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    <h3>Editing: array-set.js</h3>
    <textarea name="content" rows="20" cols="80">/* -*- Mode: js; js-indent-level: 2; -*- */
/*
 * Copyright 2011 Mozilla Foundation and contributors
 * Licensed under the New BSD license. See LICENSE or:
 * http://opensource.org/licenses/BSD-3-Clause
 */

var util = require('./util');
var has = Object.prototype.hasOwnProperty;
var hasNativeMap = typeof Map !== &quot;undefined&quot;;

/**
 * A data structure which is a combination of an array and a set. Adding a new
 * member is O(1), testing for membership is O(1), and finding the index of an
 * element is O(1). Removing elements from the set is not supported. Only
 * strings are supported for membership.
 */
function ArraySet() {
  this._array = [];
  this._set = hasNativeMap ? new Map() : Object.create(null);
}

/**
 * Static method for creating ArraySet instances from an existing array.
 */
ArraySet.fromArray = function ArraySet_fromArray(aArray, aAllowDuplicates) {
  var set = new ArraySet();
  for (var i = 0, len = aArray.length; i &lt; len; i++) {
    set.add(aArray[i], aAllowDuplicates);
  }
  return set;
};

/**
 * Return how many unique items are in this ArraySet. If duplicates have been
 * added, than those do not count towards the size.
 *
 * @returns Number
 */
ArraySet.prototype.size = function ArraySet_size() {
  return hasNativeMap ? this._set.size : Object.getOwnPropertyNames(this._set).length;
};

/**
 * Add the given string to this set.
 *
 * @param String aStr
 */
ArraySet.prototype.add = function ArraySet_add(aStr, aAllowDuplicates) {
  var sStr = hasNativeMap ? aStr : util.toSetString(aStr);
  var isDuplicate = hasNativeMap ? this.has(aStr) : has.call(this._set, sStr);
  var idx = this._array.length;
  if (!isDuplicate || aAllowDuplicates) {
    this._array.push(aStr);
  }
  if (!isDuplicate) {
    if (hasNativeMap) {
      this._set.set(aStr, idx);
    } else {
      this._set[sStr] = idx;
    }
  }
};

/**
 * Is the given string a member of this set?
 *
 * @param String aStr
 */
ArraySet.prototype.has = function ArraySet_has(aStr) {
  if (hasNativeMap) {
    return this._set.has(aStr);
  } else {
    var sStr = util.toSetString(aStr);
    return has.call(this._set, sStr);
  }
};

/**
 * What is the index of the given string in the array?
 *
 * @param String aStr
 */
ArraySet.prototype.indexOf = function ArraySet_indexOf(aStr) {
  if (hasNativeMap) {
    var idx = this._set.get(aStr);
    if (idx &gt;= 0) {
        return idx;
    }
  } else {
    var sStr = util.toSetString(aStr);
    if (has.call(this._set, sStr)) {
      return this._set[sStr];
    }
  }

  throw new Error('&quot;' + aStr + '&quot; is not in the set.');
};

/**
 * What is the element at the given index?
 *
 * @param Number aIdx
 */
ArraySet.prototype.at = function ArraySet_at(aIdx) {
  if (aIdx &gt;= 0 &amp;&amp; aIdx &lt; this._array.length) {
    return this._array[aIdx];
  }
  throw new Error('No element indexed by ' + aIdx);
};

/**
 * Returns the array representation of this set (which has the proper indices
 * indicated by indexOf). Note that this is a copy of the internal array used
 * for storing the members so that no one can mess with internal state.
 */
ArraySet.prototype.toArray = function ArraySet_toArray() {
  return this._array.slice();
};

exports.ArraySet = ArraySet;
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