observe.js

/*
 * Copyright (c) 2014 The Polymer Project Authors. All rights reserved.
 * This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
 * The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
 * The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
 * Code distributed by Google as part of the polymer project is also
 * subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 */

(function(global) {
  'use strict';

  var testingExposeCycleCount = global.testingExposeCycleCount;

  // Detect and do basic sanity checking on Object/Array.observe.
  function detectObjectObserve() {
    if (typeof Object.observe !== 'function' ||
        typeof Array.observe !== 'function') {
      return false;
    }

    var records = [];

    function callback(recs) {
      records = recs;
    }

    var test = {};
    var arr = [];
    Object.observe(test, callback);
    Array.observe(arr, callback);
    test.id = 1;
    test.id = 2;
    delete test.id;
    arr.push(1, 2);
    arr.length = 0;

    Object.deliverChangeRecords(callback);
    if (records.length !== 5)
      return false;

    if (records[0].type != 'add' ||
        records[1].type != 'update' ||
        records[2].type != 'delete' ||
        records[3].type != 'splice' ||
        records[4].type != 'splice') {
      return false;
    }

    Object.unobserve(test, callback);
    Array.unobserve(arr, callback);

    return true;
  }

  var hasObserve = detectObjectObserve();

  function detectEval() {
    // Don't test for eval if we're running in a Chrome App environment.
    // We check for APIs set that only exist in a Chrome App context.
    if (typeof chrome !== 'undefined' && chrome.app && chrome.app.runtime) {
      return false;
    }

    // Firefox OS Apps do not allow eval. This feature detection is very hacky
    // but even if some other platform adds support for this function this code
    // will continue to work.
    if (typeof navigator != 'undefined' && navigator.getDeviceStorage) {
      return false;
    }

    try {
      var f = new Function('', 'return true;');
      return f();
    } catch (ex) {
      return false;
    }
  }

  var hasEval = detectEval();

  function isIndex(s) {
    return +s === s >>> 0 && s !== '';
  }

  function toNumber(s) {
    return +s;
  }

  function isObject(obj) {
    return obj === Object(obj);
  }

  var numberIsNaN = global.Number.isNaN || function(value) {
    return typeof value === 'number' && global.isNaN(value);
  };

  function areSameValue(left, right) {
    if (left === right)
      return left !== 0 || 1 / left === 1 / right;
    if (numberIsNaN(left) && numberIsNaN(right))
      return true;

    return left !== left && right !== right;
  }

  var createObject = ('__proto__' in {}) ?
    function(obj) { return obj; } :
    function(obj) {
      var proto = obj.__proto__;
      if (!proto)
        return obj;
      var newObject = Object.create(proto);
      Object.getOwnPropertyNames(obj).forEach(function(name) {
        Object.defineProperty(newObject, name,
                             Object.getOwnPropertyDescriptor(obj, name));
      });
      return newObject;
    };

  var identStart = '[\$_a-zA-Z]';
  var identPart = '[\$_a-zA-Z0-9]';
  var identRegExp = new RegExp('^' + identStart + '+' + identPart + '*' + '$');

  function getPathCharType(char) {
    if (char === undefined)
      return 'eof';

    var code = char.charCodeAt(0);

    switch(code) {
      case 0x5B: // [
      case 0x5D: // ]
      case 0x2E: // .
      case 0x22: // "
      case 0x27: // '
      case 0x30: // 0
        return char;

      case 0x5F: // _
      case 0x24: // $
        return 'ident';

      case 0x20: // Space
      case 0x09: // Tab
      case 0x0A: // Newline
      case 0x0D: // Return
      case 0xA0:  // No-break space
      case 0xFEFF:  // Byte Order Mark
      case 0x2028:  // Line Separator
      case 0x2029:  // Paragraph Separator
        return 'ws';
    }

    // a-z, A-Z
    if ((0x61 <= code && code <= 0x7A) || (0x41 <= code && code <= 0x5A))
      return 'ident';

    // 1-9
    if (0x31 <= code && code <= 0x39)
      return 'number';

    return 'else';
  }

  var pathStateMachine = {
    'beforePath': {
      'ws': ['beforePath'],
      'ident': ['inIdent', 'append'],
      '[': ['beforeElement'],
      'eof': ['afterPath']
    },

    'inPath': {
      'ws': ['inPath'],
      '.': ['beforeIdent'],
      '[': ['beforeElement'],
      'eof': ['afterPath']
    },

    'beforeIdent': {
      'ws': ['beforeIdent'],
      'ident': ['inIdent', 'append']
    },

    'inIdent': {
      'ident': ['inIdent', 'append'],
      '0': ['inIdent', 'append'],
      'number': ['inIdent', 'append'],
      'ws': ['inPath', 'push'],
      '.': ['beforeIdent', 'push'],
      '[': ['beforeElement', 'push'],
      'eof': ['afterPath', 'push']
    },

    'beforeElement': {
      'ws': ['beforeElement'],
      '0': ['afterZero', 'append'],
      'number': ['inIndex', 'append'],
      "'": ['inSingleQuote', 'append', ''],
      '"': ['inDoubleQuote', 'append', '']
    },

    'afterZero': {
      'ws': ['afterElement', 'push'],
      ']': ['inPath', 'push']
    },

