com>
  // MIT License
  var keys = ["source", "protocol", "authority", "userInfo", "user", "password",
      "host", "port", "relative", "path", "directory", "file", "query", "anchor"];
  var qName ="queryKey";
  var qParser = /(?:^|&)([^&=]*)=?([^&]*)/g;
  
  // use the "loose" parser
  /* jshint maxlen: false */
  var parser = /^(?:(?![^:@]+:[^:@\/]*@)([^:\/?#.]+):)?(?:\/\/)?((?:(([^:@]*)(?::([^:@]*))?)?@)?([^:\/?#]*)(?::(\d*))?)(((\/(?:[^?#](?![^?#\/]*\.[^?#\/.]+(?:[?#]|$)))*\/?)?([^?#\/]*))(?:\?([^#]*))?(?:#(.*))?)/;
  
  function parseUri(str) {
    var m = parser.exec(str);
    var uri = {};
    var i = 14;
  
    while (i--) {
      var key = keys[i];
      var value = m[i] || "";
      var encoded = ['user', 'password'].indexOf(key) !== -1;
      uri[key] = encoded ? decodeURIComponent(value) : value;
    }
  
    uri[qName] = {};
    uri[keys[12]].replace(qParser, function ($0, $1, $2) {
      if ($1) {
        uri[qName][$1] = $2;
      }
    });
  
    return uri;
  }
  
  // this is essentially the "update sugar" function from daleharvey/pouchdb#1388
  // the diffFun tells us what delta to apply to the doc.  it either returns
  // the doc, or false if it doesn't need to do an update after all
  function upsert(db, docId, diffFun) {
    return new PouchPromise(function (fulfill, reject) {
      db.get(docId, function (err, doc) {
        if (err) {
          /* istanbul ignore next */
          if (err.status !== 404) {
            return reject(err);
          }
          doc = {};
        }
  
        // the user might change the _rev, so save it for posterity
        var docRev = doc._rev;
        var newDoc = diffFun(doc);
  
        if (!newDoc) {
          // if the diffFun returns falsy, we short-circuit as
          // an optimization
          return fulfill({updated: false, rev: docRev});
        }
  
        // users aren't allowed to modify these values,
        // so reset them here
        newDoc._id = docId;
        newDoc._rev = docRev;
        fulfill(tryAndPut(db, newDoc, diffFun));
      });
    });
  }
  
  function tryAndPut(db, doc, diffFun) {
    return db.put(doc).then(function (res) {
      return {
        updated: true,
        rev: res.rev
      };
    }, function (err) {
      /* istanbul ignore next */
      if (err.status !== 409) {
        throw err;
      }
      return upsert(db, doc._id, diffFun);
    });
  }
  
  // BEGIN Math.uuid.js
  
  /*!
  Math.uuid.js (v1.4)
  http://www.broofa.com
  mailto:robert@broofa.com
  
  Copyright (c) 2010 Robert Kieffer
  Dual licensed under the MIT and GPL licenses.
  */
  
  /*
   * Generate a random uuid.
   *
   * USAGE: Math.uuid(length, radix)
   *   length - the desired number of characters
   *   radix  - the number of allowable values for each character.
   *
   * EXAMPLES:
   *   // No arguments  - returns RFC4122, version 4 ID
   *   >>> Math.uuid()
   *   "92329D39-6F5C-4520-ABFC-AAB64544E172"
   *
   *   // One argument - returns ID of the specified length
   *   >>> Math.uuid(15)     // 15 character ID (default base=62)
   *   "VcydxgltxrVZSTV"
   *
   *   // Two arguments - returns ID of the specified length, and radix. 
   *   // (Radix must be <= 62)
   *   >>> Math.uuid(8, 2)  // 8 character ID (base=2)
   *   "01001010"
   *   >>> Math.uuid(8, 10) // 8 character ID (base=10)
   *   "47473046"
   *   >>> Math.uuid(8, 16) // 8 character ID (base=16)
   *   "098F4D35"
   */
  var chars = (
    '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ' +
    'abcdefghijklmnopqrstuvwxyz'
  ).split('');
  function getValue(radix) {
    return 0 | Math.random() * radix;
  }
  function uuid(len, radix) {
    radix = radix || chars.length;
    var out = '';
    var i = -1;
  
    if (len) {
      // Compact form
      while (++i < len) {
        out += chars[getValue(radix)];
      }
      return out;
    }
      // rfc4122, version 4 form
      // Fill in random data.  At i==19 set the high bits of clock sequence as
      // per rfc4122, sec. 4.1.5
    while (++i < 36) {
      switch (i) {
        case 8:
        case 13:
        case 18:
        case 23:
          out += '-';
          break;
        case 19:
          out += chars[(getValue(16) & 0x3) | 0x8];
          break;
        default:
          out += chars[getValue(16)];
      }
    }
  
    return out;
  }
  
  // based on https://github.com/montagejs/collections
  function mangle(key) {
    return '$' + key;
  }
  function unmangle(key) {
    return key.substring(1);
  }
  function _Map() {
    this.store = {};
  }
  _Map.prototype.get = function (key) {
    var mangled = mangle(key);
    return this.store[mangled];
  };
  _Map.prototype.set = function (key, value) {
    var mangled = mangle(key);
    this.store[mangled] = value;
    return true;
  };
  _Map.prototype.has = function (key) {
    var mangled = mangle(key);
    return mangled in this.store;
  };
  _Map.prototype.delete = function (key) {
    var mangled = mangle(key);
    var res = mangled in this.store;
    delete this.store[mangled];
    return res;
  };
  _Map.prototype.forEach = function (cb) {
    var keys = Object.keys(this.store);
    for (var i = 0, len = keys.length; i < len; i++) {
      var key = keys[i];
      var value = this.store[key];
      key = unmangle(key);
      cb(value, key);
    }
  };
  
  function _Set(array) {
    this.store = new _Map();
  
    // init with an array
    if (array && Array.isArray(array)) {
      for (var i = 0, len = array.length; i < len; i++) {
        this.add(array[i]);
      }
    }
  }
  _Set.prototype.add = function (key) {
    return this.store.set(key, true);
  };
  _Set.prototype.has = function (key) {
    return this.store.has(key);
  };
  
  // We fetch all leafs of the revision tree, and sort them based on tree length
  // and whether they were deleted, undeleted documents with the longest revision
  // tree (most edits) win
  // The final sort algorithm is slightly documented in a sidebar here:
  // http://guide.couchdb.org/draft/conflicts.html
  function winningRev(metadata) {
    var winningId;
    var winningPos;
    var winningDeleted;
    var toVisit = metadata.rev_tree.slice();
    var node;
    while ((node = toVisit.pop())) {
      var tree = node.ids;
      var branches = tree[2];
      var pos = node.pos;
      if (branches.length) { // non-leaf
        for (var i = 0, len = branches.length; i < len; i++) {
          toVisit.push({pos: pos + 1, ids: branches[i]});
        }
        continue;
      }
      var deleted = !!tree[1].deleted;
      var id = tree[0];
      // sort by deleted, then pos, then id
      if (!winningId || (winningDeleted !== deleted ? winningDeleted :
          winningPos !== pos ? winningPos < pos : winningId < id)) {
        winningId = id;
        winningPos = pos;
        winningDeleted = deleted;
      }
    }
  
    return winningPos + '-' + winningId;
  }
  
  // Pretty much all below can be combined into a higher order function to
  // traverse revisions
  // The return value from the callback will be passed as context to all
  // children of that node
  function traverseRevTree(revs, callback) {
    var toVisit = revs.slice();
  
    var node;
    while ((node = toVisit.pop())) {
      var pos = node.pos;
      var tree = node.ids;
      var branches = tree[2];
      var newCtx =
        callback(branches.length === 0, pos, tree[0], node.ctx, tree[1]);
      for (var i = 0, len = branches.length; i < len; i++) {
        toVisit.push({pos: pos + 1, ids: branches[i], ctx: newCtx});
      }
    }
  }
  
  function sortByPos(a, b) {
    return a.pos - b.pos;
  }
  
  function collectLeaves(revs) {
    var leaves = [];
    traverseRevTree(revs, function (isLeaf, pos, id, acc, opts) {
      if (isLeaf) {
        leaves.push({rev: pos + "-" + id, pos: pos, opts: opts});
      }
    });
    leaves.sort(sortByPos).reverse();
    for (var i = 0, len = leaves.length; i < len; i++) {
      delete leaves[i].pos;
    }
    return leaves;
  }
  
  // returns revs of all conflicts that is leaves such that
  // 1. are not deleted and
  // 2. are different than winning revision
  function collectConflicts(metadata) {
    var win = winningRev(metadata);
    var leaves = collectLeaves(metadata.rev_tree);
    var conflicts = [];
    for (var i = 0, len = leaves.length; i < len; i++) {
      var leaf = leaves[i];
      if (leaf.rev !== win && !leaf.opts.deleted) {
        conflicts.push(leaf.rev);
      }
    }
    return conflicts;
  }
  
  // compact a tree by marking its non-leafs as missing,
  // and return a list of revs to delete
  function compactTree(metadata) {
    var revs = [];
    traverseRevTree(metadata.rev_tree, function (isLeaf, pos,
                                                 revHash, ctx, opts) {
      if (opts.status === 'available' && !isLeaf) {
        revs.push(pos + '-' + revHash);
        opts.status = 'missing';
      }
    });
    return revs;
  }
  
  // build up a list of all the paths to the leafs in this revision tree
  function rootToLeaf(revs) {
    var paths = [];
    var toVisit = revs.slice();
    var node;
    while ((node = toVisit.pop())) {
      var pos = node.pos;
      var tree = node.ids;
      var id = tree[0];
      var opts = tree[1];
      var branches = tree[2];
      var isLeaf = branches.length === 0;
  
      var history = node.history ? node.history.slice() : [];
      history.push({id: id, opts: opts});
      if (isLeaf) {
        paths.push({pos: (pos + 1 - history.length), ids: history});
      }
      for (var i = 0, len = branches.length; i < len; i++) {
        toVisit.push({pos: pos + 1, ids: branches[i], history: history});
      }
    }
    return paths.reverse();
  }
  
  // for a better overview of what this is doing, read:
  // https://github.com/apache/couchdb-couch/blob/master/src/couch_key_tree.erl
  //
  // But for a quick intro, CouchDB uses a revision tree to store a documents
  // history, A -> B -> C, when a document has conflicts, that is a branch in the
  // tree, A -> (B1 | B2 -> C), We store these as a nested array in the format
  //
  // KeyTree = [Path ... ]
  // Path = {pos: position_from_root, ids: Tree}
  // Tree = [Key, Opts, [Tree, ...]], in particular single node: [Key, []]
  
  function sortByPos$1(a, b) {
    return a.pos - b.pos;
  }
  
  // classic binary search
  function binarySearch(arr, item, comparator) {
    var low = 0;
    var high = arr.length;
    var mid;
    while (low < high) {
      mid = (low + high) >>> 1;
      if (comparator(arr[mid], item) < 0) {
        low = mid + 1;
      } else {
        high = mid;
      }
    }
    return low;
  }
  
  // assuming the arr is sorted, insert the item in the proper place
  function insertSorted(arr, item, comparator) {
    var idx = binarySearch(arr, item, comparator);
    arr.splice(idx, 0, item);
  }
  
  // Turn a path as a flat array into a tree with a single branch.
  // If any should be stemmed from the beginning of the array, that's passed
  // in as the second argument
  function pathToTree(path, numStemmed) {
    var root;
    var leaf;
    for (var i = numStemmed, len = path.length; i < len; i++) {
      var node = path[i];
      var currentLeaf = [node.id, node.opts, []];
      if (leaf) {
        leaf[2].push(currentLeaf);
        leaf = currentLeaf;
      } else {
        root = leaf = currentLeaf;
      }
    }
    return root;
  }
  
  // compare the IDs of two trees
  function compareTree(a, b) {
    return a[0] < b[0] ? -1 : 1;
  }
  
  // Merge two trees together
  // The roots of tree1 and tree2 must be the same revision
  function mergeTree(in_tree1, in_tree2) {
    var queue = [{tree1: in_tree1, tree2: in_tree2}];
    var conflicts = false;
    while (queue.length > 0) {
      var item = queue.pop();
      var tree1 = item.tree1;
      var tree2 = item.tree2;
  
      if (tree1[1].status || tree2[1].status) {
        tree1[1].status =
          (tree1[1].status ===  'available' ||
          tree2[1].status === 'available') ? 'available' : 'missing';
      }
  
      for (var i = 0; i < tree2[2].length; i++) {
        if (!tree1[2][0]) {
          conflicts = 'new_leaf';
          tree1[2][0] = tree2[2][i];
          continue;
        }
  
        var merged = false;
        for (var j = 0; j < tree1[2].length; j++) {
          if (tree1[2][j][0] === tree2[2][i][0]) {
            queue.push({tree1: tree1[2][j], tree2: tree2[2][i]});
            merged = true;
          }
        }
        if (!merged) {
          conflicts = 'new_branch';
          insertSorted(tree1[2], tree2[2][i], compareTree);
        }
      }
    }
    return {conflicts: conflicts, tree: in_tree1};
  }
  
  function doMerge(tree, path, dontExpand) {
    var restree = [];
    var conflicts = false;
    var merged = false;
    var res;
  
    if (!tree.length) {
      return {tree: [path], conflicts: 'new_leaf'};
    }
  
    for (var i = 0, len = tree.length; i < len; i++) {
      var branch = tree[i];
      if (branch.pos === path.pos && branch.ids[0] === path.ids[0]) {
        // Paths start at the same position and have the same root, so they need
        // merged
        res = mergeTree(branch.ids, path.ids);
        restree.push({pos: branch.pos, ids: res.tree});
        conflicts = conflicts || res.conflicts;
        merged = true;
      } else if (dontExpand !== true) {
        // The paths start at a different position, take the earliest path and
        // traverse up until it as at the same point from root as the path we
        // want to merge.  If the keys match we return the longer path with the
        // other merged After stemming we dont want to expand the trees
  
        var t1 = branch.pos < path.pos ? branch : path;
        var t2 = branch.pos < path.pos ? path : branch;
        var diff = t2.pos - t1.pos;
  
        var candidateParents = [];
  
        var trees = [];
        trees.push({ids: t1.ids, diff: diff, parent: null, parentIdx: null});
        while (trees.length > 0) {
          var item = trees.pop();
          if (item.diff === 0) {
            if (item.ids[0] === t2.ids[0]) {
              candidateParents.push(item);
            }
            continue;
          }
          var elements = item.ids[2];
          for (var j = 0, elementsLen = elements.length; j < elementsLen; j++) {
            trees.push({
              ids: elements[j],
              diff: item.diff - 1,
              parent: item.ids,
              parentIdx: j
            });
          }
        }
  
        var el = candidateParents[0];
  
        if (!el) {
          restree.push(branch);
        } else {
          res = mergeTree(el.ids, t2.ids);
          el.parent[2][el.parentIdx] = res.tree;
          restree.push({pos: t1.pos, ids: t1.ids});
          conflicts = conflicts || res.conflicts;
          merged = true;
        }
      } else {
        restree.push(branch);
      }
    }
  
    // We didnt find
    if (!merged) {
      restree.push(path);
    }
  
    restree.sort(sortByPos$1);
  
    return {
      tree: restree,
      conflicts: conflicts || 'internal_node'
    };
  }
  
  // To ensure we dont grow the revision tree infinitely, we stem old revisions
  function stem(tree, depth) {
    // First we break out the tree into a complete list of root to leaf paths
    var paths = rootToLeaf(tree);
    var maybeStem = {};
  
    var result;
    for (var i = 0, len = paths.length; i < len; i++) {
      // Then for each path, we cut off the start of the path based on the
      // `depth` to stem to, and generate a new set of flat trees
      var path = paths[i];
      var stemmed = path.ids;
      var numStemmed = Math.max(0, stemmed.length - depth);
      var stemmedNode = {
        pos: path.pos + numStemmed,
        ids: pathToTree(stemmed, numStemmed)
      };
  
      for (var s = 0; s < numStemmed; s++) {
        var rev = (path.pos + s) + '-' + stemmed[s].id;
        maybeStem[rev] = true;
      }
  
      // Then we remerge all those flat trees together, ensuring that we dont
      // connect trees that would go beyond the depth limit
      if (result) {
        result = doMerge(result, stemmedNode, true).tree;
      } else {
        result = [stemmedNode];
      }
    }
  
    traverseRevTree(result, function (isLeaf, pos, revHash) {
      // some revisions may have been removed in a branch but not in another
      delete maybeStem[pos + '-' + revHash];
    });
  
    return {
      tree: result,
      revs: Object.keys(maybeStem)
    };
  }
  
  function merge(tree, path, depth) {
    var newTree = doMerge(tree, path);
    var stemmed = stem(newTree.tree, depth);
    return {
      tree: stemmed.tree,
      stemmedRevs: stemmed.revs,
      conflicts: newTree.conflicts
    };
  }
  
  // return true if a rev exists in the rev tree, false otherwise
  function revExists(revs, rev) {
    var toVisit = revs.slice();
    var splitRev = rev.split('-');
    var targetPos = parseInt(splitRev[0], 10);
    var targetId = splitRev[1];
  
    var node;
    while ((node = toVisit.pop())) {
      if (node.pos === targetPos && node.ids[0] === targetId) {
        return true;
      }
      var branches = node.ids[2];
      for (var i = 0, len = branches.length; i < len; i++) {
        toVisit.push({pos: node.pos + 1, ids: branches[i]});
      }
    }
    return false;
  }
  
  function getTrees(node) {
    return node.ids;
  }
  
  // check if a specific revision of a doc has been deleted
  //  - metadata: the metadata object from the doc store
  //  - rev: (optional) the revision to check. defaults to winning revision
  function isDeleted(metadata, rev) {
    if (!rev) {
      rev = winningRev(metadata);
    }
    var id = rev.substring(rev.indexOf('-') + 1);
    var toVisit = metadata.rev_tree.map(getTrees);
  
    var tree;
    while ((tree = toVisit.pop())) {
      if (tree[0] === id) {
        return !!tree[1].deleted;
      }
      toVisit = toVisit.concat(tree[2]);
    }
  }
  
  function isLocalId(id) {
    return (/^_local/).test(id);
  }
  
  function evalFilter(input) {
    var code = '(function() {\n"use strict";\nreturn ' + input + '\n})()';
  
    return vm.runInNewContext(code);
  }
  
  function evalView(input) {
    var code = [
      '"use strict";',
      'var emitted = false;',
      'var emit = function (a, b) {',
      '  emitted = true;',
      '};',
      'var view = ' + input + ';',
      'view(doc);',
      'if (emitted) {',
      '  return true;',
      '}'
    ].join('\n');
  
    return vm.runInNewContext('(function(doc) {\n' + code + '\n})');
  }
  
  inherits(Changes$1, EventEmitter);
  
  function tryCatchInChangeListener(self, change) {
    // isolate try/catches to avoid V8 deoptimizations
    try {
      self.emit('change', change);
    } catch (e) {
      guardedConsole('error', 'Error in .on("change", function):', e);
    }
  }
  
  function Changes$1(db, opts, callback) {
    EventEmitter.call(this);
    var self = this;
    this.db = db;
    opts = opts ? clone(opts) : {};
    var complete = opts.complete = once(function (err, resp) {
      if (err) {
        if (listenerCount(self, 'error') > 0) {
          self.emit('error', err);
        }
      } else {
        self.emit('complete', resp);
      }
      self.removeAllListeners();
      db.removeListener('destroyed', onDestroy);
    });
    if (callback) {
      self.on('complete', function (resp) {
        callback(null, resp);
      });
      self.on('error', callback);
    }
    function onDestroy() {
      self.cancel();
    }
    db.once('destroyed', onDestroy);
  
    opts.onChange = function (change) {
      /* istanbul ignore if */
      if (opts.isCancelled) {
        return;
      }
      tryCatchInChangeListener(self, change);
    };
  
    var promise = new PouchPromise(function (fulfill, reject) {
      opts.complete = function (err, res) {
        if (err) {
          reject(err);
        } else {
          fulfill(res);
        }
      };
    });
    self.once('cancel', function () {
      db.removeListener('destroyed', onDestroy);
      opts.complete(null, {status: 'cancelled'});
    });
    this.then = promise.then.bind(promise);
    this['catch'] = promise['catch'].bind(promise);
    this.then(function (result) {
      complete(null, result);
    }, complete);
  
  
  
    if (!db.taskqueue.isReady) {
      db.taskqueue.addTask(function (failed) {
        if (failed) {
          opts.complete(failed);
        } else if (self.isCancelled) {
          self.emit('cancel');
        } else {
          self.doChanges(opts);
        }
      });
    } else {
      self.doChanges(opts);
    }
  }
  Changes$1.prototype.cancel = function () {
    this.isCancelled = true;
    if (this.db.taskqueue.isReady) {
      this.emit('cancel');
    }
  };
  function processChange(doc, metadata, opts) {
    var changeList = [{rev: doc._rev}];
    if (opts.style === 'all_docs') {
      changeList = collectLeaves(metadata.rev_tree)
      .map(function (x) { return {rev: x.rev}; });
    }
    var change = {
      id: metadata.id,
      changes: changeList,
      doc: doc
    };
  
    if (isDeleted(metadata, doc._rev)) {
      change.deleted = true;
    }
    if (opts.conflicts) {
      change.doc._conflicts = collectConflicts(metadata);
      if (!change.doc._conflicts.length) {
        delete change.doc._conflicts;
      }
    }
    return change;
  }
  
  Changes$1.prototype.doChanges = function (opts) {
    var self = this;
    var callback = opts.complete;
  
    opts = clone(opts);
    if ('live' in opts && !('continuous' in opts)) {
      opts.continuous = opts.live;
    }
    opts.processChange = processChange;
  
    if (opts.since === 'latest') {
      opts.since = 'now';
    }
    if (!opts.since) {
      opts.since = 0;
    }
    if (opts.since === 'now') {
      this.db.info().then(function (info) {
        /* istanbul ignore if */
        if (self.isCancelled) {
          callback(null, {status: 'cancelled'});
          return;
        }
        opts.since = info.update_seq;
        self.doChanges(opts);
      }, callback);
      return;
    }
  
  
    if (opts.view && !opts.filter) {
      opts.filter = '_view';
    }
  
    if (opts.filter && typeof opts.filter === 'string') {
      if (opts.filter === '_view') {
        opts.view = normalizeDesignDocFunctionName(opts.view);
      } else {
        opts.filter = normalizeDesignDocFunctionName(opts.filter);
      }
  
      if (this.db.type() !== 'http' && !opts.doc_ids) {
        return this.filterChanges(opts);
      }
    }
  
    if (!('descending' in opts)) {
      opts.descending = false;
    }
  
    // 0 and 1 should return 1 document
    opts.limit = opts.limit === 0 ? 1 : opts.limit;
    opts.complete = callback;
    var newPromise = this.db._changes(opts);
    /* istanbul ignore else */
    if (newPromise && typeof newPromise.cancel === 'function') {
      var cancel = self.cancel;
      self.cancel = getArguments(function (args) {
        newPromise.cancel();
        cancel.apply(this, args);
      });
    }
  };
  
  Changes$1.prototype.filterChanges = function (opts) {
    var self = this;
    var callback = opts.complete;
    if (opts.filter === '_view') {
      if (!opts.view || typeof opts.view !== 'string') {
        var err = createError(BAD_REQUEST,
          '`view` filter parameter not found or invalid.');
        return callback(err);
      }
      // fetch a view from a design doc, make it behave like a filter
      var viewName = parseDesignDocFunctionName(opts.view);
      this.db.get('_design/' + viewName[0], function (err, ddoc) {
        /* istanbul ignore if */
        if (self.isCancelled) {
          return callback(null, {status: 'cancelled'});
        }
        /* istanbul ignore next */
        if (err) {
          return callback(generateErrorFromResponse(err));
        }
        var mapFun = ddoc && ddoc.views && ddoc.views[viewName[1]] &&
          ddoc.views[viewName[1]].map;
        if (!mapFun) {
          return callback(createError(MISSING_DOC,
            (ddoc.views ? 'missing json key: ' + viewName[1] :
              'missing json key: views')));
        }
        opts.filter = evalView(mapFun);
        self.doChanges(opts);
      });
    } else {
      // fetch a filter from a design doc
      var filterName = parseDesignDocFunctionName(opts.filter);
      if (!filterName) {
        return self.doChanges(opts);
      }
      this.db.get('_design/' + filterName[0], function (err, ddoc) {
        /* istanbul ignore if */
        if (self.isCancelled) {
          return callback(null, {status: 'cancelled'});
        }
        /* istanbul ignore next */
        if (err) {
          return callback(generateErrorFromResponse(err));
        }
        var filterFun = ddoc && ddoc.filters && ddoc.filters[filterName[1]];
        if (!filterFun) {
          return callback(createError(MISSING_DOC,
            ((ddoc && ddoc.filters) ? 'missing json key: ' + filterName[1]
              : 'missing json key: filters')));
        }
        opts.filter = evalFilter(filterFun);
        self.doChanges(opts);
      });
    }
  };
  
  /*
   * A generic pouch adapter
   */
  
  function compare(left, right) {
    return left < right ? -1 : left > right ? 1 : 0;
  }
  
  // returns first element of arr satisfying callback predicate
  function arrayFirst(arr, callback) {
    for (var i = 0; i < arr.length; i++) {
      if (callback(arr[i], i) === true) {
        return arr[i];
      }
    }
  }
  
  // Wrapper for functions that call the bulkdocs api with a single doc,
  // if the first result is an error, return an error
  function yankError(callback) {
    return function (err, results) {
      if (err || (results[0] && results[0].error)) {
        callback(err || results[0]);
      } else {
        callback(null, results.length ? results[0]  : results);
      }
    };
  }
  
  // clean docs given to us by the user
  function cleanDocs(docs) {
    for (var i = 0; i < docs.length; i++) {
      var doc = docs[i];
      if (doc._deleted) {
        delete doc._attachments; // ignore atts for deleted docs
      } else if (doc._attachments) {
        // filter out extraneous keys from _attachments
        var atts = Object.keys(doc._attachments);
        for (var j = 0; j < atts.length; j++) {
          var att = atts[j];
          doc._attachments[att] = pick(doc._attachments[att],
            ['data', 'digest', 'content_type', 'length', 'revpos', 'stub']);
        }
      }
    }
  }
  
  // compare two docs, first by _id then by _rev
  function compareByIdThenRev(a, b) {
    var idCompare = compare(a._id, b._id);
    if (idCompare !== 0) {
      return idCompare;
    }
    var aStart = a._revisions ? a._revisions.start : 0;
    var bStart = b._revisions ? b._revisions.start : 0;
    return compare(aStart, bStart);
  }
  
  // for every node in a revision tree computes its distance from the closest
  // leaf
  function computeHeight(revs) {
    var height = {};
    var edges = [];
    traverseRevTree(revs, function (isLeaf, pos, id, prnt) {
      var rev = pos + "-" + id;
      if (isLeaf) {
        height[rev] = 0;
      }
      if (prnt !== undefined) {
        edges.push({from: prnt, to: rev});
      }
      return rev;
    });
  
    edges.reverse();
    edges.forEach(function (edge) {
      if (height[edge.from] === undefined) {
        height[edge.from] = 1 + height[edge.to];
      } else {
        height[edge.from] = Math.min(height[edge.from], 1 + height[edge.to]);
      }
    });
    return height;
  }
  
  function allDocsKeysQuery(api, opts, callback) {
    var keys =  ('limit' in opts) ?
        opts.keys.slice(opts.skip, opts.limit + opts.skip) :
        (opts.skip > 0) ? opts.keys.slice(opts.skip) : opts.keys;
    if (opts.descending) {
      keys.reverse();
    }
    if (!keys.length) {
      return api._allDocs({limit: 0}, callback);
    }
    var finalResults = {
      offset: opts.skip
    };
    return PouchPromise.all(keys.map(function (key) {
      var subOpts = extend$1({key: key, deleted: 'ok'}, opts);
      ['limit', 'skip', 'keys'].forEach(function (optKey) {
        delete subOpts[optKey];
      });
      return new PouchPromise(function (resolve, reject) {
        api._allDocs(subOpts, function (err, res) {
          /* istanbul ignore if */
          if (err) {
            return reject(err);
          }
          finalResults.total_rows = res.total_rows;
          resolve(res.rows[0] || {key: key, error: 'not_found'});
        });
      });
    })).then(function (results) {
      finalResults.rows = results;
      return finalResults;
    });
  }
  
