text: /*
   Copyright (c) 2012, Yahoo! Inc.  All rights reserved.
   Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
   */
  
  /**
   * provides a mechanism to transform code in the scope of `require` or `vm.createScript`.
   * This mechanism is general and relies on a user-supplied `matcher` function that determines when transformations should be
   * performed and a user-supplied `transformer` function that performs the actual transform.
   * Instrumenting code for coverage is one specific example of useful hooking.
   *
   * Note that both the `matcher` and `transformer` must execute synchronously.
   *
   * For the common case of matching filesystem paths based on inclusion/ exclusion patterns, use the `matcherFor`
   * function in the istanbul API to get a matcher.
   *
   * It is up to the transformer to perform processing with side-effects, such as caching, storing the original
   * source code to disk in case of dynamically generated scripts etc. The `Store` class can help you with this.
   *
   * Usage
   * -----
   *
   *      var hook = require('istanbul').hook,
   *          myMatcher = function (file) { return file.match(/foo/); },
   *          myTransformer = function (code, file) { return 'console.log("' + file + '");' + code; };
   *
   *      hook.hookRequire(myMatcher, myTransformer);
   *
   *      var foo = require('foo'); //will now print foo's module path to console
   *
   * @class Hook
   * @module main
   */
  var path = require('path'),
      fs = require('fs'),
      Module = require('module'),
      vm = require('vm'),
      originalLoaders = {},
      originalCreateScript = vm.createScript,
      originalRunInThisContext = vm.runInThisContext;
  
  function transformFn(matcher, transformer, verbose) {
  
      return function (code, filename) {
          var shouldHook = typeof filename === 'string' && matcher(path.resolve(filename)),
              transformed,
              changed = false;
  
          if (shouldHook) {
              if (verbose) {
                  console.error('Module load hook: transform [' + filename + ']');
              }
              try {
                  transformed = transformer(code, filename);
                  changed = true;
              } catch (ex) {
                  console.error('Transformation error; return original code');
                  console.error(ex);
                  transformed = code;
              }
          } else {
              transformed = code;
          }
          return { code: transformed, changed: changed };
      };
  }
  
  function unloadRequireCache(matcher) {
      if (matcher && typeof require !== 'undefined' && require && require.cache) {
          Object.keys(require.cache).forEach(function (filename) {
              if (matcher(filename)) {
                  delete require.cache[filename];
              }
          });
      }
  }
  /**
   * hooks `require` to return transformed code to the node module loader.
   * Exceptions in the transform result in the original code being used instead.
   * @method hookRequire
   * @static
   * @param matcher {Function(filePath)} a function that is called with the absolute path to the file being
   *  `require`-d. Should return a truthy value when transformations need to be applied to the code, a falsy value otherwise
   * @param transformer {Function(code, filePath)} a function called with the original code and the associated path of the file
   *  from where the code was loaded. Should return the transformed code.
   * @param options {Object} options Optional.
   * @param {Boolean} [options.verbose] write a line to standard error every time the transformer is called
   * @param {Function} [options.postLoadHook] a function that is called with the name of the file being
   *  required. This is called after the require is processed irrespective of whether it was transformed.
   */
  function hookRequire(matcher, transformer, options) {
      options = options || {};
      var extensions,
          fn = transformFn(matcher, transformer, options.verbose),
          postLoadHook = options.postLoadHook &&
              typeof options.postLoadHook === 'function' ? options.postLoadHook : null;
  
      extensions = options.extensions || ['.js'];
  
      extensions.forEach(function(ext){
          if (!(ext in originalLoaders)) { 
              originalLoaders[ext] = Module._extensions[ext];
          } 
          Module._extensions[ext] = function (module, filename) {
              var ret = fn(fs.readFileSync(filename, 'utf8'), filename);
              if (ret.changed) {
                  module._compile(ret.code, filename);
              } else {
                  originalLoaders[ext](module, filename);
              }
              if (postLoadHook) {
                  postLoadHook(filename);
              }
          };
      });
  }
  /**
   * unhook `require` to restore it to its original state.
   * @method unhookRequire
   * @static
   */
  function unhookRequire() {
      Object.keys(originalLoaders).forEach(function(ext) {
          Module._extensions[ext] = originalLoaders[ext];
      });
  }
  /**
   * hooks `vm.createScript` to return transformed code out of which a `Script` object will be created.
   * Exceptions in the transform result in the original code being used instead.
   * @method hookCreateScript
   * @static
   * @param matcher {Function(filePath)} a function that is called with the filename passed to `vm.createScript`
   *  Should return a truthy value when transformations need to be applied to the code, a falsy value otherwise
   * @param transformer {Function(code, filePath)} a function called with the original code and the filename passed to
   *  `vm.createScript`. Should return the transformed code.
   * @param options {Object} options Optional.
   * @param {Boolean} [options.verbose] write a line to standard error every time the transformer is called
   */
  function hookCreateScript(matcher, transformer, opts) {
      opts = opts || {};
      var fn = transformFn(matcher, transformer, opts.verbose);
      vm.createScript = function (code, file) {
          var ret = fn(code, file);
          return originalCreateScript(ret.code, file);
      };
  }
  
  /**
   * unhooks vm.createScript, restoring it to its original state.
   * @method unhookCreateScript
   * @static
   */
  function unhookCreateScript() {
      vm.createScript = originalCreateScript;
  }
  
  
  /**
   * hooks `vm.runInThisContext` to return transformed code.
   * @method hookRunInThisContext
   * @static
   * @param matcher {Function(filePath)} a function that is called with the filename passed to `vm.createScript`
   *  Should return a truthy value when transformations need to be applied to the code, a falsy value otherwise
   * @param transformer {Function(code, filePath)} a function called with the original code and the filename passed to
   *  `vm.createScript`. Should return the transformed code.
   * @param options {Object} options Optional.
   * @param {Boolean} [options.verbose] write a line to standard error every time the transformer is called
   */
  function hookRunInThisContext(matcher, transformer, opts) {
      opts = opts || {};
      var fn = transformFn(matcher, transformer, opts.verbose);
      vm.runInThisContext = function (code, file) {
          var ret = fn(code, file);
          return originalRunInThisContext(ret.code, file);
      };
  }
  
  /**
   * unhooks vm.runInThisContext, restoring it to its original state.
   * @method unhookRunInThisContext
   * @static
   */
  function unhookRunInThisContext() {
      vm.runInThisContext = originalRunInThisContext;
  }
  
  
  module.exports = {
      hookRequire: hookRequire,
      unhookRequire: unhookRequire,
      hookCreateScript: hookCreateScript,
      unhookCreateScript: unhookCreateScript,
      hookRunInThisContext : hookRunInThisContext,
      unhookRunInThisContext : unhookRunInThisContext,
      unloadRequireCache: unloadRequireCache
  };
  