    'inIndex': {
      '0': ['inIndex', 'append'],
      'number': ['inIndex', 'append'],
      'ws': ['afterElement'],
      ']': ['inPath', 'push']
    },

    'inSingleQuote': {
      "'": ['afterElement'],
      'eof': ['error'],
      'else': ['inSingleQuote', 'append']
    },

    'inDoubleQuote': {
      '"': ['afterElement'],
      'eof': ['error'],
      'else': ['inDoubleQuote', 'append']
    },

    'afterElement': {
      'ws': ['afterElement'],
      ']': ['inPath', 'push']
    }
  };

  function noop() {}

  function parsePath(path) {
    var keys = [];
    var index = -1;
    var c, newChar, key, type, transition, action, typeMap, mode = 'beforePath';

    var actions = {
      push: function() {
        if (key === undefined)
          return;

        keys.push(key);
        key = undefined;
      },

      append: function() {
        if (key === undefined)
          key = newChar;
        else
          key += newChar;
      }
    };

    function maybeUnescapeQuote() {
      if (index >= path.length)
        return;

      var nextChar = path[index + 1];
      if ((mode == 'inSingleQuote' && nextChar == "'") ||
          (mode == 'inDoubleQuote' && nextChar == '"')) {
        index++;
        newChar = nextChar;
        actions.append();
        return true;
      }
    }

    while (mode) {
      index++;
      c = path[index];

      if (c == '\\' && maybeUnescapeQuote(mode))
        continue;

      type = getPathCharType(c);
      typeMap = pathStateMachine[mode];
      transition = typeMap[type] || typeMap['else'] || 'error';

      if (transition == 'error')
        return; // parse error;

      mode = transition[0];
      action = actions[transition[1]] || noop;
      newChar = transition[2] === undefined ? c : transition[2];
      action();

      if (mode === 'afterPath') {
        return keys;
      }
    }

    return; // parse error
  }

  function isIdent(s) {
    return identRegExp.test(s);
  }

  var constructorIsPrivate = {};

  function Path(parts, privateToken) {
    if (privateToken !== constructorIsPrivate)
      throw Error('Use Path.get to retrieve path objects');

    for (var i = 0; i < parts.length; i++) {
      this.push(String(parts[i]));
    }

    if (hasEval && this.length) {
      this.getValueFrom = this.compiledGetValueFromFn();
    }
  }

  // TODO(rafaelw): Make simple LRU cache
  var pathCache = {};

  function getPath(pathString) {
    if (pathString instanceof Path)
      return pathString;

    if (pathString == null || pathString.length == 0)
      pathString = '';

    if (typeof pathString != 'string') {
      if (isIndex(pathString.length)) {
        // Constructed with array-like (pre-parsed) keys
        return new Path(pathString, constructorIsPrivate);
      }

      pathString = String(pathString);
    }

    var path = pathCache[pathString];
    if (path)
      return path;

    var parts = parsePath(pathString);
    if (!parts)
      return invalidPath;

    path = new Path(parts, constructorIsPrivate);
    pathCache[pathString] = path;
    return path;
  }

  Path.get = getPath;

  function formatAccessor(key) {
    if (isIndex(key)) {
      return '[' + key + ']';
    } else {
      return '["' + key.replace(/"/g, '\\"') + '"]';
    }
  }

  Path.prototype = createObject({
    __proto__: [],
    valid: true,

    toString: function() {
      var pathString = '';
      for (var i = 0; i < this.length; i++) {
        var key = this[i];
        if (isIdent(key)) {
          pathString += i ? '.' + key : key;
        } else {
          pathString += formatAccessor(key);
        }
      }

      return pathString;
    },

    getValueFrom: function(obj, defaultValue) {
      for (var i = 0; i < this.length; i++) {
        var key = this[i];
        if (obj == null || !(key in obj))
          return defaultValue;
        obj = obj[key];
      }
      return obj;
    },

    iterateObjects: function(obj, observe) {
      for (var i = 0; i < this.length; i++) {
        if (i)
          obj = obj[this[i - 1]];
        if (!isObject(obj))
          return;
        observe(obj, this[i]);
      }
    },

    compiledGetValueFromFn: function() {
      var str = '';
      var pathString = 'obj';
      str += 'if (obj != null';
      var i = 0;
      var key;
      for (; i < (this.length - 1); i++) {
        key = this[i];
        pathString += isIdent(key) ? '.' + key : formatAccessor(key);
        str += ' &&\n    ' + pathString + ' != null';
      }

      key = this[i];
      var keyIsIdent = isIdent(key);
      var keyForInOperator = keyIsIdent ? '"' + key.replace(/"/g, '\\"') + '"' : key;
      str += ' &&\n    ' + keyForInOperator + ' in ' + pathString + ')\n';
      pathString += keyIsIdent ? '.' + key : formatAccessor(key);

      str += '  return ' + pathString + ';\nelse\n  return defaultValue;';
      return new Function('obj', 'defaultValue', str);
    },

    setValueFrom: function(obj, value) {
      if (!this.length)
        return false;

      for (var i = 0; i < this.length - 1; i++) {
        if (!isObject(obj))
          return false;
        obj = obj[this[i]];
      }

      if (!isObject(obj))
        return false;

      obj[this[i]] = value;
      return true;
    }
  });

  var invalidPath = new Path('', constructorIsPrivate);
  invalidPath.valid = false;
  invalidPath.getValueFrom = invalidPath.setValueFrom = function() {};