  // all compaction is done in a queue, to avoid attaching
  // too many listeners at once
  function doNextCompaction(self) {
    var task = self._compactionQueue[0];
    var opts = task.opts;
    var callback = task.callback;
    self.get('_local/compaction').catch(function () {
      return false;
    }).then(function (doc) {
      if (doc && doc.last_seq) {
        opts.last_seq = doc.last_seq;
      }
      self._compact(opts, function (err, res) {
        /* istanbul ignore if */
        if (err) {
          callback(err);
        } else {
          callback(null, res);
        }
        process.nextTick(function () {
          self._compactionQueue.shift();
          if (self._compactionQueue.length) {
            doNextCompaction(self);
          }
        });
      });
    });
  }
  
  function attachmentNameError(name) {
    if (name.charAt(0) === '_') {
      return name + 'is not a valid attachment name, attachment ' +
        'names cannot start with \'_\'';
    }
    return false;
  }
  
  inherits(AbstractPouchDB, EventEmitter);
  
  function AbstractPouchDB() {
    EventEmitter.call(this);
  }
  
  AbstractPouchDB.prototype.post =
    adapterFun('post', function (doc, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    if (typeof doc !== 'object' || Array.isArray(doc)) {
      return callback(createError(NOT_AN_OBJECT));
    }
    this.bulkDocs({docs: [doc]}, opts, yankError(callback));
  });
  
  AbstractPouchDB.prototype.put = adapterFun('put', function (doc, opts, cb) {
    if (typeof opts === 'function') {
      cb = opts;
      opts = {};
    }
    if (typeof doc !== 'object' || Array.isArray(doc)) {
      return cb(createError(NOT_AN_OBJECT));
    }
    invalidIdError(doc._id);
    if (isLocalId(doc._id) && typeof this._putLocal === 'function') {
      if (doc._deleted) {
        return this._removeLocal(doc, cb);
      } else {
        return this._putLocal(doc, cb);
      }
    }
    if (typeof this._put === 'function' && opts.new_edits !== false) {
      this._put(doc, opts, cb);
    } else {
      this.bulkDocs({docs: [doc]}, opts, yankError(cb));
    }
  });
  
  AbstractPouchDB.prototype.putAttachment =
    adapterFun('putAttachment', function (docId, attachmentId, rev,
                                                blob, type) {
    var api = this;
    if (typeof type === 'function') {
      type = blob;
      blob = rev;
      rev = null;
    }
    // Lets fix in https://github.com/pouchdb/pouchdb/issues/3267
    /* istanbul ignore if */
    if (typeof type === 'undefined') {
      type = blob;
      blob = rev;
      rev = null;
    }
  
    function createAttachment(doc) {
      var prevrevpos = '_rev' in doc ? parseInt(doc._rev, 10) : 0;
      doc._attachments = doc._attachments || {};
      doc._attachments[attachmentId] = {
        content_type: type,
        data: blob,
        revpos: ++prevrevpos
      };
      return api.put(doc);
    }
  
    return api.get(docId).then(function (doc) {
      if (doc._rev !== rev) {
        throw createError(REV_CONFLICT);
      }
  
      return createAttachment(doc);
    }, function (err) {
       // create new doc
      /* istanbul ignore else */
      if (err.reason === MISSING_DOC.message) {
        return createAttachment({_id: docId});
      } else {
        throw err;
      }
    });
  });
  
  AbstractPouchDB.prototype.removeAttachment =
    adapterFun('removeAttachment', function (docId, attachmentId, rev,
                                                   callback) {
    var self = this;
    self.get(docId, function (err, obj) {
      /* istanbul ignore if */
      if (err) {
        callback(err);
        return;
      }
      if (obj._rev !== rev) {
        callback(createError(REV_CONFLICT));
        return;
      }
      /* istanbul ignore if */
      if (!obj._attachments) {
        return callback();
      }
      delete obj._attachments[attachmentId];
      if (Object.keys(obj._attachments).length === 0) {
        delete obj._attachments;
      }
      self.put(obj, callback);
    });
  });
  
  AbstractPouchDB.prototype.remove =
    adapterFun('remove', function (docOrId, optsOrRev, opts, callback) {
    var doc;
    if (typeof optsOrRev === 'string') {
      // id, rev, opts, callback style
      doc = {
        _id: docOrId,
        _rev: optsOrRev
      };
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
    } else {
      // doc, opts, callback style
      doc = docOrId;
      if (typeof optsOrRev === 'function') {
        callback = optsOrRev;
        opts = {};
      } else {
        callback = opts;
        opts = optsOrRev;
      }
    }
    opts = opts || {};
    opts.was_delete = true;
    var newDoc = {_id: doc._id, _rev: (doc._rev || opts.rev)};
    newDoc._deleted = true;
    if (isLocalId(newDoc._id) && typeof this._removeLocal === 'function') {
      return this._removeLocal(doc, callback);
    }
    this.bulkDocs({docs: [newDoc]}, opts, yankError(callback));
  });
  
  AbstractPouchDB.prototype.revsDiff =
    adapterFun('revsDiff', function (req, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    var ids = Object.keys(req);
  
    if (!ids.length) {
      return callback(null, {});
    }
  
    var count = 0;
    var missing = new _Map();
  
    function addToMissing(id, revId) {
      if (!missing.has(id)) {
        missing.set(id, {missing: []});
      }
      missing.get(id).missing.push(revId);
    }
  
    function processDoc(id, rev_tree) {
      // Is this fast enough? Maybe we should switch to a set simulated by a map
      var missingForId = req[id].slice(0);
      traverseRevTree(rev_tree, function (isLeaf, pos, revHash, ctx,
        opts) {
          var rev = pos + '-' + revHash;
          var idx = missingForId.indexOf(rev);
          if (idx === -1) {
            return;
          }
  
          missingForId.splice(idx, 1);
          /* istanbul ignore if */
          if (opts.status !== 'available') {
            addToMissing(id, rev);
          }
        });
  
      // Traversing the tree is synchronous, so now `missingForId` contains
      // revisions that were not found in the tree
      missingForId.forEach(function (rev) {
        addToMissing(id, rev);
      });
    }
  
    ids.map(function (id) {
      this._getRevisionTree(id, function (err, rev_tree) {
        if (err && err.status === 404 && err.message === 'missing') {
          missing.set(id, {missing: req[id]});
        } else if (err) {
          /* istanbul ignore next */
          return callback(err);
        } else {
          processDoc(id, rev_tree);
        }
  
        if (++count === ids.length) {
          // convert LazyMap to object
          var missingObj = {};
          missing.forEach(function (value, key) {
            missingObj[key] = value;
          });
          return callback(null, missingObj);
        }
      });
    }, this);
  });
  
  // _bulk_get API for faster replication, as described in
  // https://github.com/apache/couchdb-chttpd/pull/33
  // At the "abstract" level, it will just run multiple get()s in
  // parallel, because this isn't much of a performance cost
  // for local databases (except the cost of multiple transactions, which is
  // small). The http adapter overrides this in order
  // to do a more efficient single HTTP request.
  AbstractPouchDB.prototype.bulkGet =
    adapterFun('bulkGet', function (opts, callback) {
    bulkGet(this, opts, callback);
  });
  
  // compact one document and fire callback
  // by compacting we mean removing all revisions which
  // are further from the leaf in revision tree than max_height
  AbstractPouchDB.prototype.compactDocument =
    adapterFun('compactDocument', function (docId, maxHeight, callback) {
    var self = this;
    this._getRevisionTree(docId, function (err, revTree) {
      /* istanbul ignore if */
      if (err) {
        return callback(err);
      }
      var height = computeHeight(revTree);
      var candidates = [];
      var revs = [];
      Object.keys(height).forEach(function (rev) {
        if (height[rev] > maxHeight) {
          candidates.push(rev);
        }
      });
  
      traverseRevTree(revTree, function (isLeaf, pos, revHash, ctx, opts) {
        var rev = pos + '-' + revHash;
        if (opts.status === 'available' && candidates.indexOf(rev) !== -1) {
          revs.push(rev);
        }
      });
      self._doCompaction(docId, revs, callback);
    });
  });
  
  // compact the whole database using single document
  // compaction
  AbstractPouchDB.prototype.compact =
    adapterFun('compact', function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
  
    var self = this;
    opts = opts || {};
  
    self._compactionQueue = self._compactionQueue || [];
    self._compactionQueue.push({opts: opts, callback: callback});
    if (self._compactionQueue.length === 1) {
      doNextCompaction(self);
    }
  });
  AbstractPouchDB.prototype._compact = function (opts, callback) {
    var self = this;
    var changesOpts = {
      return_docs: false,
      last_seq: opts.last_seq || 0
    };
    var promises = [];
  
    function onChange(row) {
      promises.push(self.compactDocument(row.id, 0));
    }
    function onComplete(resp) {
      var lastSeq = resp.last_seq;
      PouchPromise.all(promises).then(function () {
        return upsert(self, '_local/compaction', function deltaFunc(doc) {
          if (!doc.last_seq || doc.last_seq < lastSeq) {
            doc.last_seq = lastSeq;
            return doc;
          }
          return false; // somebody else got here first, don't update
        });
      }).then(function () {
        callback(null, {ok: true});
      }).catch(callback);
    }
    self.changes(changesOpts)
      .on('change', onChange)
      .on('complete', onComplete)
      .on('error', callback);
  };
  
  /* Begin api wrappers. Specific functionality to storage belongs in the
     _[method] */
  AbstractPouchDB.prototype.get = adapterFun('get', function (id, opts, cb) {
    if (typeof opts === 'function') {
      cb = opts;
      opts = {};
    }
    if (typeof id !== 'string') {
      return cb(createError(INVALID_ID));
    }
    if (isLocalId(id) && typeof this._getLocal === 'function') {
      return this._getLocal(id, cb);
    }
    var leaves = [], self = this;
  
    function finishOpenRevs() {
      var result = [];
      var count = leaves.length;
      /* istanbul ignore if */
      if (!count) {
        return cb(null, result);
      }
      // order with open_revs is unspecified
      leaves.forEach(function (leaf) {
        self.get(id, {
          rev: leaf,
          revs: opts.revs,
          attachments: opts.attachments
        }, function (err, doc) {
          if (!err) {
            result.push({ok: doc});
          } else {
            result.push({missing: leaf});
          }
          count--;
          if (!count) {
            cb(null, result);
          }
        });
      });
    }
  
    if (opts.open_revs) {
      if (opts.open_revs === "all") {
        this._getRevisionTree(id, function (err, rev_tree) {
          if (err) {
            return cb(err);
          }
          leaves = collectLeaves(rev_tree).map(function (leaf) {
            return leaf.rev;
          });
          finishOpenRevs();
        });
      } else {
        if (Array.isArray(opts.open_revs)) {
          leaves = opts.open_revs;
          for (var i = 0; i < leaves.length; i++) {
            var l = leaves[i];
            // looks like it's the only thing couchdb checks
            if (!(typeof (l) === "string" && /^\d+-/.test(l))) {
              return cb(createError(INVALID_REV));
            }
          }
          finishOpenRevs();
        } else {
          return cb(createError(UNKNOWN_ERROR, 'function_clause'));
        }
      }
      return; // open_revs does not like other options
    }
  
    return this._get(id, opts, function (err, result) {
      if (err) {
        return cb(err);
      }
  
      var doc = result.doc;
      var metadata = result.metadata;
      var ctx = result.ctx;
  
      if (opts.conflicts) {
        var conflicts = collectConflicts(metadata);
        if (conflicts.length) {
          doc._conflicts = conflicts;
        }
      }
  
      if (isDeleted(metadata, doc._rev)) {
        doc._deleted = true;
      }
  
      if (opts.revs || opts.revs_info) {
        var paths = rootToLeaf(metadata.rev_tree);
        var path = arrayFirst(paths, function (arr) {
          return arr.ids.map(function (x) { return x.id; })
            .indexOf(doc._rev.split('-')[1]) !== -1;
        });
  
        var indexOfRev = path.ids.map(function (x) {return x.id; })
          .indexOf(doc._rev.split('-')[1]) + 1;
        var howMany = path.ids.length - indexOfRev;
        path.ids.splice(indexOfRev, howMany);
        path.ids.reverse();
  
        if (opts.revs) {
          doc._revisions = {
            start: (path.pos + path.ids.length) - 1,
            ids: path.ids.map(function (rev) {
              return rev.id;
            })
          };
        }
        if (opts.revs_info) {
          var pos =  path.pos + path.ids.length;
          doc._revs_info = path.ids.map(function (rev) {
            pos--;
            return {
              rev: pos + '-' + rev.id,
              status: rev.opts.status
            };
          });
        }
      }
  
      if (opts.attachments && doc._attachments) {
        var attachments = doc._attachments;
        var count = Object.keys(attachments).length;
        if (count === 0) {
          return cb(null, doc);
        }
        Object.keys(attachments).forEach(function (key) {
          this._getAttachment(doc._id, key, attachments[key], {
            // Previously the revision handling was done in adapter.js
            // getAttachment, however since idb-next doesnt we need to
            // pass the rev through
            rev: doc._rev,
            binary: opts.binary,
            ctx: ctx
          }, function (err, data) {
            var att = doc._attachments[key];
            att.data = data;
            delete att.stub;
            delete att.length;
            if (!--count) {
              cb(null, doc);
            }
          });
        }, self);
      } else {
        if (doc._attachments) {
          for (var key in doc._attachments) {
            /* istanbul ignore else */
            if (doc._attachments.hasOwnProperty(key)) {
              doc._attachments[key].stub = true;
            }
          }
        }
        cb(null, doc);
      }
    });
  });
  
  // TODO: I dont like this, it forces an extra read for every
  // attachment read and enforces a confusing api between
  // adapter.js and the adapter implementation
  AbstractPouchDB.prototype.getAttachment =
    adapterFun('getAttachment', function (docId, attachmentId, opts, callback) {
    var self = this;
    if (opts instanceof Function) {
      callback = opts;
      opts = {};
    }
    this._get(docId, opts, function (err, res) {
      if (err) {
        return callback(err);
      }
      if (res.doc._attachments && res.doc._attachments[attachmentId]) {
        opts.ctx = res.ctx;
        opts.binary = true;
        self._getAttachment(docId, attachmentId,
                            res.doc._attachments[attachmentId], opts, callback);
      } else {
        return callback(createError(MISSING_DOC));
      }
    });
  });
  
  AbstractPouchDB.prototype.allDocs =
    adapterFun('allDocs', function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts.skip = typeof opts.skip !== 'undefined' ? opts.skip : 0;
    if (opts.start_key) {
      opts.startkey = opts.start_key;
    }
    if (opts.end_key) {
      opts.endkey = opts.end_key;
    }
    if ('keys' in opts) {
      if (!Array.isArray(opts.keys)) {
        return callback(new TypeError('options.keys must be an array'));
      }
      var incompatibleOpt =
        ['startkey', 'endkey', 'key'].filter(function (incompatibleOpt) {
        return incompatibleOpt in opts;
      })[0];
      if (incompatibleOpt) {
        callback(createError(QUERY_PARSE_ERROR,
          'Query parameter `' + incompatibleOpt +
          '` is not compatible with multi-get'
        ));
        return;
      }
      if (this.type() !== 'http') {
        return allDocsKeysQuery(this, opts, callback);
      }
    }
  
    return this._allDocs(opts, callback);
  });
  
  AbstractPouchDB.prototype.changes = function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    return new Changes$1(this, opts, callback);
  };
  
  AbstractPouchDB.prototype.close = adapterFun('close', function (callback) {
    this._closed = true;
    return this._close(callback);
  });
  
  AbstractPouchDB.prototype.info = adapterFun('info', function (callback) {
    var self = this;
    this._info(function (err, info) {
      if (err) {
        return callback(err);
      }
      // assume we know better than the adapter, unless it informs us
      info.db_name = info.db_name || self.name;
      info.auto_compaction = !!(self.auto_compaction && self.type() !== 'http');
      info.adapter = self.type();
      callback(null, info);
    });
  });
  
  AbstractPouchDB.prototype.id = adapterFun('id', function (callback) {
    return this._id(callback);
  });
  
  /* istanbul ignore next */
  AbstractPouchDB.prototype.type = function () {
    return (typeof this._type === 'function') ? this._type() : this.adapter;
  };
  
  AbstractPouchDB.prototype.bulkDocs =
    adapterFun('bulkDocs', function (req, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
  
    opts = opts || {};
  
    if (Array.isArray(req)) {
      req = {
        docs: req
      };
    }
  
    if (!req || !req.docs || !Array.isArray(req.docs)) {
      return callback(createError(MISSING_BULK_DOCS));
    }
  
    for (var i = 0; i < req.docs.length; ++i) {
      if (typeof req.docs[i] !== 'object' || Array.isArray(req.docs[i])) {
        return callback(createError(NOT_AN_OBJECT));
      }
    }
  
    var attachmentError;
    req.docs.forEach(function (doc) {
      if (doc._attachments) {
        Object.keys(doc._attachments).forEach(function (name) {
          attachmentError = attachmentError || attachmentNameError(name);
        });
      }
    });
  
    if (attachmentError) {
      return callback(createError(BAD_REQUEST, attachmentError));
    }
  
    if (!('new_edits' in opts)) {
      if ('new_edits' in req) {
        opts.new_edits = req.new_edits;
      } else {
        opts.new_edits = true;
      }
    }
  
    if (!opts.new_edits && this.type() !== 'http') {
      // ensure revisions of the same doc are sorted, so that
      // the local adapter processes them correctly (#2935)
      req.docs.sort(compareByIdThenRev);
    }
  
    cleanDocs(req.docs);
  
    return this._bulkDocs(req, opts, function (err, res) {
      if (err) {
        return callback(err);
      }
      if (!opts.new_edits) {
        // this is what couch does when new_edits is false
        res = res.filter(function (x) {
          return x.error;
        });
      }
      callback(null, res);
    });
  });
  
  AbstractPouchDB.prototype.registerDependentDatabase =
    adapterFun('registerDependentDatabase', function (dependentDb,
                                                            callback) {
    var depDB = new this.constructor(dependentDb, this.__opts);
  
    function diffFun(doc) {
      doc.dependentDbs = doc.dependentDbs || {};
      if (doc.dependentDbs[dependentDb]) {
        return false; // no update required
      }
      doc.dependentDbs[dependentDb] = true;
      return doc;
    }
    upsert(this, '_local/_pouch_dependentDbs', diffFun)
      .then(function () {
        callback(null, {db: depDB});
      }).catch(callback);
  });
  
  AbstractPouchDB.prototype.destroy =
    adapterFun('destroy', function (opts, callback) {
  
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
  
    var self = this;
    var usePrefix = 'use_prefix' in self ? self.use_prefix : true;
  
    function destroyDb() {
      // call destroy method of the particular adaptor
      self._destroy(opts, function (err, resp) {
        if (err) {
          return callback(err);
        }
        self._destroyed = true;
        self.emit('destroyed');
        callback(null, resp || { 'ok': true });
      });
    }
  
    if (self.type() === 'http') {
      // no need to check for dependent DBs if it's a remote DB
      return destroyDb();
    }
  
    self.get('_local/_pouch_dependentDbs', function (err, localDoc) {
      if (err) {
        /* istanbul ignore if */
        if (err.status !== 404) {
          return callback(err);
        } else { // no dependencies
          return destroyDb();
        }
      }
      var dependentDbs = localDoc.dependentDbs;
      var PouchDB = self.constructor;
      var deletedMap = Object.keys(dependentDbs).map(function (name) {
        // use_prefix is only false in the browser
        /* istanbul ignore next */
        var trueName = usePrefix ?
          name.replace(new RegExp('^' + PouchDB.prefix), '') : name;
        return new PouchDB(trueName, self.__opts).destroy();
      });
      PouchPromise.all(deletedMap).then(destroyDb, callback);
    });
  });
  
  function TaskQueue() {
    this.isReady = false;
    this.failed = false;
    this.queue = [];
  }
  
  TaskQueue.prototype.execute = function () {
    var fun;
    if (this.failed) {
      while ((fun = this.queue.shift())) {
        fun(this.failed);
      }
    } else {
      while ((fun = this.queue.shift())) {
        fun();
      }
    }
  };
  
  TaskQueue.prototype.fail = function (err) {
    this.failed = err;
    this.execute();
  };
  
  TaskQueue.prototype.ready = function (db) {
    this.isReady = true;
    this.db = db;
    this.execute();
  };
  
  TaskQueue.prototype.addTask = function (fun) {
    this.queue.push(fun);
    if (this.failed) {
      this.execute();
    }
  };
  
  function parseAdapter(name, opts) {
    var match = name.match(/([a-z\-]*):\/\/(.*)/);
    if (match) {
      // the http adapter expects the fully qualified name
      name = /http(s?)/.test(match[1]) ? match[1] + '://' + match[2] : match[2];
      return {name: name, adapter: match[1]};
    }
  
    // check for browsers that have been upgraded from websql-only to websql+idb
    var skipIdb = 'idb' in PouchDB.adapters && 'websql' in PouchDB.adapters &&
      hasLocalStorage() &&
      localStorage['_pouch__websqldb_' + PouchDB.prefix + name];
  
    var adapterName;
  
    if (opts.adapter) {
      adapterName = opts.adapter;
    } else if (typeof opts !== 'undefined' && opts.db) {
      adapterName = 'leveldb';
    } else { // automatically determine adapter
      for (var i = 0; i < PouchDB.preferredAdapters.length; ++i) {
        adapterName = PouchDB.preferredAdapters[i];
        /* istanbul ignore if */
        if (skipIdb && adapterName === 'idb') {
          // log it, because this can be confusing during development
          guardedConsole('log', 'PouchDB is downgrading "' + name + '" to WebSQL to' +
            ' avoid data loss, because it was already opened with WebSQL.');
          continue; // keep using websql to avoid user data loss
        }
        break;
      }
    }
  
    var adapter = PouchDB.adapters[adapterName];
  
    // if adapter is invalid, then an error will be thrown later
    var usePrefix = (adapter && 'use_prefix' in adapter) ?
      adapter.use_prefix : true;
  
    return {
      name: usePrefix ? (PouchDB.prefix + name) : name,
      adapter: adapterName
    };
  }
  
  // OK, so here's the deal. Consider this code:
  //     var db1 = new PouchDB('foo');
  //     var db2 = new PouchDB('foo');
  //     db1.destroy();
  // ^ these two both need to emit 'destroyed' events,
  // as well as the PouchDB constructor itself.
  // So we have one db object (whichever one got destroy() called on it)
  // responsible for emitting the initial event, which then gets emitted
  // by the constructor, which then broadcasts it to any other dbs
  // that may have been created with the same name.
  function prepareForDestruction(self) {
  
    var destructionListeners = self.constructor._destructionListeners;
  
    function onDestroyed() {
      self.constructor.emit('destroyed', self.name);
    }
  
    function onConstructorDestroyed() {
      self.removeListener('destroyed', onDestroyed);
      self.emit('destroyed', self);
    }
  
    self.once('destroyed', onDestroyed);
  
    // in setup.js, the constructor is primed to listen for destroy events
    if (!destructionListeners.has(self.name)) {
      destructionListeners.set(self.name, []);
    }
    destructionListeners.get(self.name).push(onConstructorDestroyed);
  }
  
  inherits(PouchDB, AbstractPouchDB);
  function PouchDB(name, opts) {
    // In Node our test suite only tests this for PouchAlt unfortunately
    /* istanbul ignore if */
    if (!(this instanceof PouchDB)) {
      return new PouchDB(name, opts);
    }
  
    var self = this;
    opts = opts || {};
  
    if (name && typeof name === 'object') {
      opts = name;
      name = opts.name;
      delete opts.name;
    }
  
    this.__opts = opts = clone(opts);
  
    self.auto_compaction = opts.auto_compaction;
    self.prefix = PouchDB.prefix;
  
    if (typeof name !== 'string') {
      throw new Error('Missing/invalid DB name');
    }
  
    var prefixedName = (opts.prefix || '') + name;
    var backend = parseAdapter(prefixedName, opts);
  
    opts.name = backend.name;
    opts.adapter = opts.adapter || backend.adapter;
  
    self.name = name;
    self._adapter = opts.adapter;
    debug('pouchdb:adapter')('Picked adapter: ' + opts.adapter);
  
    if (!PouchDB.adapters[opts.adapter] ||
        !PouchDB.adapters[opts.adapter].valid()) {
      throw new Error('Invalid Adapter: ' + opts.adapter);
    }
  
    AbstractPouchDB.call(self);
    self.taskqueue = new TaskQueue();
  
    self.adapter = opts.adapter;
  
    PouchDB.adapters[opts.adapter].call(self, opts, function (err) {
      if (err) {
        return self.taskqueue.fail(err);
      }
      prepareForDestruction(self);
  
      self.emit('created', self);
      PouchDB.emit('created', self.name);
      self.taskqueue.ready(self);
    });
  
  }
  
  PouchDB.debug = debug;
  
  PouchDB.adapters = {};
  PouchDB.preferredAdapters = [];
  
  PouchDB.prefix = '_pouch_';
  
  var eventEmitter = new EventEmitter();
  
  function setUpEventEmitter(Pouch) {
    Object.keys(EventEmitter.prototype).forEach(function (key) {
      if (typeof EventEmitter.prototype[key] === 'function') {
        Pouch[key] = eventEmitter[key].bind(eventEmitter);
      }
    });
  
    // these are created in constructor.js, and allow us to notify each DB with
    // the same name that it was destroyed, via the constructor object
    var destructListeners = Pouch._destructionListeners = new _Map();
    Pouch.on('destroyed', function onConstructorDestroyed(name) {
      destructListeners.get(name).forEach(function (callback) {
        callback();
      });
      destructListeners.delete(name);
    });
  }
  
  setUpEventEmitter(PouchDB);
  
  PouchDB.adapter = function (id, obj, addToPreferredAdapters) {
    /* istanbul ignore else */
    if (obj.valid()) {
      PouchDB.adapters[id] = obj;
      if (addToPreferredAdapters) {
        PouchDB.preferredAdapters.push(id);
      }
    }
  };
  
  PouchDB.plugin = function (obj) {
    if (typeof obj === 'function') { // function style for plugins
      obj(PouchDB);
    } else if (typeof obj !== 'object' || Object.keys(obj).length === 0){
      throw new Error('Invalid plugin: object passed in is empty or not an object');
    } else {
      Object.keys(obj).forEach(function (id) { // object style for plugins
        PouchDB.prototype[id] = obj[id];
      });
    }
    return PouchDB;
  };
  
  PouchDB.defaults = function (defaultOpts) {
    function PouchAlt(name, opts) {
      if (!(this instanceof PouchAlt)) {
        return new PouchAlt(name, opts);
      }
  
      opts = opts || {};
  
      if (name && typeof name === 'object') {
        opts = name;
        name = opts.name;
        delete opts.name;
      }
  
      opts = extend$1({}, defaultOpts, opts);
      PouchDB.call(this, name, opts);
    }
  
    inherits(PouchAlt, PouchDB);
  
    PouchAlt.preferredAdapters = PouchDB.preferredAdapters.slice();
    Object.keys(PouchDB).forEach(function (key) {
      if (!(key in PouchAlt)) {
        PouchAlt[key] = PouchDB[key];
      }
    });
  
    return PouchAlt;
  };
  
  // managed automatically by set-version.js
  var version = "6.0.7";
  
  PouchDB.version = version;
  
  function isFunction(f) {
    return 'function' === typeof f;
  }
  
  function getPrefix(db) {
    if (isFunction(db.prefix)) {
      return db.prefix();
    }
    return db;
  }
  
  function clone$1(_obj) {
    var obj = {};
    for(var k in _obj) {
      obj[k] = _obj[k];
    }
    return obj;
  }
  
  function nut(db, precodec, codec) {
    function encodePrefix(prefix, key, opts1, opts2) {
      return precodec.encode([ prefix, codec.encodeKey(key, opts1, opts2 ) ]);
    }
  
    function addEncodings(op, prefix) {
      if(prefix && prefix.options) {
        op.keyEncoding =
          op.keyEncoding || prefix.options.keyEncoding;
        op.valueEncoding =
          op.valueEncoding || prefix.options.valueEncoding;
      }
      return op;
    }
  
    db.open(function () { /* no-op */});
  
    return {
      apply: function (ops, opts, cb) {
        opts = opts || {};
  
        var batch = [];
        var i = -1;
        var len = ops.length;
  
        while (++i < len) {
          var op = ops[i];
          addEncodings(op, op.prefix);
          op.prefix = getPrefix(op.prefix);
          batch.push({
            key: encodePrefix(op.prefix, op.key, opts, op),
            value: op.type !== 'del' && codec.encodeValue(op.value, opts, op),
            type: op.type
          });
        }
        db.db.batch(batch, opts, cb);
      },
      get: function (key, prefix, opts, cb) {
        opts.asBuffer = codec.valueAsBuffer(opts);
        return db.db.get(
          encodePrefix(prefix, key, opts),
          opts,
          function (err, value) {
            if (err) {
              cb(err);
            } else {
              cb(null, codec.decodeValue(value, opts));
            }
          }
        );
      },
      createDecoder: function (opts) {
        return function (key, value) {
          return {
            key: codec.decodeKey(precodec.decode(key)[1], opts),
            value: codec.decodeValue(value, opts)
          };
        };
      },
      isClosed: function isClosed() {
        return db.isClosed();
      },
      close: function close(cb) {
        return db.close(cb);
      },
      iterator: function (_opts) {
        var opts = clone$1(_opts || {});
        var prefix = _opts.prefix || [];
  
        function encodeKey(key) {
          return encodePrefix(prefix, key, opts, {});
        }
  
        ltgt.toLtgt(_opts, opts, encodeKey, precodec.lowerBound, precodec.upperBound);
  
