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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2016 The Closure Compiler Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

'require es6/conformance';
'require es6/map';
'require es6/symbol';
'require util/defines';
'require util/polyfill';

$jscomp.polyfill('Set',
    /**
     * @param {*} NativeSet
     * @return {*}
     * @suppress {reportUnknownTypes}
     */
    function(NativeSet) {

  /**
   * Checks conformance of the existing Set.
   * @return {boolean} True if the browser's implementation conforms.
   * @suppress {missingProperties} "entries" unknown prototype
   */
  function isConformant() {
    if ($jscomp.ASSUME_NO_NATIVE_SET ||
        !NativeSet ||
        typeof NativeSet != "function" ||
        !NativeSet.prototype.entries ||
        typeof Object.seal != 'function') {
      return false;
    }
    // Some implementations don't support constructor arguments.
    try {
      NativeSet = /** @type {function(new: Set, !Iterator=)} */ (NativeSet);
      var value = Object.seal({x: 4});
      var set = new NativeSet($jscomp.makeIterator([value]));
      if (!set.has(value) || set.size != 1 || set.add(value) != set ||
          set.size != 1 || set.add({x: 4}) != set || set.size != 2) {
        return false;
      }
      var iter = set.entries();
      var item = iter.next();
      if (item.done || item.value[0] != value || item.value[1] != value) {
        return false;
      }
      item = iter.next();
      if (item.done || item.value[0] == value || item.value[0].x != 4 ||
          item.value[1] != item.value[0]) {
        return false;
      }
      return iter.next().done;
    } catch (err) { // This should hopefully never happen, but let's be safe.
      return false;
    }
  }

  if ($jscomp.USE_PROXY_FOR_ES6_CONFORMANCE_CHECKS) {
    if (NativeSet && $jscomp.ES6_CONFORMANCE) return NativeSet;
  } else {
    if (isConformant()) return NativeSet;
  }

  /**
   * Polyfill for the global Map data type.
   * @constructor
   * @struct
   * @extends {Set}
   * @implements {Iterable}
   * @template KEY, VALUE
   * @param {!Iterable|!Array|null=} opt_iterable
   *     Optional data to populate the set.
   */
  // TODO(sdh): fix param type if heterogeneous arrays ever supported.
  var PolyfillSet = function(opt_iterable) {
    /** @private @const {!Map} */
    this.map_ = new Map();
    if (opt_iterable) {
      var iter = $jscomp.makeIterator(opt_iterable);
      var entry;
      while (!(entry = iter.next()).done) {
        var item = /** @type {!IIterableResult} */ (entry).value;
        this.add(item);
      }
    }
    // Note: this property should not be changed.  If we're willing to give up
    // ES3 support, we could define it as a property directly.  It should be
    // marked readonly if such an annotation ever comes into existence.
    this.size = this.map_.size;
  };


  /** @override */
  PolyfillSet.prototype.add = function(value) {
    // normalize -0/+0 to +0
    value = value === 0 ? 0 : value;
    this.map_.set(value, value);
    this.size = this.map_.size;
    return this;
  };


  /** @override */
  PolyfillSet.prototype.delete = function(value) {
    var result = this.map_.delete(value);
    this.size = this.map_.size;
    return result;
  };


  /** @override */
  PolyfillSet.prototype.clear = function() {
    this.map_.clear();
    this.size = 0;
  };


  /** @override */
  PolyfillSet.prototype.has = function(value) {
    return this.map_.has(value);
  };


  /** @override */
  PolyfillSet.prototype.entries = function() {
    return this.map_.entries();
  };


  /** @override */
  PolyfillSet.prototype.values = function() {
    return this.map_.values();
  };


  /** @override */
  PolyfillSet.prototype.keys = PolyfillSet.prototype.values;


  /** @type {?} */ (PolyfillSet.prototype)[Symbol.iterator] =
      PolyfillSet.prototype.values;


  /** @override */
  PolyfillSet.prototype.forEach = function(callback, opt_thisArg) {
    var set = this;
    this.map_.forEach(function(value) {
      return callback.call(/** @type {?} */ (opt_thisArg), value, value, set);
    });
  };


  return PolyfillSet;
}, 'es6', 'es3');