  var MAX_DIRTY_CHECK_CYCLES = 1000;

  function dirtyCheck(observer) {
    var cycles = 0;
    while (cycles < MAX_DIRTY_CHECK_CYCLES && observer.check_()) {
      cycles++;
    }
    if (testingExposeCycleCount)
      global.dirtyCheckCycleCount = cycles;

    return cycles > 0;
  }

  function objectIsEmpty(object) {
    for (var prop in object)
      return false;
    return true;
  }

  function diffIsEmpty(diff) {
    return objectIsEmpty(diff.added) &&
           objectIsEmpty(diff.removed) &&
           objectIsEmpty(diff.changed);
  }

  function diffObjectFromOldObject(object, oldObject) {
    var added = {};
    var removed = {};
    var changed = {};
    var prop;

    for (prop in oldObject) {
      var newValue = object[prop];

      if (newValue !== undefined && newValue === oldObject[prop])
        continue;

      if (!(prop in object)) {
        removed[prop] = undefined;
        continue;
      }

      if (newValue !== oldObject[prop])
        changed[prop] = newValue;
    }

    for (prop in object) {
      if (prop in oldObject)
        continue;

      added[prop] = object[prop];
    }

    if (Array.isArray(object) && object.length !== oldObject.length)
      changed.length = object.length;

    return {
      added: added,
      removed: removed,
      changed: changed
    };
  }

  var eomTasks = [];
  function runEOMTasks() {
    if (!eomTasks.length)
      return false;

    for (var i = 0; i < eomTasks.length; i++) {
      eomTasks[i]();
    }
    eomTasks.length = 0;
    return true;
  }

  var runEOM = hasObserve ? (function(){
    return function(fn) {
      return Promise.resolve().then(fn);
    };
  })() :
  (function() {
    return function(fn) {
      eomTasks.push(fn);
    };
  })();

  var observedObjectCache = [];

  function newObservedObject() {
    var observer;
    var object;
    var discardRecords = false;
    var first = true;

    function callback(records) {
      if (observer && observer.state_ === OPENED && !discardRecords)
        observer.check_(records);
    }

    return {
      open: function(obs) {
        if (observer)
          throw Error('ObservedObject in use');

        if (!first)
          Object.deliverChangeRecords(callback);

        observer = obs;
        first = false;
      },
      observe: function(obj, arrayObserve) {
        object = obj;
        if (arrayObserve)
          Array.observe(object, callback);
        else
          Object.observe(object, callback);
      },
      deliver: function(discard) {
        discardRecords = discard;
        Object.deliverChangeRecords(callback);
        discardRecords = false;
      },
      close: function() {
        observer = undefined;
        Object.unobserve(object, callback);
        observedObjectCache.push(this);
      }
    };
  }

  /*
   * The observedSet abstraction is a perf optimization which reduces the total
   * number of Object.observe observations of a set of objects. The idea is that
   * groups of Observers will have some object dependencies in common and this
   * observed set ensures that each object in the transitive closure of
   * dependencies is only observed once. The observedSet acts as a write barrier
   * such that whenever any change comes through, all Observers are checked for
   * changed values.
   *
   * Note that this optimization is explicitly moving work from setup-time to
   * change-time.
   *
   * TODO(rafaelw): Implement "garbage collection". In order to move work off
   * the critical path, when Observers are closed, their observed objects are
   * not Object.unobserve(d). As a result, it's possible that if the observedSet
   * is kept open, but some Observers have been closed, it could cause "leaks"
   * (prevent otherwise collectable objects from being collected). At some
   * point, we should implement incremental "gc" which keeps a list of
   * observedSets which may need clean-up and does small amounts of cleanup on a
   * timeout until all is clean.
   */

  function getObservedObject(observer, object, arrayObserve) {
    var dir = observedObjectCache.pop() || newObservedObject();
    dir.open(observer);
    dir.observe(object, arrayObserve);
    return dir;
  }

  var observedSetCache = [];

  function newObservedSet() {
    var observerCount = 0;
    var observers = [];
    var objects = [];
    var rootObj;
    var rootObjProps;

    function observe(obj, prop) {
      if (!obj)
        return;

      if (obj === rootObj)
        rootObjProps[prop] = true;

      if (objects.indexOf(obj) < 0) {
        objects.push(obj);
        Object.observe(obj, callback);
      }

      observe(Object.getPrototypeOf(obj), prop);
    }

    function allRootObjNonObservedProps(recs) {
      for (var i = 0; i < recs.length; i++) {
        var rec = recs[i];
        if (rec.object !== rootObj ||
            rootObjProps[rec.name] ||
            rec.type === 'setPrototype') {
          return false;
        }
      }
      return true;
    }

    function callback(recs) {
      if (allRootObjNonObservedProps(recs))
        return;

      var i, observer;
      for (i = 0; i < observers.length; i++) {
        observer = observers[i];
        if (observer.state_ == OPENED) {
          observer.iterateObjects_(observe);
        }
      }

      for (i = 0; i < observers.length; i++) {
        observer = observers[i];
        if (observer.state_ == OPENED) {
          observer.check_();
        }
      }
    }