        // if these legacy values are in the options, remove them
  
        opts.prefix = null;
  
        //************************************************
        //hard coded defaults, for now...
        //TODO: pull defaults and encoding out of levelup.
        opts.keyAsBuffer = opts.valueAsBuffer = false;
        //************************************************
  
  
        //this is vital, otherwise limit: undefined will
        //create an empty stream.
        /* istanbul ignore next */
        if ('number' !== typeof opts.limit) {
          opts.limit = -1;
        }
  
        opts.keyAsBuffer = precodec.buffer;
        opts.valueAsBuffer = codec.valueAsBuffer(opts);
  
        function wrapIterator(iterator) {
          return {
            next: function (cb) {
              return iterator.next(cb);
            },
            end: function (cb) {
              iterator.end(cb);
            }
          };
        }
  
        return wrapIterator(db.db.iterator(opts));
      }
    };
  }
  
  function NotFoundError() {
    Error.call(this);
  }
  
  inherits(NotFoundError, Error);
  
  NotFoundError.prototype.name = 'NotFoundError';
  
  var EventEmitter$1 = events.EventEmitter;
  var version$1 = "6.5.4";
  
  var NOT_FOUND_ERROR = new NotFoundError();
  
  var sublevel = function (nut, prefix, createStream, options) {
    var emitter = new EventEmitter$1();
    emitter.sublevels = {};
    emitter.options = options;
  
    emitter.version = version$1;
  
    emitter.methods = {};
    prefix = prefix || [];
  
    function mergeOpts(opts) {
      var o = {};
      var k;
      if (options) {
        for (k in options) {
          if (typeof options[k] !== 'undefined') {
            o[k] = options[k];
          }
        }
      }
      if (opts) {
        for (k in opts) {
          if (typeof opts[k] !== 'undefined') {
            o[k] = opts[k];
          }
        }
      }
      return o;
    }
  
    emitter.put = function (key, value, opts, cb) {
      if ('function' === typeof opts) {
        cb = opts;
        opts = {};
      }
  
      nut.apply([{
        key: key, value: value,
        prefix: prefix.slice(), type: 'put'
      }], mergeOpts(opts), function (err) {
        /* istanbul ignore next */
        if (err) {
          return cb(err);
        }
        emitter.emit('put', key, value);
        cb(null);
      });
    };
  
    emitter.prefix = function () {
      return prefix.slice();
    };
  
    emitter.batch = function (ops, opts, cb) {
      if ('function' === typeof opts) {
        cb = opts;
        opts = {};
      }
  
      ops = ops.map(function (op) {
        return {
          key: op.key,
          value: op.value,
          prefix: op.prefix || prefix,
          keyEncoding: op.keyEncoding,    // *
          valueEncoding: op.valueEncoding,  // * (TODO: encodings on sublevel)
          type: op.type
        };
      });
  
      nut.apply(ops, mergeOpts(opts), function (err) {
        /* istanbul ignore next */
        if (err) {
          return cb(err);
        }
        emitter.emit('batch', ops);
        cb(null);
      });
    };
  
    emitter.get = function (key, opts, cb) {
      /* istanbul ignore else */
      if ('function' === typeof opts) {
        cb = opts;
        opts = {};
      }
      nut.get(key, prefix, mergeOpts(opts), function (err, value) {
        if (err) {
          cb(NOT_FOUND_ERROR);
        } else {
          cb(null, value);
        }
      });
    };
  
    emitter.sublevel = function (name, opts) {
      return emitter.sublevels[name] =
        emitter.sublevels[name] || sublevel(nut, prefix.concat(name), createStream, mergeOpts(opts));
    };
  
    emitter.readStream = emitter.createReadStream = function (opts) {
      opts = mergeOpts(opts);
      opts.prefix = prefix;
      var stream;
      var it = nut.iterator(opts);
  
      stream = createStream(opts, nut.createDecoder(opts));
      stream.setIterator(it);
  
      return stream;
    };
  
    emitter.close = function (cb) {
      nut.close(cb);
    };
  
    emitter.isOpen = nut.isOpen;
    emitter.isClosed = nut.isClosed;
  
    return emitter;
  };
  
  /* Copyright (c) 2012-2014 LevelUP contributors
   * See list at <https://github.com/rvagg/node-levelup#contributing>
   * MIT License <https://github.com/rvagg/node-levelup/blob/master/LICENSE.md>
   */
  
  // NOTE: we are fixed to readable-stream@1.0.x for now
  // for pure Streams2 across Node versions
  var Readable = ReadableStreamCore.Readable;
  
  function ReadStream(options, makeData) {
    if (!(this instanceof ReadStream)) {
      return new ReadStream(options, makeData);
    }
  
    Readable.call(this, { objectMode: true, highWaterMark: options.highWaterMark });
  
    // purely to keep `db` around until we're done so it's not GCed if the user doesn't keep a ref
  
    this._waiting = false;
    this._options = options;
    this._makeData = makeData;
  }
  
  inherits(ReadStream, Readable);
  
  ReadStream.prototype.setIterator = function (it) {
    this._iterator = it;
    /* istanbul ignore if */
    if (this._destroyed) {
      return it.end(function () {});
    }
    /* istanbul ignore if */
    if (this._waiting) {
      this._waiting = false;
      return this._read();
    }
    return this;
  };
  
  ReadStream.prototype._read = function read() {
    var self = this;
    /* istanbul ignore if */
    if (self._destroyed) {
      return;
    }
    /* istanbul ignore if */
    if (!self._iterator) {
      return this._waiting = true;
    }
  
    self._iterator.next(function (err, key, value) {
      if (err || (key === undefined && value === undefined)) {
        if (!err && !self._destroyed) {
          self.push(null);
        }
        return self._cleanup(err);
      }
  
  
      value = self._makeData(key, value);
      if (!self._destroyed) {
        self.push(value);
      }
    });
  };
  
  ReadStream.prototype._cleanup = function (err) {
    if (this._destroyed) {
      return;
    }
  
    this._destroyed = true;
  
    var self = this;
    /* istanbul ignore if */
    if (err) {
      self.emit('error', err);
    }
  
    /* istanbul ignore else */
    if (self._iterator) {
      self._iterator.end(function () {
        self._iterator = null;
        self.emit('close');
      });
    } else {
      self.emit('close');
    }
  };
  
  ReadStream.prototype.destroy = function () {
    this._cleanup();
  };
  
  var precodec = {
    encode: function (decodedKey) {
      return '\xff' + decodedKey[0] + '\xff' + decodedKey[1];
    },
    decode: function (encodedKeyAsBuffer) {
      var str = encodedKeyAsBuffer.toString();
      var idx = str.indexOf('\xff', 1);
      return [str.substring(1, idx), str.substring(idx + 1)];
    },
    lowerBound: '\x00',
    upperBound: '\xff'
  };
  
  var codec = new Codec();
  
  function sublevelPouch(db) {
    return sublevel(nut(db, precodec, codec), [], ReadStream, db.options);
  }
  
  function toObject(array) {
    return array.reduce(function (obj, item) {
      obj[item] = true;
      return obj;
    }, {});
  }
  // List of top level reserved words for doc
  var reservedWords = toObject([
    '_id',
    '_rev',
    '_attachments',
    '_deleted',
    '_revisions',
    '_revs_info',
    '_conflicts',
    '_deleted_conflicts',
    '_local_seq',
    '_rev_tree',
    //replication documents
    '_replication_id',
    '_replication_state',
    '_replication_state_time',
    '_replication_state_reason',
    '_replication_stats',
    // Specific to Couchbase Sync Gateway
    '_removed'
  ]);
  
  // List of reserved words that should end up the document
  var dataWords = toObject([
    '_attachments',
    //replication documents
    '_replication_id',
    '_replication_state',
    '_replication_state_time',
    '_replication_state_reason',
    '_replication_stats'
  ]);
  
  function parseRevisionInfo(rev) {
    if (!/^\d+\-./.test(rev)) {
      return createError(INVALID_REV);
    }
    var idx = rev.indexOf('-');
    var left = rev.substring(0, idx);
    var right = rev.substring(idx + 1);
    return {
      prefix: parseInt(left, 10),
      id: right
    };
  }
  
  function makeRevTreeFromRevisions(revisions, opts) {
    var pos = revisions.start - revisions.ids.length + 1;
  
    var revisionIds = revisions.ids;
    var ids = [revisionIds[0], opts, []];
  
    for (var i = 1, len = revisionIds.length; i < len; i++) {
      ids = [revisionIds[i], {status: 'missing'}, [ids]];
    }
  
    return [{
      pos: pos,
      ids: ids
    }];
  }
  
  // Preprocess documents, parse their revisions, assign an id and a
  // revision for new writes that are missing them, etc
  function parseDoc(doc, newEdits) {
  
    var nRevNum;
    var newRevId;
    var revInfo;
    var opts = {status: 'available'};
    if (doc._deleted) {
      opts.deleted = true;
    }
  
    if (newEdits) {
      if (!doc._id) {
        doc._id = uuid();
      }
      newRevId = uuid(32, 16).toLowerCase();
      if (doc._rev) {
        revInfo = parseRevisionInfo(doc._rev);
        if (revInfo.error) {
          return revInfo;
        }
        doc._rev_tree = [{
          pos: revInfo.prefix,
          ids: [revInfo.id, {status: 'missing'}, [[newRevId, opts, []]]]
        }];
        nRevNum = revInfo.prefix + 1;
      } else {
        doc._rev_tree = [{
          pos: 1,
          ids : [newRevId, opts, []]
        }];
        nRevNum = 1;
      }
    } else {
      if (doc._revisions) {
        doc._rev_tree = makeRevTreeFromRevisions(doc._revisions, opts);
        nRevNum = doc._revisions.start;
        newRevId = doc._revisions.ids[0];
      }
      if (!doc._rev_tree) {
        revInfo = parseRevisionInfo(doc._rev);
        if (revInfo.error) {
          return revInfo;
        }
        nRevNum = revInfo.prefix;
        newRevId = revInfo.id;
        doc._rev_tree = [{
          pos: nRevNum,
          ids: [newRevId, opts, []]
        }];
      }
    }
  
    invalidIdError(doc._id);
  
    doc._rev = nRevNum + '-' + newRevId;
  
    var result = {metadata : {}, data : {}};
    for (var key in doc) {
      /* istanbul ignore else */
      if (Object.prototype.hasOwnProperty.call(doc, key)) {
        var specialKey = key[0] === '_';
        if (specialKey && !reservedWords[key]) {
          var error = createError(DOC_VALIDATION, key);
          error.message = DOC_VALIDATION.message + ': ' + key;
          throw error;
        } else if (specialKey && !dataWords[key]) {
          result.metadata[key.slice(1)] = doc[key];
        } else {
          result.data[key] = doc[key];
        }
      }
    }
    return result;
  }
  
  function atob(str) {
    var base64 = new Buffer(str, 'base64');
    // Node.js will just skip the characters it can't decode instead of
    // throwing an exception
    if (base64.toString('base64') !== str) {
      throw new Error("attachment is not a valid base64 string");
    }
    return base64.toString('binary');
  }
  
  function btoa(str) {
    return new Buffer(str, 'binary').toString('base64');
  }
  
  function typedBuffer(binString, buffType, type) {
    // buffType is either 'binary' or 'base64'
    var buff = new Buffer(binString, buffType);
    buff.type = type; // non-standard, but used for consistency with the browser
    return buff;
  }
  
  function b64ToBluffer(b64, type) {
    return typedBuffer(b64, 'base64', type);
  }
  
  function binStringToBluffer(binString, type) {
    return typedBuffer(binString, 'binary', type);
  }
  
  function blobToBase64(blobOrBuffer, callback) {
    callback(blobOrBuffer.toString('base64'));
  }
  
  function binaryMd5(data, callback) {
    var base64 = crypto.createHash('md5').update(data, 'binary').digest('base64');
    callback(base64);
  }
  
  function stringMd5(string) {
    return crypto.createHash('md5').update(string, 'binary').digest('hex');
  }
  
  function updateDoc(revLimit, prev, docInfo, results,
                     i, cb, writeDoc, newEdits) {
  
    if (revExists(prev.rev_tree, docInfo.metadata.rev)) {
      results[i] = docInfo;
      return cb();
    }
  
    // sometimes this is pre-calculated. historically not always
    var previousWinningRev = prev.winningRev || winningRev(prev);
    var previouslyDeleted = 'deleted' in prev ? prev.deleted :
      isDeleted(prev, previousWinningRev);
    var deleted = 'deleted' in docInfo.metadata ? docInfo.metadata.deleted :
      isDeleted(docInfo.metadata);
    var isRoot = /^1-/.test(docInfo.metadata.rev);
  
    if (previouslyDeleted && !deleted && newEdits && isRoot) {
      var newDoc = docInfo.data;
      newDoc._rev = previousWinningRev;
      newDoc._id = docInfo.metadata.id;
      docInfo = parseDoc(newDoc, newEdits);
    }
  
    var merged = merge(prev.rev_tree, docInfo.metadata.rev_tree[0], revLimit);
  
    var inConflict = newEdits && (((previouslyDeleted && deleted) ||
      (!previouslyDeleted && merged.conflicts !== 'new_leaf') ||
      (previouslyDeleted && !deleted && merged.conflicts === 'new_branch')));
  
    if (inConflict) {
      var err = createError(REV_CONFLICT);
      results[i] = err;
      return cb();
    }
  
    var newRev = docInfo.metadata.rev;
    docInfo.metadata.rev_tree = merged.tree;
    docInfo.stemmedRevs = merged.stemmedRevs || [];
    /* istanbul ignore else */
    if (prev.rev_map) {
      docInfo.metadata.rev_map = prev.rev_map; // used only by leveldb
    }
  
    // recalculate
    var winningRev$$ = winningRev(docInfo.metadata);
    var winningRevIsDeleted = isDeleted(docInfo.metadata, winningRev$$);
  
    // calculate the total number of documents that were added/removed,
    // from the perspective of total_rows/doc_count
    var delta = (previouslyDeleted === winningRevIsDeleted) ? 0 :
      previouslyDeleted < winningRevIsDeleted ? -1 : 1;
  
    var newRevIsDeleted;
    if (newRev === winningRev$$) {
      // if the new rev is the same as the winning rev, we can reuse that value
      newRevIsDeleted = winningRevIsDeleted;
    } else {
      // if they're not the same, then we need to recalculate
      newRevIsDeleted = isDeleted(docInfo.metadata, newRev);
    }
  
    writeDoc(docInfo, winningRev$$, winningRevIsDeleted, newRevIsDeleted,
      true, delta, i, cb);
  }
  
  function rootIsMissing(docInfo) {
    return docInfo.metadata.rev_tree[0].ids[1].status === 'missing';
  }
  
  function processDocs(revLimit, docInfos, api, fetchedDocs, tx, results,
                       writeDoc, opts, overallCallback) {
  
    // Default to 1000 locally
    revLimit = revLimit || 1000;
  
    function insertDoc(docInfo, resultsIdx, callback) {
      // Cant insert new deleted documents
      var winningRev$$ = winningRev(docInfo.metadata);
      var deleted = isDeleted(docInfo.metadata, winningRev$$);
      if ('was_delete' in opts && deleted) {
        results[resultsIdx] = createError(MISSING_DOC, 'deleted');
        return callback();
      }
  
      // 4712 - detect whether a new document was inserted with a _rev
      var inConflict = newEdits && rootIsMissing(docInfo);
  
      if (inConflict) {
        var err = createError(REV_CONFLICT);
        results[resultsIdx] = err;
        return callback();
      }
  
      var delta = deleted ? 0 : 1;
  
      writeDoc(docInfo, winningRev$$, deleted, deleted, false,
        delta, resultsIdx, callback);
    }
  
    var newEdits = opts.new_edits;
    var idsToDocs = new _Map();
  
    var docsDone = 0;
    var docsToDo = docInfos.length;
  
    function checkAllDocsDone() {
      if (++docsDone === docsToDo && overallCallback) {
        overallCallback();
      }
    }
  
    docInfos.forEach(function (currentDoc, resultsIdx) {
  
      if (currentDoc._id && isLocalId(currentDoc._id)) {
        var fun = currentDoc._deleted ? '_removeLocal' : '_putLocal';
        api[fun](currentDoc, {ctx: tx}, function (err, res) {
          results[resultsIdx] = err || res;
          checkAllDocsDone();
        });
        return;
      }
  
      var id = currentDoc.metadata.id;
      if (idsToDocs.has(id)) {
        docsToDo--; // duplicate
        idsToDocs.get(id).push([currentDoc, resultsIdx]);
      } else {
        idsToDocs.set(id, [[currentDoc, resultsIdx]]);
      }
    });
  
    // in the case of new_edits, the user can provide multiple docs
    // with the same id. these need to be processed sequentially
    idsToDocs.forEach(function (docs, id) {
      var numDone = 0;
  
      function docWritten() {
        if (++numDone < docs.length) {
          nextDoc();
        } else {
          checkAllDocsDone();
        }
      }
      function nextDoc() {
        var value = docs[numDone];
        var currentDoc = value[0];
        var resultsIdx = value[1];
  
        if (fetchedDocs.has(id)) {
          updateDoc(revLimit, fetchedDocs.get(id), currentDoc, results,
            resultsIdx, docWritten, writeDoc, newEdits);
        } else {
          // Ensure stemming applies to new writes as well
          var merged = merge([], currentDoc.metadata.rev_tree[0], revLimit);
          currentDoc.metadata.rev_tree = merged.tree;
          currentDoc.stemmedRevs = merged.stemmedRevs || [];
          insertDoc(currentDoc, resultsIdx, docWritten);
        }
      }
      nextDoc();
    });
  }
  
  function slowJsonParse(str) {
    try {
      return JSON.parse(str);
    } catch (e) {
      /* istanbul ignore next */
      return vuvuzela.parse(str);
    }
  }
  
  function safeJsonParse(str) {
    // try/catch is deoptimized in V8, leading to slower
    // times than we'd like to have. Most documents are _not_
    // huge, and do not require a slower code path just to parse them.
    // We can be pretty sure that a document under 50000 characters
    // will not be so deeply nested as to throw a stack overflow error
    // (depends on the engine and available memory, though, so this is
    // just a hunch). 50000 was chosen based on the average length
    // of this string in our test suite, to try to find a number that covers
    // most of our test cases (26 over this size, 26378 under it).
    if (str.length < 50000) {
      return JSON.parse(str);
    }
    return slowJsonParse(str);
  }
  
  function safeJsonStringify(json) {
    try {
      return JSON.stringify(json);
    } catch (e) {
      /* istanbul ignore next */
      return vuvuzela.stringify(json);
    }
  }
  
  function readAsBlobOrBuffer(storedObject, type) {
    // In Node, we've stored a buffer
    storedObject.type = type; // non-standard, but used for consistency
    return storedObject;
  }
  
  // in Node, we store the buffer directly
  function prepareAttachmentForStorage(attData, cb) {
    cb(attData);
  }
  
  function createEmptyBlobOrBuffer(type) {
    return typedBuffer('', 'binary', type);
  }
  
  // similar to an idb or websql transaction object
  // designed to be passed around. basically just caches
  // things in-memory and then does a big batch() operation
  // when you're done
  
  function getCacheFor(transaction, store) {
    var prefix = store.prefix()[0];
    var cache = transaction._cache;
    var subCache = cache.get(prefix);
    if (!subCache) {
      subCache = new _Map();
      cache.set(prefix, subCache);
    }
    return subCache;
  }
  
  function LevelTransaction() {
    this._batch = [];
    this._cache = new _Map();
  }
  
  LevelTransaction.prototype.get = function (store, key, callback) {
    var cache = getCacheFor(this, store);
    var exists = cache.get(key);
    if (exists) {
      return process.nextTick(function () {
        callback(null, exists);
      });
    } else if (exists === null) { // deleted marker
      /* istanbul ignore next */
      return process.nextTick(function () {
        callback({name: 'NotFoundError'});
      });
    }
    store.get(key, function (err, res) {
      if (err) {
        /* istanbul ignore else */
        if (err.name === 'NotFoundError') {
          cache.set(key, null);
        }
        return callback(err);
      }
      cache.set(key, res);
      callback(null, res);
    });
  };
  
  LevelTransaction.prototype.batch = function (batch) {
    for (var i = 0, len = batch.length; i < len; i++) {
      var operation = batch[i];
  
      var cache = getCacheFor(this, operation.prefix);
  
      if (operation.type === 'put') {
        cache.set(operation.key, operation.value);
      } else {
        cache.set(operation.key, null);
      }
    }
    this._batch = this._batch.concat(batch);
  };
  
  LevelTransaction.prototype.execute = function (db, callback) {
  
    var keys = new _Set();
    var uniqBatches = [];
  
    // remove duplicates; last one wins
    for (var i = this._batch.length - 1; i >= 0; i--) {
      var operation = this._batch[i];
      var lookupKey = operation.prefix.prefix()[0] + '\xff' + operation.key;
      if (keys.has(lookupKey)) {
        continue;
      }
      keys.add(lookupKey);
      uniqBatches.push(operation);
    }
  
    db.batch(uniqBatches, callback);
  };
  
  var DOC_STORE = 'document-store';
  var BY_SEQ_STORE = 'by-sequence';
  var ATTACHMENT_STORE = 'attach-store';
  var BINARY_STORE = 'attach-binary-store';
  var LOCAL_STORE = 'local-store';
  var META_STORE = 'meta-store';
  
  // leveldb barks if we try to open a db multiple times
  // so we cache opened connections here for initstore()
  var dbStores = new _Map();
  
  // store the value of update_seq in the by-sequence store the key name will
  // never conflict, since the keys in the by-sequence store are integers
  var UPDATE_SEQ_KEY = '_local_last_update_seq';
  var DOC_COUNT_KEY = '_local_doc_count';
  var UUID_KEY = '_local_uuid';
  
  var MD5_PREFIX = 'md5-';
  
  var safeJsonEncoding = {
    encode: safeJsonStringify,
    decode: safeJsonParse,
    buffer: false,
    type: 'cheap-json'
  };
  
  var levelChanges = new Changes();
  
  // winningRev and deleted are performance-killers, but
  // in newer versions of PouchDB, they are cached on the metadata
  function getWinningRev(metadata) {
    return 'winningRev' in metadata ?
      metadata.winningRev : winningRev(metadata);
  }
  
  function getIsDeleted(metadata, winningRev) {
    return 'deleted' in metadata ?
      metadata.deleted : isDeleted(metadata, winningRev);
  }
  
  function fetchAttachment(att, stores, opts) {
    var type = att.content_type;
    return new PouchPromise(function (resolve, reject) {
      stores.binaryStore.get(att.digest, function (err, buffer) {
        var data;
        if (err) {
          /* istanbul ignore if */
          if (err.name !== 'NotFoundError') {
            return reject(err);
          } else {
            // empty
            if (!opts.binary) {
              data = '';
            } else {
              data = binStringToBluffer('', type);
            }
          }
        } else { // non-empty
          if (opts.binary) {
            data = readAsBlobOrBuffer(buffer, type);
          } else {
            data = buffer.toString('base64');
          }
        }
        delete att.stub;
        delete att.length;
        att.data = data;
        resolve();
      });
    });
  }
  
  function fetchAttachments(results, stores, opts) {
    var atts = [];
    results.forEach(function (row) {
      if (!(row.doc && row.doc._attachments)) {
        return;
      }
      var attNames = Object.keys(row.doc._attachments);
      attNames.forEach(function (attName) {
        var att = row.doc._attachments[attName];
        if (!('data' in att)) {
          atts.push(att);
        }
      });
    });
  
    return PouchPromise.all(atts.map(function (att) {
      return fetchAttachment(att, stores, opts);
    }));
  }
  
  function LevelPouch$1(opts, callback) {
    opts = clone(opts);
    var api = this;
    var instanceId;
    var stores = {};
    var revLimit = opts.revs_limit;
    var db;
    var name = opts.name;
    // TODO: this is undocumented and unused probably
    /* istanbul ignore else */
    if (typeof opts.createIfMissing === 'undefined') {
      opts.createIfMissing = true;
    }
  
    var leveldown = opts.db;
  
    var dbStore;
    var leveldownName = functionName(leveldown);
    if (dbStores.has(leveldownName)) {
      dbStore = dbStores.get(leveldownName);
    } else {
      dbStore = new _Map();
      dbStores.set(leveldownName, dbStore);
    }
    if (dbStore.has(name)) {
      db = dbStore.get(name);
      afterDBCreated();
    } else {
      dbStore.set(name, sublevelPouch(levelup(name, opts, function (err) {
        /* istanbul ignore if */
        if (err) {
          dbStore.delete(name);
          return callback(err);
        }
        db = dbStore.get(name);
        db._docCount  = -1;
        db._queue = new Deque();
        /* istanbul ignore else */
        if (typeof opts.migrate === 'object') { // migration for leveldown
          opts.migrate.doMigrationOne(name, db, afterDBCreated);
        } else {
          afterDBCreated();
        }
      })));
    }
  
    function afterDBCreated() {
      stores.docStore = db.sublevel(DOC_STORE, {valueEncoding: safeJsonEncoding});
      stores.bySeqStore = db.sublevel(BY_SEQ_STORE, {valueEncoding: 'json'});
      stores.attachmentStore =
        db.sublevel(ATTACHMENT_STORE, {valueEncoding: 'json'});
      stores.binaryStore = db.sublevel(BINARY_STORE, {valueEncoding: 'binary'});
      stores.localStore = db.sublevel(LOCAL_STORE, {valueEncoding: 'json'});
      stores.metaStore = db.sublevel(META_STORE, {valueEncoding: 'json'});
      /* istanbul ignore else */
      if (typeof opts.migrate === 'object') { // migration for leveldown
        opts.migrate.doMigrationTwo(db, stores, afterLastMigration);
      } else {
        afterLastMigration();
      }
    }
  
    function afterLastMigration() {
      stores.metaStore.get(UPDATE_SEQ_KEY, function (err, value) {
        if (typeof db._updateSeq === 'undefined') {
          db._updateSeq = value || 0;
        }
        stores.metaStore.get(DOC_COUNT_KEY, function (err, value) {
          db._docCount = !err ? value : 0;
          stores.metaStore.get(UUID_KEY, function (err, value) {
            instanceId = !err ? value : uuid();
            stores.metaStore.put(UUID_KEY, instanceId, function () {
              process.nextTick(function () {
                callback(null, api);
              });
            });
          });
        });
      });
    }
  
    function countDocs(callback) {
      /* istanbul ignore if */
      if (db.isClosed()) {
        return callback(new Error('database is closed'));
      }
      return callback(null, db._docCount); // use cached value
    }
  
    api.type = function () {
      return 'leveldb';
    };
  
    api._id = function (callback) {
      callback(null, instanceId);
    };
  
    api._info = function (callback) {
      var res = {
        doc_count: db._docCount,
        update_seq: db._updateSeq,
        backend_adapter: functionName(leveldown)
      };
      return process.nextTick(function () {
        callback(null, res);
      });
    };
  
    function tryCode(fun, args) {
      try {
        fun.apply(null, args);
      } catch (err) {
        args[args.length - 1](err);
      }
    }
  
    function executeNext() {
      var firstTask = db._queue.peekFront();
  
      if (firstTask.type === 'read') {
        runReadOperation(firstTask);
      } else { // write, only do one at a time
        runWriteOperation(firstTask);
      }
    }
  
    function runReadOperation(firstTask) {
      // do multiple reads at once simultaneously, because it's safe
  
      var readTasks = [firstTask];
      var i = 1;
      var nextTask = db._queue.get(i);
      while (typeof nextTask !== 'undefined' && nextTask.type === 'read') {
        readTasks.push(nextTask);
        i++;
        nextTask = db._queue.get(i);
      }
  
      var numDone = 0;
  
      readTasks.forEach(function (readTask) {
        var args = readTask.args;
        var callback = args[args.length - 1];
        args[args.length - 1] = getArguments(function (cbArgs) {
          callback.apply(null, cbArgs);
          if (++numDone === readTasks.length) {
            process.nextTick(function () {
              // all read tasks have finished
              readTasks.forEach(function () {
                db._queue.shift();
              });
              if (db._queue.length) {
                executeNext();
              }
            });
          }
        });
        tryCode(readTask.fun, args);
      });
    }
  
    function runWriteOperation(firstTask) {
      var args = firstTask.args;
      var callback = args[args.length - 1];
      args[args.length - 1] = getArguments(function (cbArgs) {
        callback.apply(null, cbArgs);
        process.nextTick(function () {
          db._queue.shift();
          if (db._queue.length) {
            executeNext();
          }
        });
      });
      tryCode(firstTask.fun, args);
    }
  
