text: const dispatch = require('../dispatch/central-dispatch');
  const log = require('../util/log');
  
  const BlockType = require('./block-type');
  
  // These extensions are currently built into the VM repository but should not be loaded at startup.
  // TODO: move these out into a separate repository?
  // TODO: change extension spec so that library info, including extension ID, can be collected through static methods
  const Scratch3PenBlocks = require('../blocks/scratch3_pen');
  const Scratch3WeDo2Blocks = require('../blocks/scratch3_wedo2');
  const Scratch3MusicBlocks = require('../blocks/scratch3_music');
  const builtinExtensions = {
      pen: Scratch3PenBlocks,
      wedo2: Scratch3WeDo2Blocks,
      music: Scratch3MusicBlocks
  };
  
  /**
   * @typedef {object} ArgumentInfo - Information about an extension block argument
   * @property {ArgumentType} type - the type of value this argument can take
   * @property {*|undefined} default - the default value of this argument (default: blank)
   */
  
  /**
   * @typedef {object} BlockInfo - Information about an extension block
   * @property {string} opcode - the block opcode
   * @property {string|object} text - the human-readable text on this block
   * @property {BlockType|undefined} blockType - the type of block (default: BlockType.COMMAND)
   * @property {int|undefined} branchCount - the number of branches this block controls, if conditional (default: 0)
   * @property {Boolean|undefined} isTerminal - true if this block ends a stack (default: false)
   * @property {Boolean|undefined} blockAllThreads - true if all threads must wait for this block to run (default: false)
   * @property {object.<string,ArgumentInfo>|undefined} arguments - information about this block's arguments, if any
   * @property {string|Function|undefined} func - the method for this block on the extension service (default: opcode)
   * @property {Array.<string>|undefined} filter - the list of targets for which this block should appear (default: all)
   * @property {Boolean|undefined} hideFromPalette - true if should not be appear in the palette. (default false)
   */
  
  /**
   * @typedef {object} CategoryInfo - Information about a block category
   * @property {string} id - the unique ID of this category
   * @property {string} color1 - the primary color for this category, in '#rrggbb' format
   * @property {string} color2 - the secondary color for this category, in '#rrggbb' format
   * @property {string} color3 - the tertiary color for this category, in '#rrggbb' format
   * @property {Array.<BlockInfo>} block - the blocks in this category
   */
  
  /**
   * @typedef {object} PendingExtensionWorker - Information about an extension worker still initializing
   * @property {string} extensionURL - the URL of the extension to be loaded by this worker
   * @property {Function} resolve - function to call on successful worker startup
   * @property {Function} reject - function to call on failed worker startup
   */
  
  class ExtensionManager {
      constructor (runtime) {
          /**
           * The ID number to provide to the next extension worker.
           * @type {int}
           */
          this.nextExtensionWorker = 0;
  
          /**
           * FIFO queue of extensions which have been requested but not yet loaded in a worker,
           * along with promise resolution functions to call once the worker is ready or failed.
           *
           * @type {Array.<PendingExtensionWorker>}
           */
          this.pendingExtensions = [];
  
          /**
           * Map of worker ID to workers which have been allocated but have not yet finished initialization.
           * @type {Array.<PendingExtensionWorker>}
           */
          this.pendingWorkers = [];
  
          /**
           * Set of loaded extension URLs/IDs (equivalent for built-in extensions).
           * @type {Set.<string>}
           * @private
           */
          this._loadedExtensions = new Set();
  
          /**
           * Keep a reference to the runtime so we can construct internal extension objects.
           * TODO: remove this in favor of extensions accessing the runtime as a service.
           * @type {Runtime}
           */
          this.runtime = runtime;
  
          dispatch.setService('extensions', this).catch(e => {
              log.error(`ExtensionManager was unable to register extension service: ${JSON.stringify(e)}`);
          });
      }
  
      /**
       * Check whether an extension is registered or is in the process of loading. This is intended to control loading or
       * adding extensions so it may return `true` before the extension is ready to be used. Use the promise returned by
       * `loadExtensionURL` if you need to wait until the extension is truly ready.
       * @param {string} extensionID - the ID of the extension.
       * @returns {boolean} - true if loaded, false otherwise.
       */
      isExtensionLoaded (extensionID) {
          return this._loadedExtensions.has(extensionID);
      }
  
      /**
       * Load an extension by URL or internal extension ID
       * @param {string} extensionURL - the URL for the extension to load OR the ID of an internal extension
       * @returns {Promise} resolved once the extension is loaded and initialized or rejected on failure
       */
      loadExtensionURL (extensionURL) {
          if (builtinExtensions.hasOwnProperty(extensionURL)) {
              /** @TODO dupe handling for non-builtin extensions. See commit 670e51d33580e8a2e852b3b038bb3afc282f81b9 */
              if (this.isExtensionLoaded(extensionURL)) {
                  const message = `Rejecting attempt to load a second extension with ID ${extensionURL}`;
                  log.warn(message);
                  return Promise.reject(new Error(message));
              }
  
              const extension = builtinExtensions[extensionURL];
              const extensionInstance = new extension(this.runtime);
              return this._registerInternalExtension(extensionInstance).then(() => {
                  this._loadedExtensions.add(extensionURL);
              });
          }
  
          return new Promise((resolve, reject) => {
              // If we `require` this at the global level it breaks non-webpack targets, including tests
              const ExtensionWorker = require('worker-loader?name=extension-worker.js!./extension-worker');
  
              this.pendingExtensions.push({extensionURL, resolve, reject});
              dispatch.addWorker(new ExtensionWorker());
          });
      }
  
      allocateWorker () {
          const id = this.nextExtensionWorker++;
          const workerInfo = this.pendingExtensions.shift();
          this.pendingWorkers[id] = workerInfo;
          return [id, workerInfo.extensionURL];
      }
  