    var record = {
      objects: objects,
      get rootObject() { return rootObj; },
      set rootObject(value) {
        rootObj = value;
        rootObjProps = {};
      },
      open: function(obs, object) {
        observers.push(obs);
        observerCount++;
        obs.iterateObjects_(observe);
      },
      close: function(obs) {
        observerCount--;
        if (observerCount > 0) {
          return;
        }

        for (var i = 0; i < objects.length; i++) {
          Object.unobserve(objects[i], callback);
          Observer.unobservedCount++;
        }

        observers.length = 0;
        objects.length = 0;
        rootObj = undefined;
        rootObjProps = undefined;
        observedSetCache.push(this);
        if (lastObservedSet === this)
          lastObservedSet = null;
      },
    };

    return record;
  }

  var lastObservedSet;

  function getObservedSet(observer, obj) {
    if (!lastObservedSet || lastObservedSet.rootObject !== obj) {
      lastObservedSet = observedSetCache.pop() || newObservedSet();
      lastObservedSet.rootObject = obj;
    }
    lastObservedSet.open(observer, obj);
    return lastObservedSet;
  }

  var UNOPENED = 0;
  var OPENED = 1;
  var CLOSED = 2;
  var RESETTING = 3;

  var nextObserverId = 1;

  function Observer() {
    this.state_ = UNOPENED;
    this.callback_ = undefined;
    this.target_ = undefined; // TODO(rafaelw): Should be WeakRef
    this.directObserver_ = undefined;
    this.value_ = undefined;
    this.id_ = nextObserverId++;
  }

  Observer.prototype = {
    open: function(callback, target) {
      if (this.state_ != UNOPENED)
        throw Error('Observer has already been opened.');

      addToAll(this);
      this.callback_ = callback;
      this.target_ = target;
      this.connect_();
      this.state_ = OPENED;
      return this.value_;
    },

    close: function() {
      if (this.state_ != OPENED)
        return;

      removeFromAll(this);
      this.disconnect_();
      this.value_ = undefined;
      this.callback_ = undefined;
      this.target_ = undefined;
      this.state_ = CLOSED;
    },

    deliver: function() {
      if (this.state_ != OPENED)
        return;

      dirtyCheck(this);
    },

    report_: function(changes) {
      try {
        this.callback_.apply(this.target_, changes);
      } catch (ex) {
        Observer._errorThrownDuringCallback = true;
        console.error('Exception caught during observer callback: ' +
                       (ex.stack || ex));
      }
    },

    discardChanges: function() {
      this.check_(undefined, true);
      return this.value_;
    }
  };

  var collectObservers = !hasObserve;
  var allObservers;
  Observer._allObserversCount = 0;

  if (collectObservers) {
    allObservers = [];
  }

  function addToAll(observer) {
    Observer._allObserversCount++;
    if (!collectObservers)
      return;

    allObservers.push(observer);
  }

  function removeFromAll(observer) {
    Observer._allObserversCount--;
  }

  var runningMicrotaskCheckpoint = false;

  global.Platform = global.Platform || {};

  global.Platform.performMicrotaskCheckpoint = function() {
    if (runningMicrotaskCheckpoint)
      return;

    if (!collectObservers)
      return;

    runningMicrotaskCheckpoint = true;

    var cycles = 0;
    var anyChanged, toCheck;

    do {
      cycles++;
      toCheck = allObservers;
      allObservers = [];
      anyChanged = false;

      for (var i = 0; i < toCheck.length; i++) {
        var observer = toCheck[i];
        if (observer.state_ != OPENED)
          continue;

        if (observer.check_())
          anyChanged = true;

        allObservers.push(observer);
      }
      if (runEOMTasks())
        anyChanged = true;
    } while (cycles < MAX_DIRTY_CHECK_CYCLES && anyChanged);

    if (testingExposeCycleCount)
      global.dirtyCheckCycleCount = cycles;

    runningMicrotaskCheckpoint = false;
  };

  if (collectObservers) {
    global.Platform.clearObservers = function() {
      allObservers = [];
    };
  }

  function ObjectObserver(object) {
    Observer.call(this);
    this.value_ = object;
    this.oldObject_ = undefined;
  }

  ObjectObserver.prototype = createObject({
    __proto__: Observer.prototype,

    arrayObserve: false,

    connect_: function(callback, target) {
      if (hasObserve) {
        this.directObserver_ = getObservedObject(this, this.value_,
                                                 this.arrayObserve);
      } else {
        this.oldObject_ = this.copyObject(this.value_);
      }

    },

    copyObject: function(object) {
      var copy = Array.isArray(object) ? [] : {};
      for (var prop in object) {
        copy[prop] = object[prop];
      }
      if (Array.isArray(object))
        copy.length = object.length;
      return copy;
    },

    check_: function(changeRecords, skipChanges) {
      var diff;
      var oldValues;
      if (hasObserve) {
        if (!changeRecords)
          return false;

        oldValues = {};
        diff = diffObjectFromChangeRecords(this.value_, changeRecords,
                                           oldValues);
      } else {
        oldValues = this.oldObject_;
        diff = diffObjectFromOldObject(this.value_, this.oldObject_);
      }

      if (diffIsEmpty(diff))
        return false;

      if (!hasObserve)
        this.oldObject_ = this.copyObject(this.value_);

      this.report_([
        diff.added || {},
        diff.removed || {},
        diff.changed || {},
        function(property) {
          return oldValues[property];
        }
      ]);

      return true;
    },

    disconnect_: function() {
      if (hasObserve) {
        this.directObserver_.close();
        this.directObserver_ = undefined;
      } else {
        this.oldObject_ = undefined;
      }
    },

    deliver: function() {
      if (this.state_ != OPENED)
        return;

      if (hasObserve)
        this.directObserver_.deliver(false);
      else
        dirtyCheck(this);
    },

    discardChanges: function() {
      if (this.directObserver_)
        this.directObserver_.deliver(true);
      else
        this.oldObject_ = this.copyObject(this.value_);

      return this.value_;
    }
  });

  function ArrayObserver(array) {
    if (!Array.isArray(array))
      throw Error('Provided object is not an Array');
    ObjectObserver.call(this, array);
  }