    // all read/write operations to the database are done in a queue,
    // similar to how websql/idb works. this avoids problems such
    // as e.g. compaction needing to have a lock on the database while
    // it updates stuff. in the future we can revisit this.
    function writeLock(fun) {
      return getArguments(function (args) {
        db._queue.push({
          fun: fun,
          args: args,
          type: 'write'
        });
  
        if (db._queue.length === 1) {
          process.nextTick(executeNext);
        }
      });
    }
  
    // same as the writelock, but multiple can run at once
    function readLock(fun) {
      return getArguments(function (args) {
        db._queue.push({
          fun: fun,
          args: args,
          type: 'read'
        });
  
        if (db._queue.length === 1) {
          process.nextTick(executeNext);
        }
      });
    }
  
    function formatSeq(n) {
      return ('0000000000000000' + n).slice(-16);
    }
  
    function parseSeq(s) {
      return parseInt(s, 10);
    }
  
    api._get = readLock(function (id, opts, callback) {
      opts = clone(opts);
  
      stores.docStore.get(id, function (err, metadata) {
  
        if (err || !metadata) {
          return callback(createError(MISSING_DOC, 'missing'));
        }
  
        var rev = getWinningRev(metadata);
        var deleted = getIsDeleted(metadata, rev);
        if (deleted && !opts.rev) {
          return callback(createError(MISSING_DOC, "deleted"));
        }
  
        rev = opts.rev ? opts.rev : rev;
  
        var seq = metadata.rev_map[rev];
  
        stores.bySeqStore.get(formatSeq(seq), function (err, doc) {
          if (!doc) {
            return callback(createError(MISSING_DOC));
          }
          /* istanbul ignore if */
          if ('_id' in doc && doc._id !== metadata.id) {
            // this failing implies something very wrong
            return callback(new Error('wrong doc returned'));
          }
          doc._id = metadata.id;
          if ('_rev' in doc) {
            /* istanbul ignore if */
            if (doc._rev !== rev) {
              // this failing implies something very wrong
              return callback(new Error('wrong doc returned'));
            }
          } else {
            // we didn't always store this
            doc._rev = rev;
          }
          return callback(null, {doc: doc, metadata: metadata});
        });
      });
    });
  
    // not technically part of the spec, but if putAttachment has its own
    // method...
    api._getAttachment = function (docId, attachId, attachment, opts, callback) {
      var digest = attachment.digest;
      var type = attachment.content_type;
  
      stores.binaryStore.get(digest, function (err, attach) {
        if (err) {
          /* istanbul ignore if */
          if (err.name !== 'NotFoundError') {
            return callback(err);
          }
          // Empty attachment
          return callback(null, opts.binary ? createEmptyBlobOrBuffer(type) : '');
        }
  
        if (opts.binary) {
          callback(null, readAsBlobOrBuffer(attach, type));
        } else {
          callback(null, attach.toString('base64'));
        }
      });
    };
  
    api._bulkDocs = writeLock(function (req, opts, callback) {
      var newEdits = opts.new_edits;
      var results = new Array(req.docs.length);
      var fetchedDocs = new _Map();
      var stemmedRevs = new _Map();
  
      var txn = new LevelTransaction();
      var docCountDelta = 0;
      var newUpdateSeq = db._updateSeq;
  
      // parse the docs and give each a sequence number
      var userDocs = req.docs;
      var docInfos = userDocs.map(function (doc) {
        if (doc._id && isLocalId(doc._id)) {
          return doc;
        }
        var newDoc = parseDoc(doc, newEdits);
  
        if (newDoc.metadata && !newDoc.metadata.rev_map) {
          newDoc.metadata.rev_map = {};
        }
  
        return newDoc;
      });
      var infoErrors = docInfos.filter(function (doc) {
        return doc.error;
      });
  
      if (infoErrors.length) {
        return callback(infoErrors[0]);
      }
  
      // verify any stub attachments as a precondition test
  
      function verifyAttachment(digest, callback) {
        txn.get(stores.attachmentStore, digest, function (levelErr) {
          if (levelErr) {
            var err = createError(MISSING_STUB,
                                  'unknown stub attachment with digest ' +
                                  digest);
            callback(err);
          } else {
            callback();
          }
        });
      }
  
      function verifyAttachments(finish) {
        var digests = [];
        userDocs.forEach(function (doc) {
          if (doc && doc._attachments) {
            Object.keys(doc._attachments).forEach(function (filename) {
              var att = doc._attachments[filename];
              if (att.stub) {
                digests.push(att.digest);
              }
            });
          }
        });
        if (!digests.length) {
          return finish();
        }
        var numDone = 0;
        var err;
  
        digests.forEach(function (digest) {
          verifyAttachment(digest, function (attErr) {
            if (attErr && !err) {
              err = attErr;
            }
  
            if (++numDone === digests.length) {
              finish(err);
            }
          });
        });
      }
  
      function fetchExistingDocs(finish) {
        var numDone = 0;
        var overallErr;
        function checkDone() {
          if (++numDone === userDocs.length) {
            return finish(overallErr);
          }
        }
  
        userDocs.forEach(function (doc) {
          if (doc._id && isLocalId(doc._id)) {
            // skip local docs
            return checkDone();
          }
          txn.get(stores.docStore, doc._id, function (err, info) {
            if (err) {
              /* istanbul ignore if */
              if (err.name !== 'NotFoundError') {
                overallErr = err;
              }
            } else {
              fetchedDocs.set(doc._id, info);
            }
            checkDone();
          });
        });
      }
  
      function compact(revsMap, callback) {
        var promise = PouchPromise.resolve();
        revsMap.forEach(function (revs, docId) {
          // TODO: parallelize, for now need to be sequential to
          // pass orphaned attachment tests
          promise = promise.then(function () {
            return new PouchPromise(function (resolve, reject) {
              api._doCompactionNoLock(docId, revs, {ctx: txn}, function (err) {
                /* istanbul ignore if */
                if (err) {
                  return reject(err);
                }
                resolve();
              });
            });
          });
        });
  
        promise.then(function () {
          callback();
        }, callback);
      }
  
      function autoCompact(callback) {
        var revsMap = new _Map();
        fetchedDocs.forEach(function (metadata, docId) {
          revsMap.set(docId, compactTree(metadata));
        });
        compact(revsMap, callback);
      }
  
      function finish() {
        compact(stemmedRevs, function (error) {
          /* istanbul ignore if */
          if (error) {
            complete(error);
          }
          if (api.auto_compaction) {
            return autoCompact(complete);
          }
          complete();
        });
      }
  
      function writeDoc(docInfo, winningRev, winningRevIsDeleted, newRevIsDeleted,
                        isUpdate, delta, resultsIdx, callback2) {
        docCountDelta += delta;
  
        var err = null;
        var recv = 0;
  
        docInfo.metadata.winningRev = winningRev;
        docInfo.metadata.deleted = winningRevIsDeleted;
  
        docInfo.data._id = docInfo.metadata.id;
        docInfo.data._rev = docInfo.metadata.rev;
  
        if (newRevIsDeleted) {
          docInfo.data._deleted = true;
        }
  
        if (docInfo.stemmedRevs.length) {
          stemmedRevs.set(docInfo.metadata.id, docInfo.stemmedRevs);
        }
  
        var attachments = docInfo.data._attachments ?
          Object.keys(docInfo.data._attachments) :
          [];
  
        function attachmentSaved(attachmentErr) {
          recv++;
          if (!err) {
            /* istanbul ignore if */
            if (attachmentErr) {
              err = attachmentErr;
              callback2(err);
            } else if (recv === attachments.length) {
              finish();
            }
          }
        }
  
        function onMD5Load(doc, key, data, attachmentSaved) {
          return function (result) {
            saveAttachment(doc, MD5_PREFIX + result, key, data, attachmentSaved);
          };
        }
  
        function doMD5(doc, key, attachmentSaved) {
          return function (data) {
            binaryMd5(data, onMD5Load(doc, key, data, attachmentSaved));
          };
        }
  
        for (var i = 0; i < attachments.length; i++) {
          var key = attachments[i];
          var att = docInfo.data._attachments[key];
  
          if (att.stub) {
            // still need to update the refs mapping
            var id = docInfo.data._id;
            var rev = docInfo.data._rev;
            saveAttachmentRefs(id, rev, att.digest, attachmentSaved);
            continue;
          }
          var data;
          if (typeof att.data === 'string') {
            // input is assumed to be a base64 string
            try {
              data = atob(att.data);
            } catch (e) {
              callback(createError(BAD_ARG,
                       'Attachment is not a valid base64 string'));
              return;
            }
            doMD5(docInfo, key, attachmentSaved)(data);
          } else {
            prepareAttachmentForStorage(att.data,
              doMD5(docInfo, key, attachmentSaved));
          }
        }
  
        function finish() {
          var seq = docInfo.metadata.rev_map[docInfo.metadata.rev];
          /* istanbul ignore if */
          if (seq) {
            // check that there aren't any existing revisions with the same
            // revision id, else we shouldn't do anything
            return callback2();
          }
          seq = ++newUpdateSeq;
          docInfo.metadata.rev_map[docInfo.metadata.rev] =
            docInfo.metadata.seq = seq;
          var seqKey = formatSeq(seq);
          var batch = [{
            key: seqKey,
            value: docInfo.data,
            prefix: stores.bySeqStore,
            type: 'put'
          }, {
            key: docInfo.metadata.id,
            value: docInfo.metadata,
            prefix: stores.docStore,
            type: 'put'
          }];
          txn.batch(batch);
          results[resultsIdx] = {
            ok: true,
            id: docInfo.metadata.id,
            rev: winningRev
          };
          fetchedDocs.set(docInfo.metadata.id, docInfo.metadata);
          callback2();
        }
  
        if (!attachments.length) {
          finish();
        }
      }
  
      // attachments are queued per-digest, otherwise the refs could be
      // overwritten by concurrent writes in the same bulkDocs session
      var attachmentQueues = {};
  
      function saveAttachmentRefs(id, rev, digest, callback) {
  
        function fetchAtt() {
          return new PouchPromise(function (resolve, reject) {
            txn.get(stores.attachmentStore, digest, function (err, oldAtt) {
              /* istanbul ignore if */
              if (err && err.name !== 'NotFoundError') {
                return reject(err);
              }
              resolve(oldAtt);
            });
          });
        }
  
        function saveAtt(oldAtt) {
          var ref = [id, rev].join('@');
          var newAtt = {};
  
          if (oldAtt) {
            if (oldAtt.refs) {
              // only update references if this attachment already has them
              // since we cannot migrate old style attachments here without
              // doing a full db scan for references
              newAtt.refs = oldAtt.refs;
              newAtt.refs[ref] = true;
            }
          } else {
            newAtt.refs = {};
            newAtt.refs[ref] = true;
          }
  
          return new PouchPromise(function (resolve) {
            txn.batch([{
              type: 'put',
              prefix: stores.attachmentStore,
              key: digest,
              value: newAtt
            }]);
            resolve(!oldAtt);
          });
        }
  
        // put attachments in a per-digest queue, to avoid two docs with the same
        // attachment overwriting each other
        var queue = attachmentQueues[digest] || PouchPromise.resolve();
        attachmentQueues[digest] = queue.then(function () {
          return fetchAtt().then(saveAtt).then(function (isNewAttachment) {
            callback(null, isNewAttachment);
          }, callback);
        });
      }
  
      function saveAttachment(docInfo, digest, key, data, callback) {
        var att = docInfo.data._attachments[key];
        delete att.data;
        att.digest = digest;
        att.length = data.length;
        var id = docInfo.metadata.id;
        var rev = docInfo.metadata.rev;
        att.revpos = parseInt(rev, 10);
  
        saveAttachmentRefs(id, rev, digest, function (err, isNewAttachment) {
          /* istanbul ignore if */
          if (err) {
            return callback(err);
          }
          // do not try to store empty attachments
          if (data.length === 0) {
            return callback(err);
          }
          if (!isNewAttachment) {
            // small optimization - don't bother writing it again
            return callback(err);
          }
          txn.batch([{
            type: 'put',
            prefix: stores.binaryStore,
            key: digest,
            value: new Buffer(data, 'binary')
          }]);
          callback();
        });
      }
  
      function complete(err) {
        /* istanbul ignore if */
        if (err) {
          return process.nextTick(function () {
            callback(err);
          });
        }
        txn.batch([
          {
            prefix: stores.metaStore,
            type: 'put',
            key: UPDATE_SEQ_KEY,
            value: newUpdateSeq
          },
          {
            prefix: stores.metaStore,
            type: 'put',
            key: DOC_COUNT_KEY,
            value: db._docCount + docCountDelta
          }
        ]);
        txn.execute(db, function (err) {
          /* istanbul ignore if */
          if (err) {
            return callback(err);
          }
          db._docCount += docCountDelta;
          db._updateSeq = newUpdateSeq;
          levelChanges.notify(name);
          process.nextTick(function () {
            callback(null, results);
          });
        });
      }
  
      if (!docInfos.length) {
        return callback(null, []);
      }
  
      verifyAttachments(function (err) {
        if (err) {
          return callback(err);
        }
        fetchExistingDocs(function (err) {
          /* istanbul ignore if */
          if (err) {
            return callback(err);
          }
          processDocs(revLimit, docInfos, api, fetchedDocs, txn, results,
                      writeDoc, opts, finish);
        });
      });
    });
    api._allDocs = readLock(function (opts, callback) {
      opts = clone(opts);
      countDocs(function (err, docCount) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        var readstreamOpts = {};
        var skip = opts.skip || 0;
        if (opts.startkey) {
          readstreamOpts.gte = opts.startkey;
        }
        if (opts.endkey) {
          readstreamOpts.lte = opts.endkey;
        }
        if (opts.key) {
          readstreamOpts.gte = readstreamOpts.lte = opts.key;
        }
        if (opts.descending) {
          readstreamOpts.reverse = true;
          // switch start and ends
          var tmp = readstreamOpts.lte;
          readstreamOpts.lte = readstreamOpts.gte;
          readstreamOpts.gte = tmp;
        }
        var limit;
        if (typeof opts.limit === 'number') {
          limit = opts.limit;
        }
        if (limit === 0 ||
            ('start' in readstreamOpts && 'end' in readstreamOpts &&
            readstreamOpts.start > readstreamOpts.end)) {
          // should return 0 results when start is greater than end.
          // normally level would "fix" this for us by reversing the order,
          // so short-circuit instead
          return callback(null, {
            total_rows: docCount,
            offset: opts.skip,
            rows: []
          });
        }
        var results = [];
        var docstream = stores.docStore.readStream(readstreamOpts);
  
        var throughStream = obj(function (entry, _, next) {
          var metadata = entry.value;
          // winningRev and deleted are performance-killers, but
          // in newer versions of PouchDB, they are cached on the metadata
          var winningRev = getWinningRev(metadata);
          var deleted = getIsDeleted(metadata, winningRev);
          if (!deleted) {
            if (skip-- > 0) {
              next();
              return;
            } else if (typeof limit === 'number' && limit-- <= 0) {
              docstream.unpipe();
              docstream.destroy();
              next();
              return;
            }
          } else if (opts.deleted !== 'ok') {
            next();
            return;
          }
          function allDocsInner(data) {
            var doc = {
              id: metadata.id,
              key: metadata.id,
              value: {
                rev: winningRev
              }
            };
            if (opts.include_docs) {
              doc.doc = data;
              doc.doc._rev = doc.value.rev;
              if (opts.conflicts) {
                doc.doc._conflicts = collectConflicts(metadata);
              }
              for (var att in doc.doc._attachments) {
                if (doc.doc._attachments.hasOwnProperty(att)) {
                  doc.doc._attachments[att].stub = true;
                }
              }
            }
            if (opts.inclusive_end === false && metadata.id === opts.endkey) {
              return next();
            } else if (deleted) {
              if (opts.deleted === 'ok') {
                doc.value.deleted = true;
                doc.doc = null;
              } else {
                /* istanbul ignore next */
                return next();
              }
            }
            results.push(doc);
            next();
          }
          if (opts.include_docs) {
            var seq = metadata.rev_map[winningRev];
            stores.bySeqStore.get(formatSeq(seq), function (err, data) {
              allDocsInner(data);
            });
          }
          else {
            allDocsInner();
          }
        }, function (next) {
          PouchPromise.resolve().then(function () {
            if (opts.include_docs && opts.attachments) {
              return fetchAttachments(results, stores, opts);
            }
          }).then(function () {
            callback(null, {
              total_rows: docCount,
              offset: opts.skip,
              rows: results
            });
          }, callback);
          next();
        }).on('unpipe', function () {
          throughStream.end();
        });
  
        docstream.on('error', callback);
  
        docstream.pipe(throughStream);
      });
    });
  
    api._changes = function (opts) {
      opts = clone(opts);
  
      if (opts.continuous) {
        var id = name + ':' + uuid();
        levelChanges.addListener(name, id, api, opts);
        levelChanges.notify(name);
        return {
          cancel: function () {
            levelChanges.removeListener(name, id);
          }
        };
      }
  
      var descending = opts.descending;
      var results = [];
      var lastSeq = opts.since || 0;
      var called = 0;
      var streamOpts = {
        reverse: descending
      };
      var limit;
      if ('limit' in opts && opts.limit > 0) {
        limit = opts.limit;
      }
      if (!streamOpts.reverse) {
        streamOpts.start = formatSeq(opts.since || 0);
      }
  
      var docIds = opts.doc_ids && new _Set(opts.doc_ids);
      var filter = filterChange(opts);
      var docIdsToMetadata = new _Map();
  
      var returnDocs;
      if ('return_docs' in opts) {
        returnDocs = opts.return_docs;
      } else if ('returnDocs' in opts) {
        // TODO: Remove 'returnDocs' in favor of 'return_docs' in a future release
        returnDocs = opts.returnDocs;
      } else {
        returnDocs = true;
      }
  
      function complete() {
        opts.done = true;
        if (returnDocs && opts.limit) {
          /* istanbul ignore if */
          if (opts.limit < results.length) {
            results.length = opts.limit;
          }
        }
        changeStream.unpipe(throughStream);
        changeStream.destroy();
        if (!opts.continuous && !opts.cancelled) {
          if (opts.include_docs && opts.attachments) {
            fetchAttachments(results, stores, opts).then(function () {
              opts.complete(null, {results: results, last_seq: lastSeq});
            });
          } else {
            opts.complete(null, {results: results, last_seq: lastSeq});
          }
        }
      }
      var changeStream = stores.bySeqStore.readStream(streamOpts);
      var throughStream = obj(function (data, _, next) {
        if (limit && called >= limit) {
          complete();
          return next();
        }
        if (opts.cancelled || opts.done) {
          return next();
        }
  
        var seq = parseSeq(data.key);
        var doc = data.value;
  
        if (seq === opts.since && !descending) {
          // couchdb ignores `since` if descending=true
          return next();
        }
  
        if (docIds && !docIds.has(doc._id)) {
          return next();
        }
  
        var metadata;
  
        function onGetMetadata(metadata) {
          var winningRev = getWinningRev(metadata);
  
          function onGetWinningDoc(winningDoc) {
  
            var change = opts.processChange(winningDoc, metadata, opts);
            change.seq = metadata.seq;
  
            var filtered = filter(change);
            if (typeof filtered === 'object') {
              return opts.complete(filtered);
            }
  
            if (filtered) {
              called++;
  
              if (opts.attachments && opts.include_docs) {
                // fetch attachment immediately for the benefit
                // of live listeners
                fetchAttachments([change], stores, opts).then(function () {
                  opts.onChange(change);
                });
              } else {
                opts.onChange(change);
              }
  
              if (returnDocs) {
                results.push(change);
              }
            }
            next();
          }
  
          if (metadata.seq !== seq) {
            // some other seq is later
            return next();
          }
  
          lastSeq = seq;
  
          if (winningRev === doc._rev) {
            return onGetWinningDoc(doc);
          }
  
          // fetch the winner
  
          var winningSeq = metadata.rev_map[winningRev];
  
          stores.bySeqStore.get(formatSeq(winningSeq), function (err, doc) {
            onGetWinningDoc(doc);
          });
        }
  
        metadata = docIdsToMetadata.get(doc._id);
        if (metadata) { // cached
          return onGetMetadata(metadata);
        }
        // metadata not cached, have to go fetch it
        stores.docStore.get(doc._id, function (err, metadata) {
          /* istanbul ignore if */
          if (opts.cancelled || opts.done || db.isClosed() ||
            isLocalId(metadata.id)) {
            return next();
          }
          docIdsToMetadata.set(doc._id, metadata);
          onGetMetadata(metadata);
        });
      }, function (next) {
        if (opts.cancelled) {
          return next();
        }
        if (returnDocs && opts.limit) {
          /* istanbul ignore if */
          if (opts.limit < results.length) {
            results.length = opts.limit;
          }
        }
  
        next();
      }).on('unpipe', function () {
        throughStream.end();
        complete();
      });
      changeStream.pipe(throughStream);
      return {
        cancel: function () {
          opts.cancelled = true;
          complete();
        }
      };
    };
  
    api._close = function (callback) {
      /* istanbul ignore if */
      if (db.isClosed()) {
        return callback(createError(NOT_OPEN));
      }
      db.close(function (err) {
        /* istanbul ignore if */
        if (err) {
          callback(err);
        } else {
          dbStore.delete(name);
          callback();
        }
      });
    };
  
    api._getRevisionTree = function (docId, callback) {
      stores.docStore.get(docId, function (err, metadata) {
        if (err) {
          callback(createError(MISSING_DOC));
        } else {
          callback(null, metadata.rev_tree);
        }
      });
    };
  
    api._doCompaction = writeLock(function (docId, revs, opts, callback) {
      api._doCompactionNoLock(docId, revs, opts, callback);
    });
  
    // the NoLock version is for use by bulkDocs
    api._doCompactionNoLock = function (docId, revs, opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
  
      if (!revs.length) {
        return callback();
      }
      var txn = opts.ctx || new LevelTransaction();
  
      txn.get(stores.docStore, docId, function (err, metadata) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        var seqs = revs.map(function (rev) {
          var seq = metadata.rev_map[rev];
          delete metadata.rev_map[rev];
          return seq;
        });
        traverseRevTree(metadata.rev_tree, function (isLeaf, pos,
                                                           revHash, ctx, opts) {
          var rev = pos + '-' + revHash;
          if (revs.indexOf(rev) !== -1) {
            opts.status = 'missing';
          }
        });
  
        var batch = [];
        batch.push({
          key: metadata.id,
          value: metadata,
          type: 'put',
          prefix: stores.docStore
        });
  
        var digestMap = {};
        var numDone = 0;
        var overallErr;
        function checkDone(err) {
          /* istanbul ignore if */
          if (err) {
            overallErr = err;
          }
          if (++numDone === revs.length) { // done
            /* istanbul ignore if */
            if (overallErr) {
              return callback(overallErr);
            }
            deleteOrphanedAttachments();
          }
        }
  
        function finish(err) {
          /* istanbul ignore if */
          if (err) {
            return callback(err);
          }
          txn.batch(batch);
          if (opts.ctx) {
            // don't execute immediately
            return callback();
          }
          txn.execute(db, callback);
        }
  
        function deleteOrphanedAttachments() {
          var possiblyOrphanedAttachments = Object.keys(digestMap);
          if (!possiblyOrphanedAttachments.length) {
            return finish();
          }
          var numDone = 0;
          var overallErr;
          function checkDone(err) {
            /* istanbul ignore if */
            if (err) {
              overallErr = err;
            }
            if (++numDone === possiblyOrphanedAttachments.length) {
              finish(overallErr);
            }
          }
          var refsToDelete = new _Map();
          revs.forEach(function (rev) {
            refsToDelete.set(docId + '@' + rev, true);
          });
          possiblyOrphanedAttachments.forEach(function (digest) {
            txn.get(stores.attachmentStore, digest, function (err, attData) {
              /* istanbul ignore if */
              if (err) {
                if (err.name === 'NotFoundError') {
                  return checkDone();
                } else {
                  return checkDone(err);
                }
              }
              var refs = Object.keys(attData.refs || {}).filter(function (ref) {
                return !refsToDelete.has(ref);
              });
              var newRefs = {};
              refs.forEach(function (ref) {
                newRefs[ref] = true;
              });
              if (refs.length) { // not orphaned
                batch.push({
                  key: digest,
                  type: 'put',
                  value: {refs: newRefs},
                  prefix: stores.attachmentStore
                });
              } else { // orphaned, can safely delete
                batch = batch.concat([{
                  key: digest,
                  type: 'del',
                  prefix: stores.attachmentStore
                }, {
                  key: digest,
                  type: 'del',
                  prefix: stores.binaryStore
                }]);
              }
              checkDone();
            });
          });
        }
  
        seqs.forEach(function (seq) {
          batch.push({
            key: formatSeq(seq),
            type: 'del',
            prefix: stores.bySeqStore
          });
          txn.get(stores.bySeqStore, formatSeq(seq), function (err, doc) {
            /* istanbul ignore if */
            if (err) {
              if (err.name === 'NotFoundError') {
                return checkDone();
              } else {
                return checkDone(err);
              }
            }
            var atts = Object.keys(doc._attachments || {});
            atts.forEach(function (attName) {
              var digest = doc._attachments[attName].digest;
              digestMap[digest] = true;
            });
            checkDone();
          });
        });
      });
    };
  
    api._getLocal = function (id, callback) {
      stores.localStore.get(id, function (err, doc) {
        if (err) {
          callback(createError(MISSING_DOC));
        } else {
          callback(null, doc);
        }
      });
    };
  
    api._putLocal = function (doc, opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      if (opts.ctx) {
        api._putLocalNoLock(doc, opts, callback);
      } else {
        api._putLocalWithLock(doc, opts, callback);
      }
    };
  
    api._putLocalWithLock = writeLock(function (doc, opts, callback) {
      api._putLocalNoLock(doc, opts, callback);
    });
  
    // the NoLock version is for use by bulkDocs
    api._putLocalNoLock = function (doc, opts, callback) {
      delete doc._revisions; // ignore this, trust the rev
      var oldRev = doc._rev;
      var id = doc._id;
  
      var txn = opts.ctx || new LevelTransaction();
  
      txn.get(stores.localStore, id, function (err, resp) {
        if (err && oldRev) {
          return callback(createError(REV_CONFLICT));
        }
        if (resp && resp._rev !== oldRev) {
          return callback(createError(REV_CONFLICT));
        }
        doc._rev =
            oldRev ? '0-' + (parseInt(oldRev.split('-')[1], 10) + 1) : '0-1';
        var batch = [
          {
            type: 'put',
            prefix: stores.localStore,
            key: id,
            value: doc
          }
        ];
  
        txn.batch(batch);
        var ret = {ok: true, id: doc._id, rev: doc._rev};
  
        if (opts.ctx) {
          // don't execute immediately
          return callback(null, ret);
        }
        txn.execute(db, function (err) {
          /* istanbul ignore if */
          if (err) {
            return callback(err);
          }
          callback(null, ret);
        });
      });
    };
  
    api._removeLocal = function (doc, opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      if (opts.ctx) {
        api._removeLocalNoLock(doc, opts, callback);
      } else {
        api._removeLocalWithLock(doc, opts, callback);
      }
    };
  
    api._removeLocalWithLock = writeLock(function (doc, opts, callback) {
      api._removeLocalNoLock(doc, opts, callback);
    });
  
    // the NoLock version is for use by bulkDocs
    api._removeLocalNoLock = function (doc, opts, callback) {
      var txn = opts.ctx || new LevelTransaction();
      txn.get(stores.localStore, doc._id, function (err, resp) {
        if (err) {
          /* istanbul ignore if */
          if (err.name !== 'NotFoundError') {
            return callback(err);
          } else {
            return callback(createError(MISSING_DOC));
          }
        }
        if (resp._rev !== doc._rev) {
          return callback(createError(REV_CONFLICT));
        }
        txn.batch([{
          prefix: stores.localStore,
          type: 'del',
          key: doc._id
        }]);
        var ret = {ok: true, id: doc._id, rev: '0-0'};
        if (opts.ctx) {
          // don't execute immediately
          return callback(null, ret);
        }
        txn.execute(db, function (err) {
          /* istanbul ignore if */
          if (err) {
            return callback(err);
          }
          callback(null, ret);
        });
      });
    };
  