      /**
       * Collect extension metadata from the specified service and begin the extension registration process.
       * @param {string} serviceName - the name of the service hosting the extension.
       */
      registerExtensionService (serviceName) {
          dispatch.call(serviceName, 'getInfo').then(info => {
              this._registerExtensionInfo(serviceName, info);
          });
      }
  
      /**
       * Called by an extension worker to indicate that the worker has finished initialization.
       * @param {int} id - the worker ID.
       * @param {*?} e - the error encountered during initialization, if any.
       */
      onWorkerInit (id, e) {
          const workerInfo = this.pendingWorkers[id];
          delete this.pendingWorkers[id];
          if (e) {
              workerInfo.reject(e);
          } else {
              workerInfo.resolve(id);
          }
      }
  
      /**
       * Register an internal (non-Worker) extension object
       * @param {object} extensionObject - the extension object to register
       * @returns {Promise} resolved once the extension is fully registered or rejected on failure
       */
      _registerInternalExtension (extensionObject) {
          const extensionInfo = extensionObject.getInfo();
          const fakeWorkerId = this.nextExtensionWorker++;
          const serviceName = `extension.${fakeWorkerId}.${extensionInfo.id}`;
          return dispatch.setService(serviceName, extensionObject)
              .then(() => dispatch.call('extensions', 'registerExtensionService', serviceName));
      }
  
      /**
       * Sanitize extension info then register its primitives with the VM.
       * @param {string} serviceName - the name of the service hosting the extension
       * @param {ExtensionInfo} extensionInfo - the extension's metadata
       * @private
       */
      _registerExtensionInfo (serviceName, extensionInfo) {
          extensionInfo = this._prepareExtensionInfo(serviceName, extensionInfo);
          dispatch.call('runtime', '_registerExtensionPrimitives', extensionInfo).catch(e => {
              log.error(`Failed to register primitives for extension on service ${serviceName}: ${JSON.stringify(e)}`);
          });
      }
  
      /**
       * Modify the provided text as necessary to ensure that it may be used as an attribute value in valid XML.
       * @param {string} text - the text to be sanitized
       * @returns {string} - the sanitized text
       * @private
       */
      _sanitizeID (text) {
          return text.toString().replace(/[<"&]/, '_');
      }
  
      /**
       * Apply minor cleanup and defaults for optional extension fields.
       * TODO: make the ID unique in cases where two copies of the same extension are loaded.
       * @param {string} serviceName - the name of the service hosting this extension block
       * @param {ExtensionInfo} extensionInfo - the extension info to be sanitized
       * @returns {ExtensionInfo} - a new extension info object with cleaned-up values
       * @private
       */
      _prepareExtensionInfo (serviceName, extensionInfo) {
          extensionInfo = Object.assign({}, extensionInfo);
          extensionInfo.id = this._sanitizeID(extensionInfo.id);
          extensionInfo.name = extensionInfo.name || extensionInfo.id;
          extensionInfo.blocks = extensionInfo.blocks || [];
          extensionInfo.targetTypes = extensionInfo.targetTypes || [];
          extensionInfo.blocks = extensionInfo.blocks.reduce((result, blockInfo) => {
              try {
                  result.push(this._prepareBlockInfo(serviceName, blockInfo));
              } catch (e) {
                  // TODO: more meaningful error reporting
                  log.error(`Error processing block: ${e.message}, Block:\n${JSON.stringify(blockInfo)}`);
              }
              return result;
          }, []);
          return extensionInfo;
      }
  
      /**
       * Apply defaults for optional block fields.
       * @param {string} serviceName - the name of the service hosting this extension block
       * @param {BlockInfo} blockInfo - the block info from the extension
       * @returns {BlockInfo} - a new block info object which has values for all relevant optional fields.
       * @private
       */
      _prepareBlockInfo (serviceName, blockInfo) {
          blockInfo = Object.assign({}, {
              blockType: BlockType.COMMAND,
              terminal: false,
              blockAllThreads: false,
              arguments: {}
          }, blockInfo);
          blockInfo.opcode = this._sanitizeID(blockInfo.opcode);
          blockInfo.func = blockInfo.func ? this._sanitizeID(blockInfo.func) : blockInfo.opcode;
          blockInfo.text = blockInfo.text || blockInfo.opcode;
  
          /**
           * This is only here because the VM performs poorly when blocks return promises.
           * @TODO make it possible for the VM to resolve a promise and continue during the same frame.
           */
          if (dispatch._isRemoteService(serviceName)) {
              blockInfo.func = dispatch.call.bind(dispatch, serviceName, blockInfo.func);
          } else {
              const serviceObject = dispatch.services[serviceName];
              blockInfo.func = serviceObject[blockInfo.func].bind(serviceObject);
          }
          return blockInfo;
      }
  }
  
  module.exports = ExtensionManager;
  