  ArrayObserver.prototype = createObject({

    __proto__: ObjectObserver.prototype,

    arrayObserve: true,

    copyObject: function(arr) {
      return arr.slice();
    },

    check_: function(changeRecords) {
      var splices;
      if (hasObserve) {
        if (!changeRecords)
          return false;
        splices = projectArraySplices(this.value_, changeRecords);
      } else {
        splices = calcSplices(this.value_, 0, this.value_.length,
                              this.oldObject_, 0, this.oldObject_.length);
      }

      if (!splices || !splices.length)
        return false;

      if (!hasObserve)
        this.oldObject_ = this.copyObject(this.value_);

      this.report_([splices]);
      return true;
    }
  });

  ArrayObserver.applySplices = function(previous, current, splices) {
    splices.forEach(function(splice) {
      var spliceArgs = [splice.index, splice.removed.length];
      var addIndex = splice.index;
      while (addIndex < splice.index + splice.addedCount) {
        spliceArgs.push(current[addIndex]);
        addIndex++;
      }

      Array.prototype.splice.apply(previous, spliceArgs);
    });
  };

  function PathObserver(object, path, defaultValue) {
    Observer.call(this);

    this.object_ = object;
    this.path_ = getPath(path);
    this.defaultValue_ = defaultValue;
    this.directObserver_ = undefined;
  }

  PathObserver.prototype = createObject({
    __proto__: Observer.prototype,

    get path() {
      return this.path_;
    },

    connect_: function() {
      if (hasObserve)
        this.directObserver_ = getObservedSet(this, this.object_);

      this.check_(undefined, true);
    },

    disconnect_: function() {
      this.value_ = undefined;

      if (this.directObserver_) {
        this.directObserver_.close(this);
        this.directObserver_ = undefined;
      }
    },

    iterateObjects_: function(observe) {
      this.path_.iterateObjects(this.object_, observe);
    },

    check_: function(changeRecords, skipChanges) {
      var oldValue = this.value_;
      this.value_ = this.path_.getValueFrom(this.object_, this.defaultValue_);
      if (skipChanges || areSameValue(this.value_, oldValue))
        return false;

      this.report_([this.value_, oldValue, this]);
      return true;
    },

    setValue: function(newValue) {
      if (this.path_)
        this.path_.setValueFrom(this.object_, newValue);
    }
  });

  function CompoundObserver(reportChangesOnOpen) {
    Observer.call(this);

    this.reportChangesOnOpen_ = reportChangesOnOpen;
    this.value_ = [];
    this.directObserver_ = undefined;
    this.observed_ = [];
  }

  var observerSentinel = {};

  CompoundObserver.prototype = createObject({
    __proto__: Observer.prototype,

    connect_: function() {
      if (hasObserve) {
        var object;
        var needsDirectObserver = false;
        for (var i = 0; i < this.observed_.length; i += 2) {
          object = this.observed_[i];
          if (object !== observerSentinel) {
            needsDirectObserver = true;
            break;
          }
        }

        if (needsDirectObserver)
          this.directObserver_ = getObservedSet(this, object);
      }

      this.check_(undefined, !this.reportChangesOnOpen_);
    },

    disconnect_: function() {
      for (var i = 0; i < this.observed_.length; i += 2) {
        if (this.observed_[i] === observerSentinel)
          this.observed_[i + 1].close();
      }
      this.observed_.length = 0;
      this.value_.length = 0;

      if (this.directObserver_) {
        this.directObserver_.close(this);
        this.directObserver_ = undefined;
      }
    },

    addPath: function(object, path) {
      if (this.state_ != UNOPENED && this.state_ != RESETTING)
        throw Error('Cannot add paths once started.');

      path = getPath(path);
      this.observed_.push(object, path);
      if (!this.reportChangesOnOpen_)
        return;
      var index = this.observed_.length / 2 - 1;
      this.value_[index] = path.getValueFrom(object);
    },

    addObserver: function(observer) {
      if (this.state_ != UNOPENED && this.state_ != RESETTING)
        throw Error('Cannot add observers once started.');

      this.observed_.push(observerSentinel, observer);
      if (!this.reportChangesOnOpen_)
        return;
      var index = this.observed_.length / 2 - 1;
      this.value_[index] = observer.open(this.deliver, this);
    },

    startReset: function() {
      if (this.state_ != OPENED)
        throw Error('Can only reset while open');

      this.state_ = RESETTING;
      this.disconnect_();
    },

    finishReset: function() {
      if (this.state_ != RESETTING)
        throw Error('Can only finishReset after startReset');
      this.state_ = OPENED;
      this.connect_();