    // close and delete open leveldb stores
    api._destroy = function (opts, callback) {
      var dbStore;
      var leveldownName = functionName(leveldown);
      /* istanbul ignore else */
      if (dbStores.has(leveldownName)) {
        dbStore = dbStores.get(leveldownName);
      } else {
        return callDestroy(name, callback);
      }
  
      /* istanbul ignore else */
      if (dbStore.has(name)) {
        levelChanges.removeAllListeners(name);
  
        dbStore.get(name).close(function () {
          dbStore.delete(name);
          callDestroy(name, callback);
        });
      } else {
        callDestroy(name, callback);
      }
    };
    function callDestroy(name, cb) {
      leveldown.destroy(name, cb);
    }
  }
  
  // require leveldown. provide verbose output on error as it is the default
  // nodejs adapter, which we do not provide for the user
  /* istanbul ignore next */
  var requireLeveldown = function () {
    try {
      return require('leveldown');
    } catch (err) {
      /* eslint no-ex-assign: 0*/
      err = err || 'leveldown import error';
      if (err.code === 'MODULE_NOT_FOUND') {
        // handle leveldown not installed case
        return new Error([
          'the \'leveldown\' package is not available. install it, or,',
          'specify another storage backend using the \'db\' option'
        ].join(' '));
      } else if (err.message && err.message.match('Module version mismatch')) {
        // handle common user enviornment error
        return new Error([
          err.message,
          'This generally implies that leveldown was built with a different',
          'version of node than that which is running now.  You may try',
          'fully removing and reinstalling PouchDB or leveldown to resolve.'
        ].join(' '));
      }
      // handle general internal nodejs require error
      return new Error(err.toString() + ': unable to import leveldown');
    }
  };
  
  var stores = [
    'document-store',
    'by-sequence',
    'attach-store',
    'attach-binary-store'
  ];
  function formatSeq(n) {
    return ('0000000000000000' + n).slice(-16);
  }
  var UPDATE_SEQ_KEY$1 = '_local_last_update_seq';
  var DOC_COUNT_KEY$1 = '_local_doc_count';
  var UUID_KEY$1 = '_local_uuid';
  
  var doMigrationOne = function (name, db, callback) {
    // local require to prevent crashing if leveldown isn't installed.
    var leveldown = require("leveldown");
  
    var base = path.resolve(name);
    function move(store, index, cb) {
      var storePath = path.join(base, store);
      var opts;
      if (index === 3) {
        opts = {
          valueEncoding: 'binary'
        };
      } else {
        opts = {
          valueEncoding: 'json'
        };
      }
      var sub = db.sublevel(store, opts);
      var orig = levelup(storePath, opts);
      var from = orig.createReadStream();
      var writeStream = new LevelWriteStream(sub);
      var to = writeStream();
      from.on('end', function () {
        orig.close(function (err) {
          cb(err, storePath);
        });
      });
      from.pipe(to);
    }
    fs.unlink(base + '.uuid', function (err) {
      if (err) {
        return callback();
      }
      var todo = 4;
      var done = [];
      stores.forEach(function (store, i) {
        move(store, i, function (err, storePath) {
          /* istanbul ignore if */
          if (err) {
            return callback(err);
          }
          done.push(storePath);
          if (!(--todo)) {
            done.forEach(function (item) {
              leveldown.destroy(item, function () {
                if (++todo === done.length) {
                  fs.rmdir(base, callback);
                }
              });
            });
          }
        });
      });
    });
  };
  var doMigrationTwo = function (db, stores, callback) {
    var batches = [];
    stores.bySeqStore.get(UUID_KEY$1, function (err, value) {
      if (err) {
        // no uuid key, so don't need to migrate;
        return callback();
      }
      batches.push({
        key: UUID_KEY$1,
        value: value,
        prefix: stores.metaStore,
        type: 'put',
        valueEncoding: 'json'
      });
      batches.push({
        key: UUID_KEY$1,
        prefix: stores.bySeqStore,
        type: 'del'
      });
      stores.bySeqStore.get(DOC_COUNT_KEY$1, function (err, value) {
        if (value) {
          // if no doc count key,
          // just skip
          // we can live with this
          batches.push({
            key: DOC_COUNT_KEY$1,
            value: value,
            prefix: stores.metaStore,
            type: 'put',
            valueEncoding: 'json'
          });
          batches.push({
            key: DOC_COUNT_KEY$1,
            prefix: stores.bySeqStore,
            type: 'del'
          });
        }
        stores.bySeqStore.get(UPDATE_SEQ_KEY$1, function (err, value) {
          if (value) {
            // if no UPDATE_SEQ_KEY
            // just skip
            // we've gone to far to stop.
            batches.push({
              key: UPDATE_SEQ_KEY$1,
              value: value,
              prefix: stores.metaStore,
              type: 'put',
              valueEncoding: 'json'
            });
            batches.push({
              key: UPDATE_SEQ_KEY$1,
              prefix: stores.bySeqStore,
              type: 'del'
            });
          }
          var deletedSeqs = {};
          stores.docStore.createReadStream({
            startKey: '_',
            endKey: '_\xFF'
          }).pipe(obj(function (ch, _, next) {
            if (!isLocalId(ch.key)) {
              return next();
            }
            batches.push({
              key: ch.key,
              prefix: stores.docStore,
              type: 'del'
            });
            var winner = winningRev(ch.value);
            Object.keys(ch.value.rev_map).forEach(function (key) {
              if (key !== 'winner') {
                this.push(formatSeq(ch.value.rev_map[key]));
              }
            }, this);
            var winningSeq = ch.value.rev_map[winner];
            stores.bySeqStore.get(formatSeq(winningSeq), function (err, value) {
              if (!err) {
                batches.push({
                  key: ch.key,
                  value: value,
                  prefix: stores.localStore,
                  type: 'put',
                  valueEncoding: 'json'
                });
              }
              next();
            });
  
          })).pipe(obj(function (seq, _, next) {
            /* istanbul ignore if */
            if (deletedSeqs[seq]) {
              return next();
            }
            deletedSeqs[seq] = true;
            stores.bySeqStore.get(seq, function (err, resp) {
              /* istanbul ignore if */
              if (err || !isLocalId(resp._id)) {
                return next();
              }
              batches.push({
                key: seq,
                prefix: stores.bySeqStore,
                type: 'del'
              });
              next();
            });
          }, function () {
            db.batch(batches, callback);
          }));
        });
      });
    });
  
  };
  
  var migrate = {
    doMigrationOne: doMigrationOne,
    doMigrationTwo: doMigrationTwo
  };
  
  function LevelDownPouch(opts, callback) {
  
    // Users can pass in their own leveldown alternative here, in which case
    // it overrides the default one. (This is in addition to the custom builds.)
    var leveldown = opts.db;
  
    /* istanbul ignore else */
    if (!leveldown) {
      leveldown = requireLeveldown();
  
      /* istanbul ignore if */
      if (leveldown instanceof Error) {
        return callback(leveldown);
      }
    }
  
    var _opts = extend$1({
      db: leveldown,
      migrate: migrate
    }, opts);
  
    LevelPouch$1.call(this, _opts, callback);
  }
  
  // overrides for normal LevelDB behavior on Node
  LevelDownPouch.valid = function () {
    return true;
  };
  LevelDownPouch.use_prefix = false;
  
  function LevelPouch (PouchDB) {
    PouchDB.adapter('leveldb', LevelDownPouch, true);
  }
  
  // May seem redundant, but this is to allow switching with
  // request-browser.js.
  var request = require('request');
  
  // non-standard, but we do this to mimic blobs in the browser
  function applyTypeToBuffer(buffer, resp) {
    buffer.type = resp.headers['content-type'];
  }
  
  function defaultBody() {
    return new Buffer('', 'binary');
  }
  
  function ajaxCore(options, callback) {
  
    options = clone(options);
  
    var defaultOptions = {
      method : "GET",
      headers: {},
      json: true,
      processData: true,
      timeout: 10000,
      cache: false
    };
  
    options = extend$1(defaultOptions, options);
  
    function onSuccess(obj, resp, cb) {
      if (!options.binary && options.json && typeof obj === 'string') {
        /* istanbul ignore next */
        try {
          obj = JSON.parse(obj);
        } catch (e) {
          // Probably a malformed JSON from server
          return cb(e);
        }
      }
      if (Array.isArray(obj)) {
        obj = obj.map(function (v) {
          if (v.error || v.missing) {
            return generateErrorFromResponse(v);
          } else {
            return v;
          }
        });
      }
      if (options.binary) {
        applyTypeToBuffer(obj, resp);
      }
      cb(null, obj, resp);
    }
  
    if (options.json) {
      if (!options.binary) {
        options.headers.Accept = 'application/json';
      }
      options.headers['Content-Type'] = options.headers['Content-Type'] ||
        'application/json';
    }
  
    if (options.binary) {
      options.encoding = null;
      options.json = false;
    }
  
    if (!options.processData) {
      options.json = false;
    }
  
    return request(options, function (err, response, body) {
  
      if (err) {
        return callback(generateErrorFromResponse(err));
      }
  
      var error;
      var content_type = response.headers && response.headers['content-type'];
      var data = body || defaultBody();
  
      // CouchDB doesn't always return the right content-type for JSON data, so
      // we check for ^{ and }$ (ignoring leading/trailing whitespace)
      if (!options.binary && (options.json || !options.processData) &&
          typeof data !== 'object' &&
          (/json/.test(content_type) ||
           (/^[\s]*\{/.test(data) && /\}[\s]*$/.test(data)))) {
        try {
          data = JSON.parse(data.toString());
        } catch (e) {}
      }
  
      if (response.statusCode >= 200 && response.statusCode < 300) {
        onSuccess(data, response, callback);
      } else {
        error = generateErrorFromResponse(data);
        error.status = response.statusCode;
        callback(error);
      }
    });
  }
  
  function ajax(opts, callback) {
    // do nothing; all the action is in prerequest-browser.js
    return ajaxCore(opts, callback);
  }
  
  var CHANGES_BATCH_SIZE = 25;
  var MAX_SIMULTANEOUS_REVS = 50;
  
  var supportsBulkGetMap = {};
  
  var log$1 = debug('pouchdb:http');
  
  function readAttachmentsAsBlobOrBuffer(row) {
    var atts = row.doc && row.doc._attachments;
    if (!atts) {
      return;
    }
    Object.keys(atts).forEach(function (filename) {
      var att = atts[filename];
      att.data = b64ToBluffer(att.data, att.content_type);
    });
  }
  
  function encodeDocId(id) {
    if (/^_design/.test(id)) {
      return '_design/' + encodeURIComponent(id.slice(8));
    }
    if (/^_local/.test(id)) {
      return '_local/' + encodeURIComponent(id.slice(7));
    }
    return encodeURIComponent(id);
  }
  
  function preprocessAttachments$1(doc) {
    if (!doc._attachments || !Object.keys(doc._attachments)) {
      return PouchPromise.resolve();
    }
  
    return PouchPromise.all(Object.keys(doc._attachments).map(function (key) {
      var attachment = doc._attachments[key];
      if (attachment.data && typeof attachment.data !== 'string') {
        return new PouchPromise(function (resolve) {
          blobToBase64(attachment.data, resolve);
        }).then(function (b64) {
          attachment.data = b64;
        });
      }
    }));
  }
  
  function hasUrlPrefix(opts) {
    if (!opts.prefix) {
      return false;
    }
  
    var protocol = parseUri(opts.prefix).protocol;
  
    return protocol === 'http' || protocol === 'https';
  }
  
  // Get all the information you possibly can about the URI given by name and
  // return it as a suitable object.
  function getHost(name, opts) {
  
    // encode db name if opts.prefix is a url (#5574)
    if (hasUrlPrefix(opts)) {
      var dbName = opts.name.substr(opts.prefix.length);
      name = opts.prefix + encodeURIComponent(dbName);
    }
  
    // Prase the URI into all its little bits
    var uri = parseUri(name);
  
    // Store the user and password as a separate auth object
    if (uri.user || uri.password) {
      uri.auth = {username: uri.user, password: uri.password};
    }
  
    // Split the path part of the URI into parts using '/' as the delimiter
    // after removing any leading '/' and any trailing '/'
    var parts = uri.path.replace(/(^\/|\/$)/g, '').split('/');
  
    // Store the first part as the database name and remove it from the parts
    // array
    uri.db = parts.pop();
    // Prevent double encoding of URI component
    if (uri.db.indexOf('%') === -1) {
      uri.db = encodeURIComponent(uri.db);
    }
  
    // Restore the path by joining all the remaining parts (all the parts
    // except for the database name) with '/'s
    uri.path = parts.join('/');
  
    return uri;
  }
  
  // Generate a URL with the host data given by opts and the given path
  function genDBUrl(opts, path) {
    return genUrl(opts, opts.db + '/' + path);
  }
  
  // Generate a URL with the host data given by opts and the given path
  function genUrl(opts, path) {
    // If the host already has a path, then we need to have a path delimiter
    // Otherwise, the path delimiter is the empty string
    var pathDel = !opts.path ? '' : '/';
  
    // If the host already has a path, then we need to have a path delimiter
    // Otherwise, the path delimiter is the empty string
    return opts.protocol + '://' + opts.host +
           (opts.port ? (':' + opts.port) : '') +
           '/' + opts.path + pathDel + path;
  }
  
  function paramsToStr(params) {
    return '?' + Object.keys(params).map(function (k) {
      return k + '=' + encodeURIComponent(params[k]);
    }).join('&');
  }
  
  // Implements the PouchDB API for dealing with CouchDB instances over HTTP
  function HttpPouch(opts, callback) {
  
    // The functions that will be publicly available for HttpPouch
    var api = this;
  
    var host = getHost(opts.name, opts);
    var dbUrl = genDBUrl(host, '');
  
    opts = clone(opts);
    var ajaxOpts = opts.ajax || {};
  
    if (opts.auth || host.auth) {
      var nAuth = opts.auth || host.auth;
      var str = nAuth.username + ':' + nAuth.password;
      var token = btoa(unescape(encodeURIComponent(str)));
      ajaxOpts.headers = ajaxOpts.headers || {};
      ajaxOpts.headers.Authorization = 'Basic ' + token;
    }
  
    // Not strictly necessary, but we do this because numerous tests
    // rely on swapping ajax in and out.
    api._ajax = ajax;
  
    function ajax$$(userOpts, options, callback) {
      var reqAjax = userOpts.ajax || {};
      var reqOpts = extend$1(clone(ajaxOpts), reqAjax, options);
      log$1(reqOpts.method + ' ' + reqOpts.url);
      return api._ajax(reqOpts, callback);
    }
  
    function ajaxPromise(userOpts, opts) {
      return new PouchPromise(function (resolve, reject) {
        ajax$$(userOpts, opts, function (err, res$$) {
          /* istanbul ignore if */
          if (err) {
            return reject(err);
          }
          resolve(res$$);
        });
      });
    }
  
    function adapterFun$$(name, fun) {
      return adapterFun(name, getArguments(function (args) {
        setup().then(function () {
          return fun.apply(this, args);
        }).catch(function (e) {
          var callback = args.pop();
          callback(e);
        });
      }));
    }
  
    var setupPromise;
  
    function setup() {
      // TODO: Remove `skipSetup` in favor of `skip_setup` in a future release
      if (opts.skipSetup || opts.skip_setup) {
        return PouchPromise.resolve();
      }
  
      // If there is a setup in process or previous successful setup
      // done then we will use that
      // If previous setups have been rejected we will try again
      if (setupPromise) {
        return setupPromise;
      }
  
      var checkExists = {method: 'GET', url: dbUrl};
      setupPromise = ajaxPromise({}, checkExists).catch(function (err) {
        if (err && err.status && err.status === 404) {
          // Doesnt exist, create it
          res(404, 'PouchDB is just detecting if the remote exists.');
          return ajaxPromise({}, {method: 'PUT', url: dbUrl});
        } else {
          return PouchPromise.reject(err);
        }
      }).catch(function (err) {
        // If we try to create a database that already exists, skipped in
        // istanbul since its catching a race condition.
        /* istanbul ignore if */
        if (err && err.status && err.status === 412) {
          return true;
        }
        return PouchPromise.reject(err);
      });
  
      setupPromise.catch(function () {
        setupPromise = null;
      });
  
      return setupPromise;
    }
  
    setTimeout(function () {
      callback(null, api);
    });
  
    api.type = function () {
      return 'http';
    };
  
    api.id = adapterFun$$('id', function (callback) {
      ajax$$({}, {method: 'GET', url: genUrl(host, '')}, function (err, result) {
        var uuid = (result && result.uuid) ?
          (result.uuid + host.db) : genDBUrl(host, '');
        callback(null, uuid);
      });
    });
  
    api.request = adapterFun$$('request', function (options, callback) {
      options.url = genDBUrl(host, options.url);
      ajax$$({}, options, callback);
    });
  
    // Sends a POST request to the host calling the couchdb _compact function
    //    version: The version of CouchDB it is running
    api.compact = adapterFun$$('compact', function (opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      opts = clone(opts);
      ajax$$(opts, {
        url: genDBUrl(host, '_compact'),
        method: 'POST'
      }, function () {
        function ping() {
          api.info(function (err, res$$) {
            if (res$$ && !res$$.compact_running) {
              callback(null, {ok: true});
            } else {
              setTimeout(ping, opts.interval || 200);
            }
          });
        }
        // Ping the http if it's finished compaction
        ping();
      });
    });
  
    api.bulkGet = adapterFun('bulkGet', function (opts, callback) {
      var self = this;
  
      function doBulkGet(cb) {
        var params = {};
        if (opts.revs) {
          params.revs = true;
        }
        if (opts.attachments) {
          /* istanbul ignore next */
          params.attachments = true;
        }
        ajax$$({}, {
          url: genDBUrl(host, '_bulk_get' + paramsToStr(params)),
          method: 'POST',
          body: { docs: opts.docs}
        }, cb);
      }
  
      function doBulkGetShim() {
        // avoid "url too long error" by splitting up into multiple requests
        var batchSize = MAX_SIMULTANEOUS_REVS;
        var numBatches = Math.ceil(opts.docs.length / batchSize);
        var numDone = 0;
        var results = new Array(numBatches);
  
        function onResult(batchNum) {
          return function (err, res$$) {
            // err is impossible because shim returns a list of errs in that case
            results[batchNum] = res$$.results;
            if (++numDone === numBatches) {
              callback(null, {results: flatten(results)});
            }
          };
        }
  
        for (var i = 0; i < numBatches; i++) {
          var subOpts = pick(opts, ['revs', 'attachments']);
          subOpts.ajax = ajaxOpts;
          subOpts.docs = opts.docs.slice(i * batchSize,
            Math.min(opts.docs.length, (i + 1) * batchSize));
          bulkGet(self, subOpts, onResult(i));
        }
      }
  
      // mark the whole database as either supporting or not supporting _bulk_get
      var dbUrl = genUrl(host, '');
      var supportsBulkGet = supportsBulkGetMap[dbUrl];
  
      if (typeof supportsBulkGet !== 'boolean') {
        // check if this database supports _bulk_get
        doBulkGet(function (err, res$$) {
          /* istanbul ignore else */
          if (err) {
            var status = Math.floor(err.status / 100);
            /* istanbul ignore else */
            if (status === 4 || status === 5) { // 40x or 50x
              supportsBulkGetMap[dbUrl] = false;
              res(
                err.status,
                'PouchDB is just detecting if the remote ' +
                'supports the _bulk_get API.'
              );
              doBulkGetShim();
            } else {
              callback(err);
            }
          } else {
            supportsBulkGetMap[dbUrl] = true;
            callback(null, res$$);
          }
        });
      } else if (supportsBulkGet) {
        /* istanbul ignore next */
        doBulkGet(callback);
      } else {
        doBulkGetShim();
      }
    });
  
    // Calls GET on the host, which gets back a JSON string containing
    //    couchdb: A welcome string
    //    version: The version of CouchDB it is running
    api._info = function (callback) {
      setup().then(function () {
        ajax$$({}, {
          method: 'GET',
          url: genDBUrl(host, '')
        }, function (err, res$$) {
          /* istanbul ignore next */
          if (err) {
          return callback(err);
          }
          res$$.host = genDBUrl(host, '');
          callback(null, res$$);
        });
      }).catch(callback);
    };
  
    // Get the document with the given id from the database given by host.
    // The id could be solely the _id in the database, or it may be a
    // _design/ID or _local/ID path
    api.get = adapterFun$$('get', function (id, opts, callback) {
      // If no options were given, set the callback to the second parameter
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      opts = clone(opts);
  
      // List of parameters to add to the GET request
      var params = {};
  
      if (opts.revs) {
        params.revs = true;
      }
  
      if (opts.revs_info) {
        params.revs_info = true;
      }
  
      if (opts.open_revs) {
        if (opts.open_revs !== "all") {
          opts.open_revs = JSON.stringify(opts.open_revs);
        }
        params.open_revs = opts.open_revs;
      }
  
      if (opts.rev) {
        params.rev = opts.rev;
      }
  
      if (opts.conflicts) {
        params.conflicts = opts.conflicts;
      }
  
      id = encodeDocId(id);
  
      // Set the options for the ajax call
      var options = {
        method: 'GET',
        url: genDBUrl(host, id + paramsToStr(params))
      };
  
      function fetchAttachments(doc) {
        var atts = doc._attachments;
        var filenames = atts && Object.keys(atts);
        if (!atts || !filenames.length) {
          return;
        }
        // we fetch these manually in separate XHRs, because
        // Sync Gateway would normally send it back as multipart/mixed,
        // which we cannot parse. Also, this is more efficient than
        // receiving attachments as base64-encoded strings.
        function fetch() {
  
          if (!filenames.length) {
            return null;
          }
  
          var filename = filenames.pop();
          var att = atts[filename];
          var path = encodeDocId(doc._id) + '/' + encodeAttachmentId(filename) +
            '?rev=' + doc._rev;
          return ajaxPromise(opts, {
            method: 'GET',
            url: genDBUrl(host, path),
            binary: true
          }).then(function (blob) {
            if (opts.binary) {
              return blob;
            }
            return new PouchPromise(function (resolve) {
              blobToBase64(blob, resolve);
            });
          }).then(function (data) {
            delete att.stub;
            delete att.length;
            att.data = data;
          });
        }
  
        // This limits the number of parallel xhr requests to 5 any time
        // to avoid issues with maximum browser request limits
        return new PromisePool(fetch, 5, {promise: PouchPromise}).start();
      }
  
      function fetchAllAttachments(docOrDocs) {
        if (Array.isArray(docOrDocs)) {
          return PouchPromise.all(docOrDocs.map(function (doc) {
            if (doc.ok) {
              return fetchAttachments(doc.ok);
            }
          }));
        }
        return fetchAttachments(docOrDocs);
      }
  
      ajaxPromise(opts, options).then(function (res$$) {
        return PouchPromise.resolve().then(function () {
          if (opts.attachments) {
            return fetchAllAttachments(res$$);
          }
        }).then(function () {
          callback(null, res$$);
        });
      }).catch(callback);
    });
  
    // Delete the document given by doc from the database given by host.
    api.remove = adapterFun$$('remove',
        function (docOrId, optsOrRev, opts, callback) {
      var doc;
      if (typeof optsOrRev === 'string') {
        // id, rev, opts, callback style
        doc = {
          _id: docOrId,
          _rev: optsOrRev
        };
        if (typeof opts === 'function') {
          callback = opts;
          opts = {};
        }
      } else {
        // doc, opts, callback style
        doc = docOrId;
        if (typeof optsOrRev === 'function') {
          callback = optsOrRev;
          opts = {};
        } else {
          callback = opts;
          opts = optsOrRev;
        }
      }
  
      var rev = (doc._rev || opts.rev);
  
      // Delete the document
      ajax$$(opts, {
        method: 'DELETE',
        url: genDBUrl(host, encodeDocId(doc._id)) + '?rev=' + rev
      }, callback);
    });
  
    function encodeAttachmentId(attachmentId) {
      return attachmentId.split("/").map(encodeURIComponent).join("/");
    }
  
    // Get the attachment
    api.getAttachment =
      adapterFun$$('getAttachment', function (docId, attachmentId, opts,
                                                  callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      var params = opts.rev ? ('?rev=' + opts.rev) : '';
      var url = genDBUrl(host, encodeDocId(docId)) + '/' +
        encodeAttachmentId(attachmentId) + params;
      ajax$$(opts, {
        method: 'GET',
        url: url,
        binary: true
      }, callback);
    });
  
    // Remove the attachment given by the id and rev
    api.removeAttachment =
      adapterFun$$('removeAttachment', function (docId, attachmentId, rev,
                                                     callback) {
  
      var url = genDBUrl(host, encodeDocId(docId) + '/' +
        encodeAttachmentId(attachmentId)) + '?rev=' + rev;
  
      ajax$$({}, {
        method: 'DELETE',
        url: url
      }, callback);
    });
  
    // Add the attachment given by blob and its contentType property
    // to the document with the given id, the revision given by rev, and
    // add it to the database given by host.
    api.putAttachment =
      adapterFun$$('putAttachment', function (docId, attachmentId, rev, blob,
                                                  type, callback) {
      if (typeof type === 'function') {
        callback = type;
        type = blob;
        blob = rev;
        rev = null;
      }
      var id = encodeDocId(docId) + '/' + encodeAttachmentId(attachmentId);
      var url = genDBUrl(host, id);
      if (rev) {
        url += '?rev=' + rev;
      }
  
      if (typeof blob === 'string') {
        // input is assumed to be a base64 string
        var binary;
        try {
          binary = atob(blob);
        } catch (err) {
          return callback(createError(BAD_ARG,
                          'Attachment is not a valid base64 string'));
        }
        blob = binary ? binStringToBluffer(binary, type) : '';
      }
  
      var opts = {
        headers: {'Content-Type': type},
        method: 'PUT',
        url: url,
        processData: false,
        body: blob,
        timeout: ajaxOpts.timeout || 60000
      };
      // Add the attachment
      ajax$$({}, opts, callback);
    });
  
    // Update/create multiple documents given by req in the database
    // given by host.
    api._bulkDocs = function (req, opts, callback) {
      // If new_edits=false then it prevents the database from creating
      // new revision numbers for the documents. Instead it just uses
      // the old ones. This is used in database replication.
      req.new_edits = opts.new_edits;
  
      setup().then(function () {
        return PouchPromise.all(req.docs.map(preprocessAttachments$1));
      }).then(function () {
        // Update/create the documents
        ajax$$(opts, {
          method: 'POST',
          url: genDBUrl(host, '_bulk_docs'),
          timeout: opts.timeout,
          body: req
        }, function (err, results) {
          if (err) {
            return callback(err);
          }
          results.forEach(function (result) {
            result.ok = true; // smooths out cloudant not adding this
          });
          callback(null, results);
        });
      }).catch(callback);
    };
  
  
    // Update/create document
    api._put = function (doc, opts, callback) {
      setup().then(function () {
        return preprocessAttachments$1(doc);
      }).then(function () {
        // Update/create the document
        ajax$$(opts, {
          method: 'PUT',
          url: genDBUrl(host, encodeDocId(doc._id)),
          body: doc
        }, function (err, result) {
          if (err) {
            return callback(err);
          }
          callback(null, result);
        });
      }).catch(callback);
    };
  
  
    // Get a listing of the documents in the database given
    // by host and ordered by increasing id.
    api.allDocs = adapterFun$$('allDocs', function (opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      opts = clone(opts);
  
      // List of parameters to add to the GET request
      var params = {};
      var body;
      var method = 'GET';
  
      if (opts.conflicts) {
        params.conflicts = true;
      }
  
      if (opts.descending) {
        params.descending = true;
      }
  
      if (opts.include_docs) {
        params.include_docs = true;
      }
  
      // added in CouchDB 1.6.0
      if (opts.attachments) {
        params.attachments = true;
      }
  
      if (opts.key) {
        params.key = JSON.stringify(opts.key);
      }
  
      if (opts.start_key) {
        opts.startkey = opts.start_key;
      }
  
      if (opts.startkey) {
        params.startkey = JSON.stringify(opts.startkey);
      }
  
      if (opts.end_key) {
        opts.endkey = opts.end_key;
      }
  
      if (opts.endkey) {
        params.endkey = JSON.stringify(opts.endkey);
      }
  
      if (typeof opts.inclusive_end !== 'undefined') {
        params.inclusive_end = !!opts.inclusive_end;
      }
  
      if (typeof opts.limit !== 'undefined') {
        params.limit = opts.limit;
      }
  
      if (typeof opts.skip !== 'undefined') {
        params.skip = opts.skip;
      }
  
      var paramStr = paramsToStr(params);
  
      if (typeof opts.keys !== 'undefined') {
        method = 'POST';
        body = {keys: opts.keys};
      }
  
      // Get the document listing
      ajaxPromise(opts, {
        method: method,
        url: genDBUrl(host, '_all_docs' + paramStr),
        body: body
      }).then(function (res$$) {
        if (opts.include_docs && opts.attachments && opts.binary) {
          res$$.rows.forEach(readAttachmentsAsBlobOrBuffer);
        }
        callback(null, res$$);
      }).catch(callback);
    });
  
    // Get a list of changes made to documents in the database given by host.
    // TODO According to the README, there should be two other methods here,
    // api.changes.addListener and api.changes.removeListener.
    api._changes = function (opts) {
  
      // We internally page the results of a changes request, this means
      // if there is a large set of changes to be returned we can start
      // processing them quicker instead of waiting on the entire
      // set of changes to return and attempting to process them at once
      var batchSize = 'batch_size' in opts ? opts.batch_size : CHANGES_BATCH_SIZE;
  
      opts = clone(opts);
      opts.timeout = ('timeout' in opts) ? opts.timeout :
        ('timeout' in ajaxOpts) ? ajaxOpts.timeout :
        30 * 1000;
  
      // We give a 5 second buffer for CouchDB changes to respond with
      // an ok timeout (if a timeout it set)
      var params = opts.timeout ? {timeout: opts.timeout - (5 * 1000)} : {};
      var limit = (typeof opts.limit !== 'undefined') ? opts.limit : false;
      var returnDocs;
      if ('return_docs' in opts) {
        returnDocs = opts.return_docs;
      } else if ('returnDocs' in opts) {
        // TODO: Remove 'returnDocs' in favor of 'return_docs' in a future release
        returnDocs = opts.returnDocs;
      } else {
        returnDocs = true;
      }
      //
      var leftToFetch = limit;
  
      if (opts.style) {
        params.style = opts.style;
      }
  
      if (opts.include_docs || opts.filter && typeof opts.filter === 'function') {
        params.include_docs = true;
      }
  
      if (opts.attachments) {
        params.attachments = true;
      }
  
      if (opts.continuous) {
        params.feed = 'longpoll';
      }
  
      if (opts.conflicts) {
        params.conflicts = true;
      }
  
      if (opts.descending) {
        params.descending = true;
      }
  
      if ('heartbeat' in opts) {
        // If the heartbeat value is false, it disables the default heartbeat
        if (opts.heartbeat) {
          params.heartbeat = opts.heartbeat;
        }
      } else {
        // Default heartbeat to 10 seconds
        params.heartbeat = 10000;
      }
  
      if (opts.filter && typeof opts.filter === 'string') {
        params.filter = opts.filter;
      }
  
      if (opts.view && typeof opts.view === 'string') {
        params.filter = '_view';
        params.view = opts.view;
      }
  
      // If opts.query_params exists, pass it through to the changes request.
      // These parameters may be used by the filter on the source database.
      if (opts.query_params && typeof opts.query_params === 'object') {
        for (var param_name in opts.query_params) {
          /* istanbul ignore else */
          if (opts.query_params.hasOwnProperty(param_name)) {
            params[param_name] = opts.query_params[param_name];
          }
        }
      }
  
      var method = 'GET';
      var body;
  
      if (opts.doc_ids) {
        // set this automagically for the user; it's annoying that couchdb
        // requires both a "filter" and a "doc_ids" param.
        params.filter = '_doc_ids';
        method = 'POST';
        body = {doc_ids: opts.doc_ids };
      }
  
      var xhr;
      var lastFetchedSeq;
  
      // Get all the changes starting wtih the one immediately after the
      // sequence number given by since.
      var fetch = function (since, callback) {
        if (opts.aborted) {
          return;
        }
        params.since = since;
        // "since" can be any kind of json object in Coudant/CouchDB 2.x
        /* istanbul ignore next */
        if (typeof params.since === "object") {
          params.since = JSON.stringify(params.since);
        }
  
        if (opts.descending) {
          if (limit) {
            params.limit = leftToFetch;
          }
        } else {
          params.limit = (!limit || leftToFetch > batchSize) ?
            batchSize : leftToFetch;
        }
  
        // Set the options for the ajax call
        var xhrOpts = {
          method: method,
          url: genDBUrl(host, '_changes' + paramsToStr(params)),
          timeout: opts.timeout,
          body: body
        };
        lastFetchedSeq = since;
  
        /* istanbul ignore if */
        if (opts.aborted) {
          return;
        }
  
        // Get the changes
        setup().then(function () {
          xhr = ajax$$(opts, xhrOpts, callback);
        }).catch(callback);
      };
  
      // If opts.since exists, get all the changes from the sequence
      // number given by opts.since. Otherwise, get all the changes
      // from the sequence number 0.
      var results = {results: []};
  
      var fetched = function (err, res$$) {
        if (opts.aborted) {
          return;
        }
        var raw_results_length = 0;
        // If the result of the ajax call (res) contains changes (res.results)
        if (res$$ && res$$.results) {
          raw_results_length = res$$.results.length;
          results.last_seq = res$$.last_seq;
          // For each change
          var req = {};
          req.query = opts.query_params;
          res$$.results = res$$.results.filter(function (c) {
            leftToFetch--;
            var ret = filterChange(opts)(c);
            if (ret) {
              if (opts.include_docs && opts.attachments && opts.binary) {
                readAttachmentsAsBlobOrBuffer(c);
              }
              if (returnDocs) {
                results.results.push(c);
              }
              opts.onChange(c);
            }
            return ret;
          });
        } else if (err) {
          // In case of an error, stop listening for changes and call
          // opts.complete
          opts.aborted = true;
          opts.complete(err);
          return;
        }
  
        // The changes feed may have timed out with no results
        // if so reuse last update sequence
        if (res$$ && res$$.last_seq) {
          lastFetchedSeq = res$$.last_seq;
        }
  
        var finished = (limit && leftToFetch <= 0) ||
          (res$$ && raw_results_length < batchSize) ||
          (opts.descending);
  
        if ((opts.continuous && !(limit && leftToFetch <= 0)) || !finished) {
          // Queue a call to fetch again with the newest sequence number
          setTimeout(function () { fetch(lastFetchedSeq, fetched); }, 0);
        } else {
          // We're done, call the callback
          opts.complete(null, results);
        }
      };
  
      fetch(opts.since || 0, fetched);
  
      // Return a method to cancel this method from processing any more
      return {
        cancel: function () {
          opts.aborted = true;
          if (xhr) {
            xhr.abort();
          }
        }
      };
    };
  
    // Given a set of document/revision IDs (given by req), tets the subset of
    // those that do NOT correspond to revisions stored in the database.
    // See http://wiki.apache.org/couchdb/HttpPostRevsDiff
    api.revsDiff = adapterFun$$('revsDiff', function (req, opts, callback) {
      // If no options were given, set the callback to be the second parameter
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
  
      // Get the missing document/revision IDs
      ajax$$(opts, {
        method: 'POST',
        url: genDBUrl(host, '_revs_diff'),
        body: req
      }, callback);
    });
  
    api._close = function (callback) {
      callback();
    };
  
    api._destroy = function (options, callback) {
      ajax$$(options, {
        url: genDBUrl(host, ''),
        method: 'DELETE'
      }, function (err, resp) {
        if (err && err.status && err.status !== 404) {
          return callback(err);
        }
        callback(null, resp);
      });
    };
  }
  
  // HttpPouch is a valid adapter.
  HttpPouch.valid = function () {
    return true;
  };
  
  function HttpPouch$1 (PouchDB) {
    PouchDB.adapter('http', HttpPouch, false);
    PouchDB.adapter('https', HttpPouch, false);
  }
  
  function pad(str, padWith, upToLength) {
    var padding = '';
    var targetLength = upToLength - str.length;
    /* istanbul ignore next */
    while (padding.length < targetLength) {
      padding += padWith;
    }
    return padding;
  }
  
  function padLeft(str, padWith, upToLength) {
    var padding = pad(str, padWith, upToLength);
    return padding + str;
  }
  
  var MIN_MAGNITUDE = -324; // verified by -Number.MIN_VALUE
  var MAGNITUDE_DIGITS = 3; // ditto
  var SEP = ''; // set to '_' for easier debugging 
  
  function collate(a, b) {
  
    if (a === b) {
      return 0;
    }
  
    a = normalizeKey(a);
    b = normalizeKey(b);
  
    var ai = collationIndex(a);
    var bi = collationIndex(b);
    if ((ai - bi) !== 0) {
      return ai - bi;
    }
    if (a === null) {
      return 0;
    }
    switch (typeof a) {
      case 'number':
        return a - b;
      case 'boolean':
        return a === b ? 0 : (a < b ? -1 : 1);
      case 'string':
        return stringCollate(a, b);
    }
    return Array.isArray(a) ? arrayCollate(a, b) : objectCollate(a, b);
  }
  
  // couch considers null/NaN/Infinity/-Infinity === undefined,
  // for the purposes of mapreduce indexes. also, dates get stringified.
  function normalizeKey(key) {
    switch (typeof key) {
      case 'undefined':
        return null;
      case 'number':
        if (key === Infinity || key === -Infinity || isNaN(key)) {
          return null;
        }
        return key;
      case 'object':
        var origKey = key;
        if (Array.isArray(key)) {
          var len = key.length;
          key = new Array(len);
          for (var i = 0; i < len; i++) {
            key[i] = normalizeKey(origKey[i]);
          }
        /* istanbul ignore next */
        } else if (key instanceof Date) {
          return key.toJSON();
        } else if (key !== null) { // generic object
          key = {};
          for (var k in origKey) {
            if (origKey.hasOwnProperty(k)) {
              var val = origKey[k];
              if (typeof val !== 'undefined') {
                key[k] = normalizeKey(val);
              }
            }
          }
        }
    }
    return key;
  }
  
  function indexify(key) {
    if (key !== null) {
      switch (typeof key) {
        case 'boolean':
          return key ? 1 : 0;
        case 'number':
          return numToIndexableString(key);
        case 'string':
          // We've to be sure that key does not contain \u0000
          // Do order-preserving replacements:
          // 0 -> 1, 1
          // 1 -> 1, 2
          // 2 -> 2, 2
          return key
            .replace(/\u0002/g, '\u0002\u0002')
            .replace(/\u0001/g, '\u0001\u0002')
            .replace(/\u0000/g, '\u0001\u0001');
        case 'object':
          var isArray = Array.isArray(key);
          var arr = isArray ? key : Object.keys(key);
          var i = -1;
          var len = arr.length;
          var result = '';
          if (isArray) {
            while (++i < len) {
              result += toIndexableString(arr[i]);
            }
          } else {
            while (++i < len) {
              var objKey = arr[i];
              result += toIndexableString(objKey) +
                  toIndexableString(key[objKey]);
            }
          }
          return result;
      }
    }
    return '';
  }
  
  // convert the given key to a string that would be appropriate
  // for lexical sorting, e.g. within a database, where the
  // sorting is the same given by the collate() function.
  function toIndexableString(key) {
    var zero = '\u0000';
    key = normalizeKey(key);
    return collationIndex(key) + SEP + indexify(key) + zero;
  }
  
  function parseNumber(str, i) {
    var originalIdx = i;
    var num;
    var zero = str[i] === '1';
    if (zero) {
      num = 0;
      i++;
    } else {
      var neg = str[i] === '0';
      i++;
      var numAsString = '';
      var magAsString = str.substring(i, i + MAGNITUDE_DIGITS);
      var magnitude = parseInt(magAsString, 10) + MIN_MAGNITUDE;
      /* istanbul ignore next */
      if (neg) {
        magnitude = -magnitude;
      }
      i += MAGNITUDE_DIGITS;
      while (true) {
        var ch = str[i];
        if (ch === '\u0000') {
          break;
        } else {
          numAsString += ch;
        }
        i++;
      }
      numAsString = numAsString.split('.');
      if (numAsString.length === 1) {
        num = parseInt(numAsString, 10);
      } else {
        /* istanbul ignore next */
        num = parseFloat(numAsString[0] + '.' + numAsString[1]);
      }
      /* istanbul ignore next */
      if (neg) {
        num = num - 10;
      }
      /* istanbul ignore next */
      if (magnitude !== 0) {
        // parseFloat is more reliable than pow due to rounding errors
        // e.g. Number.MAX_VALUE would return Infinity if we did
        // num * Math.pow(10, magnitude);
        num = parseFloat(num + 'e' + magnitude);
      }
    }
    return {num: num, length : i - originalIdx};
  }
  
  // move up the stack while parsing
  // this function moved outside of parseIndexableString for performance
  function pop(stack, metaStack) {
    var obj = stack.pop();
  
    if (metaStack.length) {
      var lastMetaElement = metaStack[metaStack.length - 1];
      if (obj === lastMetaElement.element) {
        // popping a meta-element, e.g. an object whose value is another object
        metaStack.pop();
        lastMetaElement = metaStack[metaStack.length - 1];
      }
      var element = lastMetaElement.element;
      var lastElementIndex = lastMetaElement.index;
      if (Array.isArray(element)) {
        element.push(obj);
      } else if (lastElementIndex === stack.length - 2) { // obj with key+value
        var key = stack.pop();
        element[key] = obj;
      } else {
        stack.push(obj); // obj with key only
      }
    }
  }
  
  function parseIndexableString(str) {
    var stack = [];
    var metaStack = []; // stack for arrays and objects
    var i = 0;
  
    /*eslint no-constant-condition: ["error", { "checkLoops": false }]*/
    while (true) {
      var collationIndex = str[i++];
      if (collationIndex === '\u0000') {
        if (stack.length === 1) {
          return stack.pop();
        } else {
          pop(stack, metaStack);
          continue;
        }
      }
      switch (collationIndex) {
        case '1':
          stack.push(null);
          break;
        case '2':
          stack.push(str[i] === '1');
          i++;
          break;
        case '3':
          var parsedNum = parseNumber(str, i);
          stack.push(parsedNum.num);
          i += parsedNum.length;
          break;
        case '4':
          var parsedStr = '';
          /*eslint no-constant-condition: ["error", { "checkLoops": false }]*/
          while (true) {
            var ch = str[i];
            if (ch === '\u0000') {
              break;
            }
            parsedStr += ch;
            i++;
          }
          // perform the reverse of the order-preserving replacement
          // algorithm (see above)
          parsedStr = parsedStr.replace(/\u0001\u0001/g, '\u0000')
            .replace(/\u0001\u0002/g, '\u0001')
            .replace(/\u0002\u0002/g, '\u0002');
          stack.push(parsedStr);
          break;
        case '5':
          var arrayElement = { element: [], index: stack.length };
          stack.push(arrayElement.element);
          metaStack.push(arrayElement);
          break;
        case '6':
          var objElement = { element: {}, index: stack.length };
          stack.push(objElement.element);
          metaStack.push(objElement);
          break;
        /* istanbul ignore next */
        default:
          throw new Error(
            'bad collationIndex or unexpectedly reached end of input: ' +
              collationIndex);
      }
    }
  }
  
  function arrayCollate(a, b) {
    var len = Math.min(a.length, b.length);
    for (var i = 0; i < len; i++) {
      var sort = collate(a[i], b[i]);
      if (sort !== 0) {
        return sort;
      }
    }
    return (a.length === b.length) ? 0 :
      (a.length > b.length) ? 1 : -1;
  }
  function stringCollate(a, b) {
    // See: https://github.com/daleharvey/pouchdb/issues/40
    // This is incompatible with the CouchDB implementation, but its the
    // best we can do for now
    return (a === b) ? 0 : ((a > b) ? 1 : -1);
  }
  function objectCollate(a, b) {
    var ak = Object.keys(a), bk = Object.keys(b);
    var len = Math.min(ak.length, bk.length);
    for (var i = 0; i < len; i++) {
      // First sort the keys
      var sort = collate(ak[i], bk[i]);
      if (sort !== 0) {
        return sort;
      }
      // if the keys are equal sort the values
      sort = collate(a[ak[i]], b[bk[i]]);
      if (sort !== 0) {
        return sort;
      }
  
    }
    return (ak.length === bk.length) ? 0 :
      (ak.length > bk.length) ? 1 : -1;
  }
  // The collation is defined by erlangs ordered terms
  // the atoms null, true, false come first, then numbers, strings,
  // arrays, then objects
  // null/undefined/NaN/Infinity/-Infinity are all considered null
  function collationIndex(x) {
    var id = ['boolean', 'number', 'string', 'object'];
    var idx = id.indexOf(typeof x);
    //false if -1 otherwise true, but fast!!!!1
    if (~idx) {
      if (x === null) {
        return 1;
      }
      if (Array.isArray(x)) {
        return 5;
      }
      return idx < 3 ? (idx + 2) : (idx + 3);
    }
    /* istanbul ignore next */
    if (Array.isArray(x)) {
      return 5;
    }
  }
  
  // conversion:
  // x yyy zz...zz
  // x = 0 for negative, 1 for 0, 2 for positive
  // y = exponent (for negative numbers negated) moved so that it's >= 0
  // z = mantisse
  function numToIndexableString(num) {
  
    if (num === 0) {
      return '1';
    }
  
    // convert number to exponential format for easier and
    // more succinct string sorting
    var expFormat = num.toExponential().split(/e\+?/);
    var magnitude = parseInt(expFormat[1], 10);
  
    var neg = num < 0;
  
    var result = neg ? '0' : '2';
  
    // first sort by magnitude
    // it's easier if all magnitudes are positive
    var magForComparison = ((neg ? -magnitude : magnitude) - MIN_MAGNITUDE);
    var magString = padLeft((magForComparison).toString(), '0', MAGNITUDE_DIGITS);
  
    result += SEP + magString;
  
    // then sort by the factor
    var factor = Math.abs(parseFloat(expFormat[0])); // [1..10)
    /* istanbul ignore next */
    if (neg) { // for negative reverse ordering
      factor = 10 - factor;
    }
  
    var factorStr = factor.toFixed(20);
  
    // strip zeros from the end
    factorStr = factorStr.replace(/\.?0+$/, '');
  
    result += SEP + factorStr;
  
    return result;
  }
  
  /*
   * Simple task queue to sequentialize actions. Assumes
   * callbacks will eventually fire (once).
   */
  
  function TaskQueue$1() {
    this.promise = new PouchPromise(function (fulfill) {fulfill(); });
  }
  TaskQueue$1.prototype.add = function (promiseFactory) {
    this.promise = this.promise.catch(function () {
      // just recover
    }).then(function () {
      return promiseFactory();
    });
    return this.promise;
  };
  TaskQueue$1.prototype.finish = function () {
    return this.promise;
  };
  
  function createView(opts) {
    var sourceDB = opts.db;
    var viewName = opts.viewName;
    var mapFun = opts.map;
    var reduceFun = opts.reduce;
    var temporary = opts.temporary;
  
    // the "undefined" part is for backwards compatibility
    var viewSignature = mapFun.toString() + (reduceFun && reduceFun.toString()) +
      'undefined';
  
    var cachedViews;
    if (!temporary) {
      // cache this to ensure we don't try to update the same view twice
      cachedViews = sourceDB._cachedViews = sourceDB._cachedViews || {};
      if (cachedViews[viewSignature]) {
        return cachedViews[viewSignature];
      }
    }
  
    var promiseForView = sourceDB.info().then(function (info) {
  
      var depDbName = info.db_name + '-mrview-' +
        (temporary ? 'temp' : stringMd5(viewSignature));
  
      // save the view name in the source db so it can be cleaned up if necessary
      // (e.g. when the _design doc is deleted, remove all associated view data)
      function diffFunction(doc) {
        doc.views = doc.views || {};
        var fullViewName = viewName;
        if (fullViewName.indexOf('/') === -1) {
          fullViewName = viewName + '/' + viewName;
        }
        var depDbs = doc.views[fullViewName] = doc.views[fullViewName] || {};
        /* istanbul ignore if */
        if (depDbs[depDbName]) {
          return; // no update necessary
        }
        depDbs[depDbName] = true;
        return doc;
      }
      return upsert(sourceDB, '_local/mrviews', diffFunction).then(function () {
        return sourceDB.registerDependentDatabase(depDbName).then(function (res) {
          var db = res.db;
          db.auto_compaction = true;
          var view = {
            name: depDbName,
            db: db,
            sourceDB: sourceDB,
            adapter: sourceDB.adapter,
            mapFun: mapFun,
            reduceFun: reduceFun
          };
          return view.db.get('_local/lastSeq').catch(function (err) {
            /* istanbul ignore if */
            if (err.status !== 404) {
              throw err;
            }
          }).then(function (lastSeqDoc) {
            view.seq = lastSeqDoc ? lastSeqDoc.seq : 0;
            if (cachedViews) {
              view.db.once('destroyed', function () {
                delete cachedViews[viewSignature];
              });
            }
            return view;
          });
        });
      });
    });
  
    if (cachedViews) {
      cachedViews[viewSignature] = promiseForView;
    }
    return promiseForView;
  }
  
  function QueryParseError(message) {
    this.status = 400;
    this.name = 'query_parse_error';
    this.message = message;
    this.error = true;
    try {
      Error.captureStackTrace(this, QueryParseError);
    } catch (e) {}
  }
  
  inherits(QueryParseError, Error);
  
  function NotFoundError$1(message) {
    this.status = 404;
    this.name = 'not_found';
    this.message = message;
    this.error = true;
    try {
      Error.captureStackTrace(this, NotFoundError$1);
    } catch (e) {}
  }
  
  inherits(NotFoundError$1, Error);
  
  function BuiltInError(message) {
    this.status = 500;
    this.name = 'invalid_value';
    this.message = message;
    this.error = true;
    try {
      Error.captureStackTrace(this, BuiltInError);
    } catch (e) {}
  }
  
  inherits(BuiltInError, Error);
  
  function createBuiltInError(name) {
    var message = 'builtin ' + name +
      ' function requires map values to be numbers' +
      ' or number arrays';
    return new BuiltInError(message);
  }
  
  function sum(values) {
    var result = 0;
    for (var i = 0, len = values.length; i < len; i++) {
      var num = values[i];
      if (typeof num !== 'number') {
        if (Array.isArray(num)) {
          // lists of numbers are also allowed, sum them separately
          result = typeof result === 'number' ? [result] : result;
          for (var j = 0, jLen = num.length; j < jLen; j++) {
            var jNum = num[j];
            if (typeof jNum !== 'number') {
              throw createBuiltInError('_sum');
            } else if (typeof result[j] === 'undefined') {
              result.push(jNum);
            } else {
              result[j] += jNum;
            }
          }
        } else { // not array/number
          throw createBuiltInError('_sum');
        }
      } else if (typeof result === 'number') {
        result += num;
      } else { // add number to array
        result[0] += num;
      }
    }
    return result;
  }
  
  // Inside of 'vm' for Node, we need a way to translate a pseudo-error
  // back into a real error once it's out of the VM.
  function createBuiltInErrorInVm(name) {
    return {
      builtInError: true,
      name: name
    };
  }
  
  function convertToTrueError(err) {
    return createBuiltInError(err.name);
  }
  
  function isBuiltInError(obj) {
    return obj && obj.builtInError;
  }
  
  // All of this vm hullaballoo is to be able to run arbitrary code in a sandbox
  // for security reasons.
  function evalFunctionInVm(func, emit) {
    return function (arg1, arg2, arg3) {
      var code = '(function() {"use strict";' +
        'var createBuiltInError = ' + createBuiltInErrorInVm.toString() + ';' +
        'var sum = ' + sum.toString() + ';' +
        'var log = function () {};' +
        'var isArray = Array.isArray;' +
        'var toJSON = JSON.parse;' +
        'var __emitteds__ = [];' +
        'var emit = function (key, value) {__emitteds__.push([key, value]);};' +
        'var __result__ = (' +
        func.replace(/;\s*$/, '') + ')' + '(' +
        JSON.stringify(arg1) + ',' +
        JSON.stringify(arg2) + ',' +
        JSON.stringify(arg3) + ');' +
        'return {result: __result__, emitteds: __emitteds__};' +
        '})()';
  
      var output = vm.runInNewContext(code);
  
      output.emitteds.forEach(function (emitted) {
        emit(emitted[0], emitted[1]);
      });
      if (isBuiltInError(output.result)) {
        output.result = convertToTrueError(output.result);
      }
      return output.result;
    };
  }
  
  var log$2 = guardedConsole.bind(null, 'log');
  
  // The "stringify, then execute in a VM" strategy totally breaks Istanbul due
  // to missing __coverage global objects. As a solution, export different
  // code during coverage testing and during regular execution.
  // Note that this doesn't get shipped to consumers because Rollup replaces it
  // with rollup-plugin-replace, so false is replaced with `false`
  var evalFunc;
  /* istanbul ignore else */
  if (false) {} else {
    evalFunc = evalFunctionInVm;
  }
  
  var evalFunction = evalFunc;
  
  var promisedCallback = function (promise, callback) {
    if (callback) {
      promise.then(function (res) {
        process.nextTick(function () {
          callback(null, res);
        });
      }, function (reason) {
        process.nextTick(function () {
          callback(reason);
        });
      });
    }
    return promise;
  };
  
  var callbackify = function (fun) {
    return getArguments(function (args) {
      var cb = args.pop();
      var promise = fun.apply(this, args);
      if (typeof cb === 'function') {
        promisedCallback(promise, cb);
      }
      return promise;
    });
  };
  
  // Promise finally util similar to Q.finally
  var fin = function (promise, finalPromiseFactory) {
    return promise.then(function (res) {
      return finalPromiseFactory().then(function () {
        return res;
      });
    }, function (reason) {
      return finalPromiseFactory().then(function () {
        throw reason;
      });
    });
  };
  
  var sequentialize = function (queue, promiseFactory) {
    return function () {
      var args = arguments;
      var that = this;
      return queue.add(function () {
        return promiseFactory.apply(that, args);
      });
    };
  };
  
  // uniq an array of strings, order not guaranteed
  // similar to underscore/lodash _.uniq
  var uniq = function (arr) {
    var map = {};
  
    for (var i = 0, len = arr.length; i < len; i++) {
      map['$' + arr[i]] = true;
    }
  
    var keys = Object.keys(map);
    var output = new Array(keys.length);
  
    for (i = 0, len = keys.length; i < len; i++) {
      output[i] = keys[i].substring(1);
    }
    return output;
  };
  
  var persistentQueues = {};
  var tempViewQueue = new TaskQueue$1();
  var CHANGES_BATCH_SIZE$1 = 50;
  
  function parseViewName(name) {
    // can be either 'ddocname/viewname' or just 'viewname'
    // (where the ddoc name is the same)
    return name.indexOf('/') === -1 ? [name, name] : name.split('/');
  }
  
  function isGenOne(changes) {
    // only return true if the current change is 1-
    // and there are no other leafs
    return changes.length === 1 && /^1-/.test(changes[0].rev);
  }
  
  function emitError(db, e) {
    try {
      db.emit('error', e);
    } catch (err) {
      guardedConsole('error',
        'The user\'s map/reduce function threw an uncaught error.\n' +
        'You can debug this error by doing:\n' +
        'myDatabase.on(\'error\', function (err) { debugger; });\n' +
        'Please double-check your map/reduce function.');
      guardedConsole('error', e);
    }
  }
  
  function tryCode(db, fun, args) {
    // emit an event if there was an error thrown by a map/reduce function.
    // putting try/catches in a single function also avoids deoptimizations.
    try {
      return {
        output : fun.apply(null, args)
      };
    } catch (e) {
      emitError(db, e);
      return {error: e};
    }
  }
  
  function sortByKeyThenValue(x, y) {
    var keyCompare = collate(x.key, y.key);
    return keyCompare !== 0 ? keyCompare : collate(x.value, y.value);
  }
  
  function sliceResults(results, limit, skip) {
    skip = skip || 0;
    if (typeof limit === 'number') {
      return results.slice(skip, limit + skip);
    } else if (skip > 0) {
      return results.slice(skip);
    }
    return results;
  }
  
  function rowToDocId(row) {
    var val = row.value;
    // Users can explicitly specify a joined doc _id, or it
    // defaults to the doc _id that emitted the key/value.
    var docId = (val && typeof val === 'object' && val._id) || row.id;
    return docId;
  }
  
  function readAttachmentsAsBlobOrBuffer$1(res) {
    res.rows.forEach(function (row) {
      var atts = row.doc && row.doc._attachments;
      if (!atts) {
        return;
      }
      Object.keys(atts).forEach(function (filename) {
        var att = atts[filename];
        atts[filename].data = b64ToBluffer(att.data, att.content_type);
      });
    });
  }
  
  function postprocessAttachments(opts) {
    return function (res) {
      if (opts.include_docs && opts.attachments && opts.binary) {
        readAttachmentsAsBlobOrBuffer$1(res);
      }
      return res;
    };
  }
  
  var builtInReduce = {
    _sum: function (keys, values) {
      return sum(values);
    },
  