      return this.value_;
    },

    iterateObjects_: function(observe) {
      var object;
      for (var i = 0; i < this.observed_.length; i += 2) {
        object = this.observed_[i];
        if (object !== observerSentinel)
          this.observed_[i + 1].iterateObjects(object, observe);
      }
    },

    check_: function(changeRecords, skipChanges) {
      var oldValues;
      for (var i = 0; i < this.observed_.length; i += 2) {
        var object = this.observed_[i];
        var path = this.observed_[i+1];
        var value;
        if (object === observerSentinel) {
          var observable = path;
          value = this.state_ === UNOPENED ?
              observable.open(this.deliver, this) :
              observable.discardChanges();
        } else {
          value = path.getValueFrom(object);
        }

        if (skipChanges) {
          this.value_[i / 2] = value;
          continue;
        }

        if (areSameValue(value, this.value_[i / 2]))
          continue;

        oldValues = oldValues || [];
        oldValues[i / 2] = this.value_[i / 2];
        this.value_[i / 2] = value;
      }

      if (!oldValues)
        return false;

      // TODO(rafaelw): Having observed_ as the third callback arg here is
      // pretty lame API. Fix.
      this.report_([this.value_, oldValues, this.observed_]);
      return true;
    }
  });

  function identFn(value) { return value; }

  function ObserverTransform(observable, getValueFn, setValueFn,
                             dontPassThroughSet) {
    this.callback_ = undefined;
    this.target_ = undefined;
    this.value_ = undefined;
    this.observable_ = observable;
    this.getValueFn_ = getValueFn || identFn;
    this.setValueFn_ = setValueFn || identFn;
    // TODO(rafaelw): This is a temporary hack. PolymerExpressions needs this
    // at the moment because of a bug in it's dependency tracking.
    this.dontPassThroughSet_ = dontPassThroughSet;
  }

  ObserverTransform.prototype = {
    open: function(callback, target) {
      this.callback_ = callback;
      this.target_ = target;
      this.value_ =
          this.getValueFn_(this.observable_.open(this.observedCallback_, this));
      return this.value_;
    },

    observedCallback_: function(value) {
      value = this.getValueFn_(value);
      if (areSameValue(value, this.value_))
        return;
      var oldValue = this.value_;
      this.value_ = value;
      this.callback_.call(this.target_, this.value_, oldValue);
    },

    discardChanges: function() {
      this.value_ = this.getValueFn_(this.observable_.discardChanges());
      return this.value_;
    },

    deliver: function() {
      return this.observable_.deliver();
    },

    setValue: function(value) {
      value = this.setValueFn_(value);
      if (!this.dontPassThroughSet_ && this.observable_.setValue)
        return this.observable_.setValue(value);
    },

    close: function() {
      if (this.observable_)
        this.observable_.close();
      this.callback_ = undefined;
      this.target_ = undefined;
      this.observable_ = undefined;
      this.value_ = undefined;
      this.getValueFn_ = undefined;
      this.setValueFn_ = undefined;
    }
  };

  var expectedRecordTypes = {
    add: true,
    update: true,
    delete: true
  };

  function diffObjectFromChangeRecords(object, changeRecords, oldValues) {
    var added = {};
    var removed = {};

    for (var i = 0; i < changeRecords.length; i++) {
      var record = changeRecords[i];
      if (!expectedRecordTypes[record.type]) {
        console.error('Unknown changeRecord type: ' + record.type);
        console.error(record);
        continue;
      }

      if (!(record.name in oldValues))
        oldValues[record.name] = record.oldValue;

      if (record.type == 'update')
        continue;

      if (record.type == 'add') {
        if (record.name in removed)
          delete removed[record.name];
        else
          added[record.name] = true;

        continue;
      }

      // type = 'delete'
      if (record.name in added) {
        delete added[record.name];
        delete oldValues[record.name];
      } else {
        removed[record.name] = true;
      }
    }

    var prop;
    for (prop in added)
      added[prop] = object[prop];

    for (prop in removed)
      removed[prop] = undefined;

    var changed = {};
    for (prop in oldValues) {
      if (prop in added || prop in removed)
        continue;

      var newValue = object[prop];
      if (oldValues[prop] !== newValue)
        changed[prop] = newValue;
    }

    return {
      added: added,
      removed: removed,
      changed: changed
    };
  }

  function newSplice(index, removed, addedCount) {
    return {
      index: index,
      removed: removed,
      addedCount: addedCount
    };
  }

  var EDIT_LEAVE = 0;
  var EDIT_UPDATE = 1;
  var EDIT_ADD = 2;
  var EDIT_DELETE = 3;

  function ArraySplice() {}

  ArraySplice.prototype = {

    // Note: This function is *based* on the computation of the Levenshtein
    // "edit" distance. The one change is that "updates" are treated as two
    // edits - not one. With Array splices, an update is really a delete
    // followed by an add. By retaining this, we optimize for "keeping" the
    // maximum array items in the original array. For example:
    //
    //   'xxxx123' -> '123yyyy'
    //
    // With 1-edit updates, the shortest path would be just to update all seven
    // characters. With 2-edit updates, we delete 4, leave 3, and add 4. This
    // leaves the substring '123' intact.
    calcEditDistances: function(current, currentStart, currentEnd,
                                old, oldStart, oldEnd) {
      // "Deletion" columns
      var rowCount = oldEnd - oldStart + 1;
      var columnCount = currentEnd - currentStart + 1;
      var distances = new Array(rowCount);

      var i, j;