    _count: function (keys, values) {
      return values.length;
    },
  
    _stats: function (keys, values) {
      // no need to implement rereduce=true, because Pouch
      // will never call it
      function sumsqr(values) {
        var _sumsqr = 0;
        for (var i = 0, len = values.length; i < len; i++) {
          var num = values[i];
          _sumsqr += (num * num);
        }
        return _sumsqr;
      }
      return {
        sum     : sum(values),
        min     : Math.min.apply(null, values),
        max     : Math.max.apply(null, values),
        count   : values.length,
        sumsqr : sumsqr(values)
      };
    }
  };
  
  function addHttpParam(paramName, opts, params, asJson) {
    // add an http param from opts to params, optionally json-encoded
    var val = opts[paramName];
    if (typeof val !== 'undefined') {
      if (asJson) {
        val = encodeURIComponent(JSON.stringify(val));
      }
      params.push(paramName + '=' + val);
    }
  }
  
  function coerceInteger(integerCandidate) {
    if (typeof integerCandidate !== 'undefined') {
      var asNumber = Number(integerCandidate);
      // prevents e.g. '1foo' or '1.1' being coerced to 1
      if (!isNaN(asNumber) && asNumber === parseInt(integerCandidate, 10)) {
        return asNumber;
      } else {
        return integerCandidate;
      }
    }
  }
  
  function coerceOptions(opts) {
    opts.group_level = coerceInteger(opts.group_level);
    opts.limit = coerceInteger(opts.limit);
    opts.skip = coerceInteger(opts.skip);
    return opts;
  }
  
  function checkPositiveInteger(number) {
    if (number) {
      if (typeof number !== 'number') {
        return  new QueryParseError('Invalid value for integer: "' +
        number + '"');
      }
      if (number < 0) {
        return new QueryParseError('Invalid value for positive integer: ' +
          '"' + number + '"');
      }
    }
  }
  
  function checkQueryParseError(options, fun) {
    var startkeyName = options.descending ? 'endkey' : 'startkey';
    var endkeyName = options.descending ? 'startkey' : 'endkey';
  
    if (typeof options[startkeyName] !== 'undefined' &&
      typeof options[endkeyName] !== 'undefined' &&
      collate(options[startkeyName], options[endkeyName]) > 0) {
      throw new QueryParseError('No rows can match your key range, ' +
      'reverse your start_key and end_key or set {descending : true}');
    } else if (fun.reduce && options.reduce !== false) {
      if (options.include_docs) {
        throw new QueryParseError('{include_docs:true} is invalid for reduce');
      } else if (options.keys && options.keys.length > 1 &&
          !options.group && !options.group_level) {
        throw new QueryParseError('Multi-key fetches for reduce views must use ' +
        '{group: true}');
      }
    }
    ['group_level', 'limit', 'skip'].forEach(function (optionName) {
      var error = checkPositiveInteger(options[optionName]);
      if (error) {
        throw error;
      }
    });
  }
  
  function httpQuery(db, fun, opts) {
    // List of parameters to add to the PUT request
    var params = [];
    var body;
    var method = 'GET';
  
    // If opts.reduce exists and is defined, then add it to the list
    // of parameters.
    // If reduce=false then the results are that of only the map function
    // not the final result of map and reduce.
    addHttpParam('reduce', opts, params);
    addHttpParam('include_docs', opts, params);
    addHttpParam('attachments', opts, params);
    addHttpParam('limit', opts, params);
    addHttpParam('descending', opts, params);
    addHttpParam('group', opts, params);
    addHttpParam('group_level', opts, params);
    addHttpParam('skip', opts, params);
    addHttpParam('stale', opts, params);
    addHttpParam('conflicts', opts, params);
    addHttpParam('startkey', opts, params, true);
    addHttpParam('start_key', opts, params, true);
    addHttpParam('endkey', opts, params, true);
    addHttpParam('end_key', opts, params, true);
    addHttpParam('inclusive_end', opts, params);
    addHttpParam('key', opts, params, true);
  
    // Format the list of parameters into a valid URI query string
    params = params.join('&');
    params = params === '' ? '' : '?' + params;
  
    // If keys are supplied, issue a POST to circumvent GET query string limits
    // see http://wiki.apache.org/couchdb/HTTP_view_API#Querying_Options
    if (typeof opts.keys !== 'undefined') {
      var MAX_URL_LENGTH = 2000;
      // according to http://stackoverflow.com/a/417184/680742,
      // the de facto URL length limit is 2000 characters
  
      var keysAsString =
        'keys=' + encodeURIComponent(JSON.stringify(opts.keys));
      if (keysAsString.length + params.length + 1 <= MAX_URL_LENGTH) {
        // If the keys are short enough, do a GET. we do this to work around
        // Safari not understanding 304s on POSTs (see pouchdb/pouchdb#1239)
        params += (params[0] === '?' ? '&' : '?') + keysAsString;
      } else {
        method = 'POST';
        if (typeof fun === 'string') {
          body = {keys: opts.keys};
        } else { // fun is {map : mapfun}, so append to this
          fun.keys = opts.keys;
        }
      }
    }
  
    // We are referencing a query defined in the design doc
    if (typeof fun === 'string') {
      var parts = parseViewName(fun);
      return db.request({
        method: method,
        url: '_design/' + parts[0] + '/_view/' + parts[1] + params,
        body: body
      }).then(postprocessAttachments(opts));
    }
  
    // We are using a temporary view, terrible for performance, good for testing
    body = body || {};
    Object.keys(fun).forEach(function (key) {
      if (Array.isArray(fun[key])) {
        body[key] = fun[key];
      } else {
        body[key] = fun[key].toString();
      }
    });
    return db.request({
      method: 'POST',
      url: '_temp_view' + params,
      body: body
    }).then(postprocessAttachments(opts));
  }
  
  // custom adapters can define their own api._query
  // and override the default behavior
  /* istanbul ignore next */
  function customQuery(db, fun, opts) {
    return new PouchPromise(function (resolve, reject) {
      db._query(fun, opts, function (err, res) {
        if (err) {
          return reject(err);
        }
        resolve(res);
      });
    });
  }
  
  // custom adapters can define their own api._viewCleanup
  // and override the default behavior
  /* istanbul ignore next */
  function customViewCleanup(db) {
    return new PouchPromise(function (resolve, reject) {
      db._viewCleanup(function (err, res) {
        if (err) {
          return reject(err);
        }
        resolve(res);
      });
    });
  }
  
  function defaultsTo(value) {
    return function (reason) {
      /* istanbul ignore else */
      if (reason.status === 404) {
        return value;
      } else {
        throw reason;
      }
    };
  }
  
  // returns a promise for a list of docs to update, based on the input docId.
  // the order doesn't matter, because post-3.2.0, bulkDocs
  // is an atomic operation in all three adapters.
  function getDocsToPersist(docId, view, docIdsToChangesAndEmits) {
    var metaDocId = '_local/doc_' + docId;
    var defaultMetaDoc = {_id: metaDocId, keys: []};
    var docData = docIdsToChangesAndEmits[docId];
    var indexableKeysToKeyValues = docData.indexableKeysToKeyValues;
    var changes = docData.changes;
  
    function getMetaDoc() {
      if (isGenOne(changes)) {
        // generation 1, so we can safely assume initial state
        // for performance reasons (avoids unnecessary GETs)
        return PouchPromise.resolve(defaultMetaDoc);
      }
      return view.db.get(metaDocId).catch(defaultsTo(defaultMetaDoc));
    }
  
    function getKeyValueDocs(metaDoc) {
      if (!metaDoc.keys.length) {
        // no keys, no need for a lookup
        return PouchPromise.resolve({rows: []});
      }
      return view.db.allDocs({
        keys: metaDoc.keys,
        include_docs: true
      });
    }
  
    function processKvDocs(metaDoc, kvDocsRes) {
      var kvDocs = [];
      var oldKeysMap = {};
  
      for (var i = 0, len = kvDocsRes.rows.length; i < len; i++) {
        var row = kvDocsRes.rows[i];
        var doc = row.doc;
        if (!doc) { // deleted
          continue;
        }
        kvDocs.push(doc);
        oldKeysMap[doc._id] = true;
        doc._deleted = !indexableKeysToKeyValues[doc._id];
        if (!doc._deleted) {
          var keyValue = indexableKeysToKeyValues[doc._id];
          if ('value' in keyValue) {
            doc.value = keyValue.value;
          }
        }
      }
  
      var newKeys = Object.keys(indexableKeysToKeyValues);
      newKeys.forEach(function (key) {
        if (!oldKeysMap[key]) {
          // new doc
          var kvDoc = {
            _id: key
          };
          var keyValue = indexableKeysToKeyValues[key];
          if ('value' in keyValue) {
            kvDoc.value = keyValue.value;
          }
          kvDocs.push(kvDoc);
        }
      });
      metaDoc.keys = uniq(newKeys.concat(metaDoc.keys));
      kvDocs.push(metaDoc);
  
      return kvDocs;
    }
  
    return getMetaDoc().then(function (metaDoc) {
      return getKeyValueDocs(metaDoc).then(function (kvDocsRes) {
        return processKvDocs(metaDoc, kvDocsRes);
      });
    });
  }
  
  // updates all emitted key/value docs and metaDocs in the mrview database
  // for the given batch of documents from the source database
  function saveKeyValues(view, docIdsToChangesAndEmits, seq) {
    var seqDocId = '_local/lastSeq';
    return view.db.get(seqDocId)
    .catch(defaultsTo({_id: seqDocId, seq: 0}))
    .then(function (lastSeqDoc) {
      var docIds = Object.keys(docIdsToChangesAndEmits);
      return PouchPromise.all(docIds.map(function (docId) {
        return getDocsToPersist(docId, view, docIdsToChangesAndEmits);
      })).then(function (listOfDocsToPersist) {
        var docsToPersist = flatten(listOfDocsToPersist);
        lastSeqDoc.seq = seq;
        docsToPersist.push(lastSeqDoc);
        // write all docs in a single operation, update the seq once
        return view.db.bulkDocs({docs : docsToPersist});
      });
    });
  }
  
  function getQueue(view) {
    var viewName = typeof view === 'string' ? view : view.name;
    var queue = persistentQueues[viewName];
    if (!queue) {
      queue = persistentQueues[viewName] = new TaskQueue$1();
    }
    return queue;
  }
  
  function updateView(view) {
    return sequentialize(getQueue(view), function () {
      return updateViewInQueue(view);
    })();
  }
  
  function updateViewInQueue(view) {
    // bind the emit function once
    var mapResults;
    var doc;
  
    function emit(key, value) {
      var output = {id: doc._id, key: normalizeKey(key)};
      // Don't explicitly store the value unless it's defined and non-null.
      // This saves on storage space, because often people don't use it.
      if (typeof value !== 'undefined' && value !== null) {
        output.value = normalizeKey(value);
      }
      mapResults.push(output);
    }
  
    var mapFun;
    // for temp_views one can use emit(doc, emit), see #38
    if (typeof view.mapFun === "function" && view.mapFun.length === 2) {
      var origMap = view.mapFun;
      mapFun = function (doc) {
        return origMap(doc, emit);
      };
    } else {
      mapFun = evalFunction(view.mapFun.toString(), emit);
    }
  
    var currentSeq = view.seq || 0;
  
    function processChange(docIdsToChangesAndEmits, seq) {
      return function () {
        return saveKeyValues(view, docIdsToChangesAndEmits, seq);
      };
    }
  
    var queue = new TaskQueue$1();
    // TODO(neojski): https://github.com/daleharvey/pouchdb/issues/1521
  
    return new PouchPromise(function (resolve, reject) {
  
      function complete() {
        queue.finish().then(function () {
          view.seq = currentSeq;
          resolve();
        });
      }
  
      function processNextBatch() {
        view.sourceDB.changes({
          conflicts: true,
          include_docs: true,
          style: 'all_docs',
          since: currentSeq,
          limit: CHANGES_BATCH_SIZE$1
        }).on('complete', function (response) {
          var results = response.results;
          if (!results.length) {
            return complete();
          }
          var docIdsToChangesAndEmits = {};
          for (var i = 0, l = results.length; i < l; i++) {
            var change = results[i];
            if (change.doc._id[0] !== '_') {
              mapResults = [];
              doc = change.doc;
  
              if (!doc._deleted) {
                tryCode(view.sourceDB, mapFun, [doc]);
              }
              mapResults.sort(sortByKeyThenValue);
  
              var indexableKeysToKeyValues = {};
              var lastKey;
              for (var j = 0, jl = mapResults.length; j < jl; j++) {
                var obj = mapResults[j];
                var complexKey = [obj.key, obj.id];
                if (collate(obj.key, lastKey) === 0) {
                  complexKey.push(j); // dup key+id, so make it unique
                }
                var indexableKey = toIndexableString(complexKey);
                indexableKeysToKeyValues[indexableKey] = obj;
                lastKey = obj.key;
              }
              docIdsToChangesAndEmits[change.doc._id] = {
                indexableKeysToKeyValues: indexableKeysToKeyValues,
                changes: change.changes
              };
            }
            currentSeq = change.seq;
          }
          queue.add(processChange(docIdsToChangesAndEmits, currentSeq));
          if (results.length < CHANGES_BATCH_SIZE$1) {
            return complete();
          }
          return processNextBatch();
        }).on('error', onError);
        /* istanbul ignore next */
        function onError(err) {
          reject(err);
        }
      }
  
      processNextBatch();
    });
  }
  
  function reduceView(view, results, options) {
    if (options.group_level === 0) {
      delete options.group_level;
    }
  
    var shouldGroup = options.group || options.group_level;
  
    var reduceFun;
    if (builtInReduce[view.reduceFun]) {
      reduceFun = builtInReduce[view.reduceFun];
    } else {
      reduceFun = evalFunction(view.reduceFun.toString());
    }
  
    var groups = [];
    var lvl = isNaN(options.group_level) ? Number.POSITIVE_INFINITY :
      options.group_level;
    results.forEach(function (e) {
      var last = groups[groups.length - 1];
      var groupKey = shouldGroup ? e.key : null;
  
      // only set group_level for array keys
      if (shouldGroup && Array.isArray(groupKey)) {
        groupKey = groupKey.slice(0, lvl);
      }
  
      if (last && collate(last.groupKey, groupKey) === 0) {
        last.keys.push([e.key, e.id]);
        last.values.push(e.value);
        return;
      }
      groups.push({
        keys: [[e.key, e.id]],
        values: [e.value],
        groupKey: groupKey
      });
    });
    results = [];
    for (var i = 0, len = groups.length; i < len; i++) {
      var e = groups[i];
      var reduceTry = tryCode(view.sourceDB, reduceFun,
        [e.keys, e.values, false]);
      if (reduceTry.error && reduceTry.error instanceof BuiltInError) {
        // CouchDB returns an error if a built-in errors out
        throw reduceTry.error;
      }
      results.push({
        // CouchDB just sets the value to null if a non-built-in errors out
        value: reduceTry.error ? null : reduceTry.output,
        key: e.groupKey
      });
    }
    // no total_rows/offset when reducing
    return {rows: sliceResults(results, options.limit, options.skip)};
  }
  
  function queryView(view, opts) {
    return sequentialize(getQueue(view), function () {
      return queryViewInQueue(view, opts);
    })();
  }
  
  function queryViewInQueue(view, opts) {
    var totalRows;
    var shouldReduce = view.reduceFun && opts.reduce !== false;
    var skip = opts.skip || 0;
    if (typeof opts.keys !== 'undefined' && !opts.keys.length) {
      // equivalent query
      opts.limit = 0;
      delete opts.keys;
    }
  
    function fetchFromView(viewOpts) {
      viewOpts.include_docs = true;
      return view.db.allDocs(viewOpts).then(function (res) {
        totalRows = res.total_rows;
        return res.rows.map(function (result) {
  
          // implicit migration - in older versions of PouchDB,
          // we explicitly stored the doc as {id: ..., key: ..., value: ...}
          // this is tested in a migration test
          /* istanbul ignore next */
          if ('value' in result.doc && typeof result.doc.value === 'object' &&
              result.doc.value !== null) {
            var keys = Object.keys(result.doc.value).sort();
            // this detection method is not perfect, but it's unlikely the user
            // emitted a value which was an object with these 3 exact keys
            var expectedKeys = ['id', 'key', 'value'];
            if (!(keys < expectedKeys || keys > expectedKeys)) {
              return result.doc.value;
            }
          }
  
          var parsedKeyAndDocId = parseIndexableString(result.doc._id);
          return {
            key: parsedKeyAndDocId[0],
            id: parsedKeyAndDocId[1],
            value: ('value' in result.doc ? result.doc.value : null)
          };
        });
      });
    }
  
    function onMapResultsReady(rows) {
      var finalResults;
      if (shouldReduce) {
        finalResults = reduceView(view, rows, opts);
      } else {
        finalResults = {
          total_rows: totalRows,
          offset: skip,
          rows: rows
        };
      }
      if (opts.include_docs) {
        var docIds = uniq(rows.map(rowToDocId));
  
        return view.sourceDB.allDocs({
          keys: docIds,
          include_docs: true,
          conflicts: opts.conflicts,
          attachments: opts.attachments,
          binary: opts.binary
        }).then(function (allDocsRes) {
          var docIdsToDocs = {};
          allDocsRes.rows.forEach(function (row) {
            if (row.doc) {
              docIdsToDocs['$' + row.id] = row.doc;
            }
          });
          rows.forEach(function (row) {
            var docId = rowToDocId(row);
            var doc = docIdsToDocs['$' + docId];
            if (doc) {
              row.doc = doc;
            }
          });
          return finalResults;
        });
      } else {
        return finalResults;
      }
    }
  
    if (typeof opts.keys !== 'undefined') {
      var keys = opts.keys;
      var fetchPromises = keys.map(function (key) {
        var viewOpts = {
          startkey : toIndexableString([key]),
          endkey   : toIndexableString([key, {}])
        };
        return fetchFromView(viewOpts);
      });
      return PouchPromise.all(fetchPromises).then(flatten).then(onMapResultsReady);
    } else { // normal query, no 'keys'
      var viewOpts = {
        descending : opts.descending
      };
      if (opts.start_key) {
          opts.startkey = opts.start_key;
      }
      if (opts.end_key) {
          opts.endkey = opts.end_key;
      }
      if (typeof opts.startkey !== 'undefined') {
        viewOpts.startkey = opts.descending ?
          toIndexableString([opts.startkey, {}]) :
          toIndexableString([opts.startkey]);
      }
      if (typeof opts.endkey !== 'undefined') {
        var inclusiveEnd = opts.inclusive_end !== false;
        if (opts.descending) {
          inclusiveEnd = !inclusiveEnd;
        }
  
        viewOpts.endkey = toIndexableString(
          inclusiveEnd ? [opts.endkey, {}] : [opts.endkey]);
      }
      if (typeof opts.key !== 'undefined') {
        var keyStart = toIndexableString([opts.key]);
        var keyEnd = toIndexableString([opts.key, {}]);
        if (viewOpts.descending) {
          viewOpts.endkey = keyStart;
          viewOpts.startkey = keyEnd;
        } else {
          viewOpts.startkey = keyStart;
          viewOpts.endkey = keyEnd;
        }
      }
      if (!shouldReduce) {
        if (typeof opts.limit === 'number') {
          viewOpts.limit = opts.limit;
        }
        viewOpts.skip = skip;
      }
      return fetchFromView(viewOpts).then(onMapResultsReady);
    }
  }
  
  function httpViewCleanup(db) {
    return db.request({
      method: 'POST',
      url: '_view_cleanup'
    });
  }
  
  function localViewCleanup(db) {
    return db.get('_local/mrviews').then(function (metaDoc) {
      var docsToViews = {};
      Object.keys(metaDoc.views).forEach(function (fullViewName) {
        var parts = parseViewName(fullViewName);
        var designDocName = '_design/' + parts[0];
        var viewName = parts[1];
        docsToViews[designDocName] = docsToViews[designDocName] || {};
        docsToViews[designDocName][viewName] = true;
      });
      var opts = {
        keys : Object.keys(docsToViews),
        include_docs : true
      };
      return db.allDocs(opts).then(function (res) {
        var viewsToStatus = {};
        res.rows.forEach(function (row) {
          var ddocName = row.key.substring(8);
          Object.keys(docsToViews[row.key]).forEach(function (viewName) {
            var fullViewName = ddocName + '/' + viewName;
            /* istanbul ignore if */
            if (!metaDoc.views[fullViewName]) {
              // new format, without slashes, to support PouchDB 2.2.0
              // migration test in pouchdb's browser.migration.js verifies this
              fullViewName = viewName;
            }
            var viewDBNames = Object.keys(metaDoc.views[fullViewName]);
            // design doc deleted, or view function nonexistent
            var statusIsGood = row.doc && row.doc.views &&
              row.doc.views[viewName];
            viewDBNames.forEach(function (viewDBName) {
              viewsToStatus[viewDBName] =
                viewsToStatus[viewDBName] || statusIsGood;
            });
          });
        });
        var dbsToDelete = Object.keys(viewsToStatus).filter(
          function (viewDBName) { return !viewsToStatus[viewDBName]; });
        var destroyPromises = dbsToDelete.map(function (viewDBName) {
          return sequentialize(getQueue(viewDBName), function () {
            return new db.constructor(viewDBName, db.__opts).destroy();
          })();
        });
        return PouchPromise.all(destroyPromises).then(function () {
          return {ok: true};
        });
      });
    }, defaultsTo({ok: true}));
  }
  
  var viewCleanup = callbackify(function () {
    var db = this;
    if (db.type() === 'http') {
      return httpViewCleanup(db);
    }
    /* istanbul ignore next */
    if (typeof db._viewCleanup === 'function') {
      return customViewCleanup(db);
    }
    return localViewCleanup(db);
  });
  
  function queryPromised(db, fun, opts) {
    if (db.type() === 'http') {
      return httpQuery(db, fun, opts);
    }
  
    /* istanbul ignore next */
    if (typeof db._query === 'function') {
      return customQuery(db, fun, opts);
    }
  
    if (typeof fun !== 'string') {
      // temp_view
      checkQueryParseError(opts, fun);
  
      var createViewOpts = {
        db : db,
        viewName : 'temp_view/temp_view',
        map : fun.map,
        reduce : fun.reduce,
        temporary : true
      };
      tempViewQueue.add(function () {
        return createView(createViewOpts).then(function (view) {
          function cleanup() {
            return view.db.destroy();
          }
          return fin(updateView(view).then(function () {
            return queryView(view, opts);
          }), cleanup);
        });
      });
      return tempViewQueue.finish();
    } else {
      // persistent view
      var fullViewName = fun;
      var parts = parseViewName(fullViewName);
      var designDocName = parts[0];
      var viewName = parts[1];
      return db.get('_design/' + designDocName).then(function (doc) {
        var fun = doc.views && doc.views[viewName];
  
        if (!fun || typeof fun.map !== 'string') {
          throw new NotFoundError$1('ddoc ' + designDocName +
          ' has no view named ' + viewName);
        }
        checkQueryParseError(opts, fun);
  
        var createViewOpts = {
          db : db,
          viewName : fullViewName,
          map : fun.map,
          reduce : fun.reduce
        };
        return createView(createViewOpts).then(function (view) {
          if (opts.stale === 'ok' || opts.stale === 'update_after') {
            if (opts.stale === 'update_after') {
              process.nextTick(function () {
                updateView(view);
              });
            }
            return queryView(view, opts);
          } else { // stale not ok
            return updateView(view).then(function () {
              return queryView(view, opts);
            });
          }
        });
      });
    }
  }
  
  var query = function (fun, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = opts ? coerceOptions(opts) : {};
  
    if (typeof fun === 'function') {
      fun = {map : fun};
    }
  
    var db = this;
    var promise = PouchPromise.resolve().then(function () {
      return queryPromised(db, fun, opts);
    });
    promisedCallback(promise, callback);
    return promise;
  };
  
  
  var mapreduce = {
    query: query,
    viewCleanup: viewCleanup
  };
  
  function isGenOne$1(rev) {
    return /^1-/.test(rev);
  }
  
  function fileHasChanged(localDoc, remoteDoc, filename) {
    return !localDoc._attachments ||
           !localDoc._attachments[filename] ||
           localDoc._attachments[filename].digest !== remoteDoc._attachments[filename].digest;
  }
  
  function getDocAttachments(db, doc) {
    var filenames = Object.keys(doc._attachments);
    return PouchPromise.all(filenames.map(function (filename) {
      return db.getAttachment(doc._id, filename, {rev: doc._rev});
    }));
  }
  
  function getDocAttachmentsFromTargetOrSource(target, src, doc) {
    var doCheckForLocalAttachments = src.type() === 'http' && target.type() !== 'http';
    var filenames = Object.keys(doc._attachments);
  
    if (!doCheckForLocalAttachments) {
      return getDocAttachments(src, doc);
    }
  
    return target.get(doc._id).then(function (localDoc) {
      return PouchPromise.all(filenames.map(function (filename) {
        if (fileHasChanged(localDoc, doc, filename)) {
          return src.getAttachment(doc._id, filename);
        }
  
        return target.getAttachment(localDoc._id, filename);
      }));
    }).catch(function (error) {
      /* istanbul ignore if */
      if (error.status !== 404) {
        throw error;
      }
  
      return getDocAttachments(src, doc);
    });
  }
  
  function createBulkGetOpts(diffs) {
    var requests = [];
    Object.keys(diffs).forEach(function (id) {
      var missingRevs = diffs[id].missing;
      missingRevs.forEach(function (missingRev) {
        requests.push({
          id: id,
          rev: missingRev
        });
      });
    });
  
    return {
      docs: requests,
      revs: true
    };
  }
  
  //
  // Fetch all the documents from the src as described in the "diffs",
  // which is a mapping of docs IDs to revisions. If the state ever
  // changes to "cancelled", then the returned promise will be rejected.
  // Else it will be resolved with a list of fetched documents.
  //
  function getDocs(src, target, diffs, state) {
    diffs = clone(diffs); // we do not need to modify this
  
    var resultDocs = [],
        ok = true;
  
    function getAllDocs() {
  
      var bulkGetOpts = createBulkGetOpts(diffs);
  
      if (!bulkGetOpts.docs.length) { // optimization: skip empty requests
        return;
      }
  
      return src.bulkGet(bulkGetOpts).then(function (bulkGetResponse) {
        /* istanbul ignore if */
        if (state.cancelled) {
          throw new Error('cancelled');
        }
        return PouchPromise.all(bulkGetResponse.results.map(function (bulkGetInfo) {
          return PouchPromise.all(bulkGetInfo.docs.map(function (doc) {
            var remoteDoc = doc.ok;
  
            if (doc.error) {
              // when AUTO_COMPACTION is set, docs can be returned which look
              // like this: {"missing":"1-7c3ac256b693c462af8442f992b83696"}
              ok = false;
            }
  
            if (!remoteDoc || !remoteDoc._attachments) {
              return remoteDoc;
            }
  
            return getDocAttachmentsFromTargetOrSource(target, src, remoteDoc).then(function (attachments) {
              var filenames = Object.keys(remoteDoc._attachments);
              attachments.forEach(function (attachment, i) {
                var att = remoteDoc._attachments[filenames[i]];
                delete att.stub;
                delete att.length;
                att.data = attachment;
              });
  
              return remoteDoc;
            });
          }));
        }))
  
        .then(function (results) {
          resultDocs = resultDocs.concat(flatten(results).filter(Boolean));
        });
      });
    }
  
    function hasAttachments(doc) {
      return doc._attachments && Object.keys(doc._attachments).length > 0;
    }
  
    function hasConflicts(doc) {
      return doc._conflicts && doc._conflicts.length > 0;
    }
  
    function fetchRevisionOneDocs(ids) {
      // Optimization: fetch gen-1 docs and attachments in
      // a single request using _all_docs
      return src.allDocs({
        keys: ids,
        include_docs: true,
        conflicts: true
      }).then(function (res) {
        if (state.cancelled) {
          throw new Error('cancelled');
        }
        res.rows.forEach(function (row) {
          if (row.deleted || !row.doc || !isGenOne$1(row.value.rev) ||
              hasAttachments(row.doc) || hasConflicts(row.doc)) {
            // if any of these conditions apply, we need to fetch using get()
            return;
          }
  
          // the doc we got back from allDocs() is sufficient
          resultDocs.push(row.doc);
          delete diffs[row.id];
        });
      });
    }
  
    function getRevisionOneDocs() {
      // filter out the generation 1 docs and get them
      // leaving the non-generation one docs to be got otherwise
      var ids = Object.keys(diffs).filter(function (id) {
        var missing = diffs[id].missing;
        return missing.length === 1 && isGenOne$1(missing[0]);
      });
      if (ids.length > 0) {
        return fetchRevisionOneDocs(ids);
      }
    }
  
    function returnResult() {
      return { ok:ok, docs:resultDocs };
    }
  
    return PouchPromise.resolve()
      .then(getRevisionOneDocs)
      .then(getAllDocs)
      .then(returnResult);
  }
  
  var CHECKPOINT_VERSION = 1;
  var REPLICATOR = "pouchdb";
  // This is an arbitrary number to limit the
  // amount of replication history we save in the checkpoint.
  // If we save too much, the checkpoing docs will become very big,
  // if we save fewer, we'll run a greater risk of having to
  // read all the changes from 0 when checkpoint PUTs fail
  // CouchDB 2.0 has a more involved history pruning,
  // but let's go for the simple version for now.
  var CHECKPOINT_HISTORY_SIZE = 5;
  var LOWEST_SEQ = 0;
  
  function updateCheckpoint(db, id, checkpoint, session, returnValue) {
    return db.get(id).catch(function (err) {
      if (err.status === 404) {
        if (db.type() === 'http') {
          res(
            404, 'PouchDB is just checking if a remote checkpoint exists.'
          );
        }
        return {
          session_id: session,
          _id: id,
          history: [],
          replicator: REPLICATOR,
          version: CHECKPOINT_VERSION
        };
      }
      throw err;
    }).then(function (doc) {
      if (returnValue.cancelled) {
        return;
      }
  
      // if the checkpoint has not changed, do not update
      if (doc.last_seq === checkpoint) {
        return;
      }
  
      // Filter out current entry for this replication
      doc.history = (doc.history || []).filter(function (item) {
        return item.session_id !== session;
      });
  
      // Add the latest checkpoint to history
      doc.history.unshift({
        last_seq: checkpoint,
        session_id: session
      });
  
      // Just take the last pieces in history, to
      // avoid really big checkpoint docs.
      // see comment on history size above
      doc.history = doc.history.slice(0, CHECKPOINT_HISTORY_SIZE);
  
      doc.version = CHECKPOINT_VERSION;
      doc.replicator = REPLICATOR;
  
      doc.session_id = session;
      doc.last_seq = checkpoint;
  
      return db.put(doc).catch(function (err) {
        if (err.status === 409) {
          // retry; someone is trying to write a checkpoint simultaneously
          return updateCheckpoint(db, id, checkpoint, session, returnValue);
        }
        throw err;
      });
    });
  }
  
  function Checkpointer(src, target, id, returnValue) {
    this.src = src;
    this.target = target;
    this.id = id;
    this.returnValue = returnValue;
  }
  
  Checkpointer.prototype.writeCheckpoint = function (checkpoint, session) {
    var self = this;
    return this.updateTarget(checkpoint, session).then(function () {
      return self.updateSource(checkpoint, session);
    });
  };
  
  Checkpointer.prototype.updateTarget = function (checkpoint, session) {
    return updateCheckpoint(this.target, this.id, checkpoint,
      session, this.returnValue);
  };
  
  Checkpointer.prototype.updateSource = function (checkpoint, session) {
    var self = this;
    if (this.readOnlySource) {
      return PouchPromise.resolve(true);
    }
    return updateCheckpoint(this.src, this.id, checkpoint,
      session, this.returnValue)
      .catch(function (err) {
        if (isForbiddenError(err)) {
          self.readOnlySource = true;
          return true;
        }
        throw err;
      });
  };
  
  var comparisons = {
    "undefined": function (targetDoc, sourceDoc) {
      // This is the previous comparison function
      if (collate(targetDoc.last_seq, sourceDoc.last_seq) === 0) {
        return sourceDoc.last_seq;
      }
      /* istanbul ignore next */
      return 0;
    },
    "1": function (targetDoc, sourceDoc) {
      // This is the comparison function ported from CouchDB
      return compareReplicationLogs(sourceDoc, targetDoc).last_seq;
    }
  };
  
  Checkpointer.prototype.getCheckpoint = function () {
    var self = this;
    return self.target.get(self.id).then(function (targetDoc) {
      if (self.readOnlySource) {
        return PouchPromise.resolve(targetDoc.last_seq);
      }
  
      return self.src.get(self.id).then(function (sourceDoc) {
        // Since we can't migrate an old version doc to a new one
        // (no session id), we just go with the lowest seq in this case
        /* istanbul ignore if */
        if (targetDoc.version !== sourceDoc.version) {
          return LOWEST_SEQ;
        }
  
        var version;
        if (targetDoc.version) {
          version = targetDoc.version.toString();
        } else {
          version = "undefined";
        }
  
        if (version in comparisons) {
          return comparisons[version](targetDoc, sourceDoc);
        }
        /* istanbul ignore next */
        return LOWEST_SEQ;
      }, function (err) {
        if (err.status === 404 && targetDoc.last_seq) {
          return self.src.put({
            _id: self.id,
            last_seq: LOWEST_SEQ
          }).then(function () {
            return LOWEST_SEQ;
          }, function (err) {
            if (isForbiddenError(err)) {
              self.readOnlySource = true;
              return targetDoc.last_seq;
            }
            /* istanbul ignore next */
            return LOWEST_SEQ;
          });
        }
        throw err;
      });
    }).catch(function (err) {
      if (err.status !== 404) {
        throw err;
      }
      return LOWEST_SEQ;
    });
  };
  // This checkpoint comparison is ported from CouchDBs source
  // they come from here:
  // https://github.com/apache/couchdb-couch-replicator/blob/master/src/couch_replicator.erl#L863-L906
  
  function compareReplicationLogs(srcDoc, tgtDoc) {
    if (srcDoc.session_id === tgtDoc.session_id) {
      return {
        last_seq: srcDoc.last_seq,
        history: srcDoc.history
      };
    }
  
    return compareReplicationHistory(srcDoc.history, tgtDoc.history);
  }
  
  function compareReplicationHistory(sourceHistory, targetHistory) {
    // the erlang loop via function arguments is not so easy to repeat in JS
    // therefore, doing this as recursion
    var S = sourceHistory[0];
    var sourceRest = sourceHistory.slice(1);
    var T = targetHistory[0];
    var targetRest = targetHistory.slice(1);
  
    if (!S || targetHistory.length === 0) {
      return {
        last_seq: LOWEST_SEQ,
        history: []
      };
    }
  
    var sourceId = S.session_id;
    /* istanbul ignore if */
    if (hasSessionId(sourceId, targetHistory)) {
      return {
        last_seq: S.last_seq,
        history: sourceHistory
      };
    }
  
    var targetId = T.session_id;
    if (hasSessionId(targetId, sourceRest)) {
      return {
        last_seq: T.last_seq,
        history: targetRest
      };
    }
  
    return compareReplicationHistory(sourceRest, targetRest);
  }
  
  function hasSessionId(sessionId, history) {
    var props = history[0];
    var rest = history.slice(1);
  
    if (!sessionId || history.length === 0) {
      return false;
    }
  
    if (sessionId === props.session_id) {
      return true;
    }
  
    return hasSessionId(sessionId, rest);
  }
  
  function isForbiddenError(err) {
    return typeof err.status === 'number' && Math.floor(err.status / 100) === 4;
  }
  
  var STARTING_BACK_OFF = 0;
  
  function backOff(opts, returnValue, error, callback) {
    if (opts.retry === false) {
      returnValue.emit('error', error);
      returnValue.removeAllListeners();
      return;
    }
    if (typeof opts.back_off_function !== 'function') {
      opts.back_off_function = defaultBackOff;
    }
    returnValue.emit('requestError', error);
    if (returnValue.state === 'active' || returnValue.state === 'pending') {
      returnValue.emit('paused', error);
      returnValue.state = 'stopped';
      var backOffSet = function backoffTimeSet() {
        opts.current_back_off = STARTING_BACK_OFF;
      };
      var removeBackOffSetter = function removeBackOffTimeSet() {
        returnValue.removeListener('active', backOffSet);
      };
      returnValue.once('paused', removeBackOffSetter);
      returnValue.once('active', backOffSet);
    }
  
    opts.current_back_off = opts.current_back_off || STARTING_BACK_OFF;
    opts.current_back_off = opts.back_off_function(opts.current_back_off);
    setTimeout(callback, opts.current_back_off);
  }
  
  function sortObjectPropertiesByKey(queryParams) {
    return Object.keys(queryParams).sort(collate).reduce(function (result, key) {
      result[key] = queryParams[key];
      return result;
    }, {});
  }
  
  // Generate a unique id particular to this replication.
  // Not guaranteed to align perfectly with CouchDB's rep ids.
  function generateReplicationId(src, target, opts) {
    var docIds = opts.doc_ids ? opts.doc_ids.sort(collate) : '';
    var filterFun = opts.filter ? opts.filter.toString() : '';
    var queryParams = '';
    var filterViewName =  '';
  
    if (opts.filter && opts.query_params) {
      queryParams = JSON.stringify(sortObjectPropertiesByKey(opts.query_params));
    }
  
    if (opts.filter && opts.filter === '_view') {
      filterViewName = opts.view.toString();
    }
  
    return PouchPromise.all([src.id(), target.id()]).then(function (res) {
      var queryData = res[0] + res[1] + filterFun + filterViewName +
        queryParams + docIds;
      return new PouchPromise(function (resolve) {
        binaryMd5(queryData, resolve);
      });
    }).then(function (md5sum) {
      // can't use straight-up md5 alphabet, because
      // the char '/' is interpreted as being for attachments,
      // and + is also not url-safe
      md5sum = md5sum.replace(/\//g, '.').replace(/\+/g, '_');
      return '_local/' + md5sum;
    });
  }
  
  function replicate$1(src, target, opts, returnValue, result) {
    var batches = [];               // list of batches to be processed
    var currentBatch;               // the batch currently being processed
    var pendingBatch = {
      seq: 0,
      changes: [],
      docs: []
    }; // next batch, not yet ready to be processed
    var writingCheckpoint = false;  // true while checkpoint is being written
    var changesCompleted = false;   // true when all changes received
    var replicationCompleted = false; // true when replication has completed
    var last_seq = 0;
    var continuous = opts.continuous || opts.live || false;
    var batch_size = opts.batch_size || 100;
    var batches_limit = opts.batches_limit || 10;
    var changesPending = false;     // true while src.changes is running
    var doc_ids = opts.doc_ids;
    var repId;
    var checkpointer;
    var changedDocs = [];
    // Like couchdb, every replication gets a unique session id
    var session = uuid();
  
    result = result || {
      ok: true,
      start_time: new Date(),
      docs_read: 0,
      docs_written: 0,
      doc_write_failures: 0,
      errors: []
    };
  
    var changesOpts = {};
    returnValue.ready(src, target);
  
    function initCheckpointer() {
      if (checkpointer) {
        return PouchPromise.resolve();
      }
      return generateReplicationId(src, target, opts).then(function (res) {
        repId = res;
        checkpointer = new Checkpointer(src, target, repId, returnValue);
      });
    }
  
    function writeDocs() {
      changedDocs = [];
  
      if (currentBatch.docs.length === 0) {
        return;
      }
      var docs = currentBatch.docs;
      var bulkOpts = {timeout: opts.timeout};
      return target.bulkDocs({docs: docs, new_edits: false}, bulkOpts).then(function (res) {
        /* istanbul ignore if */
        if (returnValue.cancelled) {
          completeReplication();
          throw new Error('cancelled');
        }
  
        // `res` doesn't include full documents (which live in `docs`), so we create a map of 
        // (id -> error), and check for errors while iterating over `docs`
        var errorsById = Object.create(null);
        res.forEach(function (res) {
          if (res.error) {
            errorsById[res.id] = res;
          }
        });
  
        var errorsNo = Object.keys(errorsById).length;
        result.doc_write_failures += errorsNo;
        result.docs_written += docs.length - errorsNo;
  
        docs.forEach(function (doc) {
          var error = errorsById[doc._id];
          if (error) {
            result.errors.push(error);
            if (error.name === 'unauthorized' || error.name === 'forbidden') {
              returnValue.emit('denied', clone(error));
            } else {
              throw error;
            }
          } else {
            changedDocs.push(doc);
          }
        });
  
      }, function (err) {
        result.doc_write_failures += docs.length;
        throw err;
      });
    }
  
    function finishBatch() {
      if (currentBatch.error) {
        throw new Error('There was a problem getting docs.');
      }
      result.last_seq = last_seq = currentBatch.seq;
      var outResult = clone(result);
      if (changedDocs.length) {
        outResult.docs = changedDocs;
        returnValue.emit('change', outResult);
      }
      writingCheckpoint = true;
      return checkpointer.writeCheckpoint(currentBatch.seq,
          session).then(function () {
        writingCheckpoint = false;
        /* istanbul ignore if */
        if (returnValue.cancelled) {
          completeReplication();
          throw new Error('cancelled');
        }
        currentBatch = undefined;
        getChanges();
      }).catch(function (err) {
        onCheckpointError(err);
        throw err;
      });
    }
  
    function getDiffs() {
      var diff = {};
      currentBatch.changes.forEach(function (change) {
        // Couchbase Sync Gateway emits these, but we can ignore them
        /* istanbul ignore if */
        if (change.id === "_user/") {
          return;
        }
        diff[change.id] = change.changes.map(function (x) {
          return x.rev;
        });
      });
      return target.revsDiff(diff).then(function (diffs) {
        /* istanbul ignore if */
        if (returnValue.cancelled) {
          completeReplication();
          throw new Error('cancelled');
        }
        // currentBatch.diffs elements are deleted as the documents are written
        currentBatch.diffs = diffs;
      });
    }
  
    function getBatchDocs() {
      return getDocs(src, target, currentBatch.diffs, returnValue).then(function (got) {
        currentBatch.error = !got.ok;
        got.docs.forEach(function (doc) {
          delete currentBatch.diffs[doc._id];
          result.docs_read++;
          currentBatch.docs.push(doc);
        });
      });
    }
  
    function startNextBatch() {
      if (returnValue.cancelled || currentBatch) {
        return;
      }
      if (batches.length === 0) {
        processPendingBatch(true);
        return;
      }
      currentBatch = batches.shift();
      getDiffs()
        .then(getBatchDocs)
        .then(writeDocs)
        .then(finishBatch)
        .then(startNextBatch)
        .catch(function (err) {
          abortReplication('batch processing terminated with error', err);
        });
    }
  
  
    function processPendingBatch(immediate) {
      if (pendingBatch.changes.length === 0) {
        if (batches.length === 0 && !currentBatch) {
          if ((continuous && changesOpts.live) || changesCompleted) {
            returnValue.state = 'pending';
            returnValue.emit('paused');
          }
          if (changesCompleted) {
            completeReplication();
          }
        }
        return;
      }
      if (
        immediate ||
        changesCompleted ||
        pendingBatch.changes.length >= batch_size
      ) {
        batches.push(pendingBatch);
        pendingBatch = {
          seq: 0,
          changes: [],
          docs: []
        };
        if (returnValue.state === 'pending' || returnValue.state === 'stopped') {
          returnValue.state = 'active';
          returnValue.emit('active');
        }
        startNextBatch();
      }
    }
  
  
    function abortReplication(reason, err) {
      if (replicationCompleted) {
        return;
      }
      if (!err.message) {
        err.message = reason;
      }
      result.ok = false;
      result.status = 'aborting';
      batches = [];
      pendingBatch = {
        seq: 0,
        changes: [],
        docs: []
      };
      completeReplication(err);
    }
  
  
    function completeReplication(fatalError) {
      if (replicationCompleted) {
        return;
      }
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        result.status = 'cancelled';
        if (writingCheckpoint) {
          return;
        }
      }
      result.status = result.status || 'complete';
      result.end_time = new Date();
      result.last_seq = last_seq;
      replicationCompleted = true;
  
      if (fatalError) {
        fatalError.result = result;
  
        if (fatalError.name === 'unauthorized' || fatalError.name === 'forbidden') {
          returnValue.emit('error', fatalError);
          returnValue.removeAllListeners();
        } else {
          backOff(opts, returnValue, fatalError, function () {
            replicate$1(src, target, opts, returnValue);
          });
        }
      } else {
        returnValue.emit('complete', result);
        returnValue.removeAllListeners();
      }
    }
  
  
    function onChange(change) {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        return completeReplication();
      }
      var filter = filterChange(opts)(change);
      if (!filter) {
        return;
      }
      pendingBatch.seq = change.seq;
      pendingBatch.changes.push(change);
      processPendingBatch(batches.length === 0 && changesOpts.live);
    }
  
  
    function onChangesComplete(changes) {
      changesPending = false;
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        return completeReplication();
      }
  
      // if no results were returned then we're done,
      // else fetch more
      if (changes.results.length > 0) {
        changesOpts.since = changes.last_seq;
        getChanges();
        processPendingBatch(true);
      } else {
  
        var complete = function () {
          if (continuous) {
            changesOpts.live = true;
            getChanges();
          } else {
            changesCompleted = true;
          }
          processPendingBatch(true);
        };
  
        // update the checkpoint so we start from the right seq next time
        if (!currentBatch && changes.results.length === 0) {
          writingCheckpoint = true;
          checkpointer.writeCheckpoint(changes.last_seq,
              session).then(function () {
            writingCheckpoint = false;
            result.last_seq = last_seq = changes.last_seq;
            complete();
          })
          .catch(onCheckpointError);
        } else {
          complete();
        }
      }
    }
  
  
    function onChangesError(err) {
      changesPending = false;
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        return completeReplication();
      }
      abortReplication('changes rejected', err);
    }
  
  
    function getChanges() {
      if (!(
        !changesPending &&
        !changesCompleted &&
        batches.length < batches_limit
        )) {
        return;
      }
      changesPending = true;
      function abortChanges() {
        changes.cancel();
      }
      function removeListener() {
        returnValue.removeListener('cancel', abortChanges);
      }
  
      if (returnValue._changes) { // remove old changes() and listeners
        returnValue.removeListener('cancel', returnValue._abortChanges);
        returnValue._changes.cancel();
      }
      returnValue.once('cancel', abortChanges);
  
      var changes = src.changes(changesOpts)
        .on('change', onChange);
      changes.then(removeListener, removeListener);
      changes.then(onChangesComplete)
        .catch(onChangesError);
  
      if (opts.retry) {
        // save for later so we can cancel if necessary
        returnValue._changes = changes;
        returnValue._abortChanges = abortChanges;
      }
    }
  
  
    function startChanges() {
      initCheckpointer().then(function () {
        /* istanbul ignore if */
        if (returnValue.cancelled) {
          completeReplication();
          return;
        }
        return checkpointer.getCheckpoint().then(function (checkpoint) {
          last_seq = checkpoint;
          changesOpts = {
            since: last_seq,
            limit: batch_size,
            batch_size: batch_size,
            style: 'all_docs',
            doc_ids: doc_ids,
            return_docs: true // required so we know when we're done
          };
          if (opts.filter) {
            if (typeof opts.filter !== 'string') {
              // required for the client-side filter in onChange
              changesOpts.include_docs = true;
            } else { // ddoc filter
              changesOpts.filter = opts.filter;
            }
          }
          if ('heartbeat' in opts) {
            changesOpts.heartbeat = opts.heartbeat;
          }
          if ('timeout' in opts) {
            changesOpts.timeout = opts.timeout;
          }
          if (opts.query_params) {
            changesOpts.query_params = opts.query_params;
          }
          if (opts.view) {
            changesOpts.view = opts.view;
          }
          getChanges();
        });
      }).catch(function (err) {
        abortReplication('getCheckpoint rejected with ', err);
      });
    }
  
    /* istanbul ignore next */
    function onCheckpointError(err) {
      writingCheckpoint = false;
      abortReplication('writeCheckpoint completed with error', err);
    }
  
    /* istanbul ignore if */
    if (returnValue.cancelled) { // cancelled immediately
      completeReplication();
      return;
    }
  
    if (!returnValue._addedListeners) {
      returnValue.once('cancel', completeReplication);
  
      if (typeof opts.complete === 'function') {
        returnValue.once('error', opts.complete);
        returnValue.once('complete', function (result) {
          opts.complete(null, result);
        });
      }
      returnValue._addedListeners = true;
    }
  
    if (typeof opts.since === 'undefined') {
      startChanges();
    } else {
      initCheckpointer().then(function () {
        writingCheckpoint = true;
        return checkpointer.writeCheckpoint(opts.since, session);
      }).then(function () {
        writingCheckpoint = false;
        /* istanbul ignore if */
        if (returnValue.cancelled) {
          completeReplication();
          return;
        }
        last_seq = opts.since;
        startChanges();
      }).catch(onCheckpointError);
    }
  }
  
  // We create a basic promise so the caller can cancel the replication possibly
  // before we have actually started listening to changes etc
  inherits(Replication, EventEmitter);
  function Replication() {
    EventEmitter.call(this);
    this.cancelled = false;
    this.state = 'pending';
    var self = this;
    var promise = new PouchPromise(function (fulfill, reject) {
      self.once('complete', fulfill);
      self.once('error', reject);
    });
    self.then = function (resolve, reject) {
      return promise.then(resolve, reject);
    };
    self.catch = function (reject) {
      return promise.catch(reject);
    };
    // As we allow error handling via "error" event as well,
    // put a stub in here so that rejecting never throws UnhandledError.
    self.catch(function () {});
  }
  
  Replication.prototype.cancel = function () {
    this.cancelled = true;
    this.state = 'cancelled';
    this.emit('cancel');
  };
  
  Replication.prototype.ready = function (src, target) {
    var self = this;
    if (self._readyCalled) {
      return;
    }
    self._readyCalled = true;
  
    function onDestroy() {
      self.cancel();
    }
    src.once('destroyed', onDestroy);
    target.once('destroyed', onDestroy);
    function cleanup() {
      src.removeListener('destroyed', onDestroy);
      target.removeListener('destroyed', onDestroy);
    }
    self.once('complete', cleanup);
  };
  
  function toPouch(db, opts) {
    var PouchConstructor = opts.PouchConstructor;
    if (typeof db === 'string') {
      return new PouchConstructor(db, opts);
    } else {
      return db;
    }
  }
  
  function replicate(src, target, opts, callback) {
  
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    if (typeof opts === 'undefined') {
      opts = {};
    }
  
    if (opts.doc_ids && !Array.isArray(opts.doc_ids)) {
      throw createError(BAD_REQUEST,
                         "`doc_ids` filter parameter is not a list.");
    }
  
    opts.complete = callback;
    opts = clone(opts);
    opts.continuous = opts.continuous || opts.live;
    opts.retry = ('retry' in opts) ? opts.retry : false;
    /*jshint validthis:true */
    opts.PouchConstructor = opts.PouchConstructor || this;
    var replicateRet = new Replication(opts);
    var srcPouch = toPouch(src, opts);
    var targetPouch = toPouch(target, opts);
    replicate$1(srcPouch, targetPouch, opts, replicateRet);
    return replicateRet;
  }
  
  inherits(Sync, EventEmitter);
  function sync(src, target, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    if (typeof opts === 'undefined') {
      opts = {};
    }
    opts = clone(opts);
    /*jshint validthis:true */
    opts.PouchConstructor = opts.PouchConstructor || this;
    src = toPouch(src, opts);
    target = toPouch(target, opts);
    return new Sync(src, target, opts, callback);
  }
  
  function Sync(src, target, opts, callback) {
    var self = this;
    this.canceled = false;
  
    var optsPush = opts.push ? extend$1({}, opts, opts.push) : opts;
    var optsPull = opts.pull ? extend$1({}, opts, opts.pull) : opts;
  
    this.push = replicate(src, target, optsPush);
    this.pull = replicate(target, src, optsPull);
  
    this.pushPaused = true;
    this.pullPaused = true;
  
    function pullChange(change) {
      self.emit('change', {
        direction: 'pull',
        change: change
      });
    }
    function pushChange(change) {
      self.emit('change', {
        direction: 'push',
        change: change
      });
    }
    function pushDenied(doc) {
      self.emit('denied', {
        direction: 'push',
        doc: doc
      });
    }
    function pullDenied(doc) {
      self.emit('denied', {
        direction: 'pull',
        doc: doc
      });
    }
    function pushPaused() {
      self.pushPaused = true;
      /* istanbul ignore if */
      if (self.pullPaused) {
        self.emit('paused');
      }
    }
    function pullPaused() {
      self.pullPaused = true;
      /* istanbul ignore if */
      if (self.pushPaused) {
        self.emit('paused');
      }
    }
    function pushActive() {
      self.pushPaused = false;
      /* istanbul ignore if */
      if (self.pullPaused) {
        self.emit('active', {
          direction: 'push'
        });
      }
    }
    function pullActive() {
      self.pullPaused = false;
      /* istanbul ignore if */
      if (self.pushPaused) {
        self.emit('active', {
          direction: 'pull'
        });
      }
    }
  
    var removed = {};
  
    function removeAll(type) { // type is 'push' or 'pull'
      return function (event, func) {
        var isChange = event === 'change' &&
          (func === pullChange || func === pushChange);
        var isDenied = event === 'denied' &&
          (func === pullDenied || func === pushDenied);
        var isPaused = event === 'paused' &&
          (func === pullPaused || func === pushPaused);
        var isActive = event === 'active' &&
          (func === pullActive || func === pushActive);
  
        if (isChange || isDenied || isPaused || isActive) {
          if (!(event in removed)) {
            removed[event] = {};
          }
          removed[event][type] = true;
          if (Object.keys(removed[event]).length === 2) {
            // both push and pull have asked to be removed
            self.removeAllListeners(event);
          }
        }
      };
    }
  
    if (opts.live) {
      this.push.on('complete', self.pull.cancel.bind(self.pull));
      this.pull.on('complete', self.push.cancel.bind(self.push));
    }
  
    this.on('newListener', function (event) {
      if (event === 'change') {
        self.pull.on('change', pullChange);
        self.push.on('change', pushChange);
      } else if (event === 'denied') {
        self.pull.on('denied', pullDenied);
        self.push.on('denied', pushDenied);
      } else if (event === 'active') {
        self.pull.on('active', pullActive);
        self.push.on('active', pushActive);
      } else if (event === 'paused') {
        self.pull.on('paused', pullPaused);
        self.push.on('paused', pushPaused);
      }
    });
  
    this.on('removeListener', function (event) {
      if (event === 'change') {
        self.pull.removeListener('change', pullChange);
        self.push.removeListener('change', pushChange);
      } else if (event === 'denied') {
        self.pull.removeListener('denied', pullDenied);
        self.push.removeListener('denied', pushDenied);
      } else if (event === 'active') {
        self.pull.removeListener('active', pullActive);
        self.push.removeListener('active', pushActive);
      } else if (event === 'paused') {
        self.pull.removeListener('paused', pullPaused);
        self.push.removeListener('paused', pushPaused);
      }
    });
  
    this.pull.on('removeListener', removeAll('pull'));
    this.push.on('removeListener', removeAll('push'));
  
    var promise = PouchPromise.all([
      this.push,
      this.pull
    ]).then(function (resp) {
      var out = {
        push: resp[0],
        pull: resp[1]
      };
      self.emit('complete', out);
      if (callback) {
        callback(null, out);
      }
      self.removeAllListeners();
      return out;
    }, function (err) {
      self.cancel();
      if (callback) {
        // if there's a callback, then the callback can receive
        // the error event
        callback(err);
      } else {
        // if there's no callback, then we're safe to emit an error
        // event, which would otherwise throw an unhandled error
        // due to 'error' being a special event in EventEmitters
        self.emit('error', err);
      }
      self.removeAllListeners();
      if (callback) {
        // no sense throwing if we're already emitting an 'error' event
        throw err;
      }
    });
  
    this.then = function (success, err) {
      return promise.then(success, err);
    };
  
    this.catch = function (err) {
      return promise.catch(err);
    };
  }
  
  Sync.prototype.cancel = function () {
    if (!this.canceled) {
      this.canceled = true;
      this.push.cancel();
      this.pull.cancel();
    }
  };
  
  function replication(PouchDB) {
    PouchDB.replicate = replicate;
    PouchDB.sync = sync;
  
    Object.defineProperty(PouchDB.prototype, 'replicate', {
      get: function () {
        var self = this;
        return {
          from: function (other, opts, callback) {
            return self.constructor.replicate(other, self, opts, callback);
          },
          to: function (other, opts, callback) {
            return self.constructor.replicate(self, other, opts, callback);
          }
        };
      }
    });
  
    PouchDB.prototype.sync = function (dbName, opts, callback) {
      return this.constructor.sync(this, dbName, opts, callback);
    };
  }
  
  PouchDB.plugin(LevelPouch)
    .plugin(HttpPouch$1)
    .plugin(mapreduce)
    .plugin(replication);
  
  // Pull from src because pouchdb-node/pouchdb-browser themselves
  // are aggressively optimized and jsnext:main would normally give us this
  // aggressive bundle.
  
  export default PouchDB;