      // "Addition" rows. Initialize null column.
      for (i = 0; i < rowCount; i++) {
        distances[i] = new Array(columnCount);
        distances[i][0] = i;
      }

      // Initialize null row
      for (j = 0; j < columnCount; j++)
        distances[0][j] = j;

      for (i = 1; i < rowCount; i++) {
        for (j = 1; j < columnCount; j++) {
          if (this.equals(current[currentStart + j - 1], old[oldStart + i - 1]))
            distances[i][j] = distances[i - 1][j - 1];
          else {
            var north = distances[i - 1][j] + 1;
            var west = distances[i][j - 1] + 1;
            distances[i][j] = north < west ? north : west;
          }
        }
      }

      return distances;
    },

    // This starts at the final weight, and walks "backward" by finding
    // the minimum previous weight recursively until the origin of the weight
    // matrix.
    spliceOperationsFromEditDistances: function(distances) {
      var i = distances.length - 1;
      var j = distances[0].length - 1;
      var current = distances[i][j];
      var edits = [];
      while (i > 0 || j > 0) {
        if (i == 0) {
          edits.push(EDIT_ADD);
          j--;
          continue;
        }
        if (j == 0) {
          edits.push(EDIT_DELETE);
          i--;
          continue;
        }
        var northWest = distances[i - 1][j - 1];
        var west = distances[i - 1][j];
        var north = distances[i][j - 1];

        var min;
        if (west < north)
          min = west < northWest ? west : northWest;
        else
          min = north < northWest ? north : northWest;

        if (min == northWest) {
          if (northWest == current) {
            edits.push(EDIT_LEAVE);
          } else {
            edits.push(EDIT_UPDATE);
            current = northWest;
          }
          i--;
          j--;
        } else if (min == west) {
          edits.push(EDIT_DELETE);
          i--;
          current = west;
        } else {
          edits.push(EDIT_ADD);
          j--;
          current = north;
        }
      }

      edits.reverse();
      return edits;
    },

    /**
     * Splice Projection functions:
     *
     * A splice map is a representation of how a previous array of items
     * was transformed into a new array of items. Conceptually it is a list of
     * tuples of
     *
     *   <index, removed, addedCount>
     *
     * which are kept in ascending index order of. The tuple represents that at
     * the |index|, |removed| sequence of items were removed, and counting forward
     * from |index|, |addedCount| items were added.
     */

    /**
     * Lacking individual splice mutation information, the minimal set of
     * splices can be synthesized given the previous state and final state of an
     * array. The basic approach is to calculate the edit distance matrix and
     * choose the shortest path through it.
     *
     * Complexity: O(l * p)
     *   l: The length of the current array
     *   p: The length of the old array
     */
    calcSplices: function(current, currentStart, currentEnd,
                          old, oldStart, oldEnd) {
      var prefixCount = 0;
      var suffixCount = 0;

      var minLength = Math.min(currentEnd - currentStart, oldEnd - oldStart);
      if (currentStart == 0 && oldStart == 0)
        prefixCount = this.sharedPrefix(current, old, minLength);

      if (currentEnd == current.length && oldEnd == old.length)
        suffixCount = this.sharedSuffix(current, old, minLength - prefixCount);

      currentStart += prefixCount;
      oldStart += prefixCount;
      currentEnd -= suffixCount;
      oldEnd -= suffixCount;

      if (currentEnd - currentStart == 0 && oldEnd - oldStart == 0)
        return [];

      var splice;
      if (currentStart == currentEnd) {
        splice = newSplice(currentStart, [], 0);
        while (oldStart < oldEnd)
          splice.removed.push(old[oldStart++]);

        return [ splice ];
      } else if (oldStart == oldEnd)
        return [ newSplice(currentStart, [], currentEnd - currentStart) ];

      var ops = this.spliceOperationsFromEditDistances(
          this.calcEditDistances(current, currentStart, currentEnd,
                                 old, oldStart, oldEnd));

      var splices = [];
      var index = currentStart;
      var oldIndex = oldStart;
      for (var i = 0; i < ops.length; i++) {
        switch(ops[i]) {
          case EDIT_LEAVE:
            if (splice) {
              splices.push(splice);
              splice = undefined;
            }

            index++;
            oldIndex++;
            break;
          case EDIT_UPDATE:
            if (!splice)
              splice = newSplice(index, [], 0);

            splice.addedCount++;
            index++;

            splice.removed.push(old[oldIndex]);
            oldIndex++;
            break;
          case EDIT_ADD:
            if (!splice)
              splice = newSplice(index, [], 0);

            splice.addedCount++;
            index++;
            break;
          case EDIT_DELETE:
            if (!splice)
              splice = newSplice(index, [], 0);

            splice.removed.push(old[oldIndex]);
            oldIndex++;
            break;
        }
      }

      if (splice) {
        splices.push(splice);
      }
      return splices;
    },

    sharedPrefix: function(current, old, searchLength) {
      for (var i = 0; i < searchLength; i++)
        if (!this.equals(current[i], old[i]))
          return i;
      return searchLength;
    },

    sharedSuffix: function(current, old, searchLength) {
      var index1 = current.length;
      var index2 = old.length;
      var count = 0;
      while (count < searchLength && this.equals(current[--index1], old[--index2]))
        count++;

      return count;
    },

    calculateSplices: function(current, previous) {
      return this.calcSplices(current, 0, current.length, previous, 0,
                              previous.length);
    },

    equals: function(currentValue, previousValue) {
      return currentValue === previousValue;
    }
  };

  var arraySplice = new ArraySplice();

  function calcSplices(current, currentStart, currentEnd,
                       old, oldStart, oldEnd) {
    return arraySplice.calcSplices(current, currentStart, currentEnd,
                                   old, oldStart, oldEnd);
  }

  function intersect(start1, end1, start2, end2) {
    // Disjoint
    if (end1 < start2 || end2 < start1)
      return -1;

    // Adjacent
    if (end1 == start2 || end2 == start1)
      return 0;

    // Non-zero intersect, span1 first
    if (start1 < start2) {
      if (end1 < end2)
        return end1 - start2; // Overlap
      else
        return end2 - start2; // Contained
    } else {
      // Non-zero intersect, span2 first
      if (end2 < end1)
        return end2 - start1; // Overlap
      else
        return end1 - start1; // Contained
    }
  }

  function mergeSplice(splices, index, removed, addedCount) {

    var splice = newSplice(index, removed, addedCount);

    var inserted = false;
    var insertionOffset = 0;

    for (var i = 0; i < splices.length; i++) {
      var current = splices[i];
      current.index += insertionOffset;

      if (inserted)
        continue;

      var intersectCount = intersect(splice.index,
                                     splice.index + splice.removed.length,
                                     current.index,
                                     current.index + current.addedCount);

      if (intersectCount >= 0) {
        // Merge the two splices

        splices.splice(i, 1);
        i--;

        insertionOffset -= current.addedCount - current.removed.length;

        splice.addedCount += current.addedCount - intersectCount;
        var deleteCount = splice.removed.length +
                          current.removed.length - intersectCount;

        if (!splice.addedCount && !deleteCount) {
          // merged splice is a noop. discard.
          inserted = true;
        } else {
          removed = current.removed;

          if (splice.index < current.index) {
            // some prefix of splice.removed is prepended to current.removed.
            var prepend = splice.removed.slice(0, current.index - splice.index);
            Array.prototype.push.apply(prepend, removed);
            removed = prepend;
          }

          if (splice.index + splice.removed.length > current.index + current.addedCount) {
            // some suffix of splice.removed is appended to current.removed.
            var append = splice.removed.slice(current.index + current.addedCount - splice.index);
            Array.prototype.push.apply(removed, append);
          }

          splice.removed = removed;
          if (current.index < splice.index) {
            splice.index = current.index;
          }
        }
      } else if (splice.index < current.index) {
        // Insert splice here.

        inserted = true;

        splices.splice(i, 0, splice);
        i++;

        var offset = splice.addedCount - splice.removed.length;
        current.index += offset;
        insertionOffset += offset;
      }
    }

    if (!inserted)
      splices.push(splice);
  }

  function createInitialSplices(array, changeRecords) {
    var splices = [];

    for (var i = 0; i < changeRecords.length; i++) {
      var record = changeRecords[i];
      switch(record.type) {
        case 'splice':
          mergeSplice(splices, record.index, record.removed.slice(), record.addedCount);
          break;
        case 'add':
        case 'update':
        case 'delete':
          if (!isIndex(record.name))
            continue;
          var index = toNumber(record.name);
          if (index < 0)
            continue;
          mergeSplice(splices, index, [record.oldValue], 1);
          break;
        default:
          console.error('Unexpected record type: ' + JSON.stringify(record));
          break;
      }
    }

    return splices;
  }

  function projectArraySplices(array, changeRecords) {
    var splices = [];

    createInitialSplices(array, changeRecords).forEach(function(splice) {
      if (splice.addedCount == 1 && splice.removed.length == 1) {
        if (splice.removed[0] !== array[splice.index])
          splices.push(splice);

        return;
      }

      splices = splices.concat(calcSplices(array, splice.index, splice.index + splice.addedCount,
                                           splice.removed, 0, splice.removed.length));
    });

    return splices;
  }

  // Export the observe-js object for **Node.js**, with backwards-compatibility
  // for the old `require()` API. Also ensure `exports` is not a DOM Element.
  // If we're in the browser, export as a global object.

  var expose = global;

  if (typeof exports !== 'undefined' && !exports.nodeType) {
    if (typeof module !== 'undefined' && module.exports) {
      exports = module.exports;
    }
    expose = exports;
  }

  expose.Observer = Observer;
  expose.Observer.runEOM_ = runEOM;
  expose.Observer.observerSentinel_ = observerSentinel; // for testing.
  expose.Observer.hasObjectObserve = hasObserve;
  expose.ArrayObserver = ArrayObserver;
  expose.ArrayObserver.calculateSplices = function(current, previous) {
    return arraySplice.calculateSplices(current, previous);
  };

  expose.ArraySplice = ArraySplice;
  expose.ObjectObserver = ObjectObserver;
  expose.PathObserver = PathObserver;
  expose.CompoundObserver = CompoundObserver;
  expose.Path = Path;
  expose.ObserverTransform = ObserverTransform;

})(typeof global !== 'undefined' && global && typeof module !== 'undefined' && module ? global : this || window);