text: // Copyright 2011 The Closure Library Authors. All Rights Reserved.
  //
  // 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.
  
  /**
   * @fileoverview Provides a convenient API for data persistence with expiration.
   *
   */
  
  goog.provide('goog.storage.ExpiringStorage');
  
  goog.require('goog.storage.RichStorage');
  
  
  
  /**
   * Provides a storage with expirning keys.
   *
   * @param {!goog.storage.mechanism.Mechanism} mechanism The underlying
   *     storage mechanism.
   * @constructor
   * @struct
   * @extends {goog.storage.RichStorage}
   */
  goog.storage.ExpiringStorage = function(mechanism) {
    goog.storage.ExpiringStorage.base(this, 'constructor', mechanism);
  };
  goog.inherits(goog.storage.ExpiringStorage, goog.storage.RichStorage);
  
  
  /**
   * Metadata key under which the expiration time is stored.
   *
   * @type {string}
   * @protected
   */
  goog.storage.ExpiringStorage.EXPIRATION_TIME_KEY = 'expiration';
  
  
  /**
   * Metadata key under which the creation time is stored.
   *
   * @type {string}
   * @protected
   */
  goog.storage.ExpiringStorage.CREATION_TIME_KEY = 'creation';
  
  
  /**
   * Returns the wrapper creation time.
   *
   * @param {!Object} wrapper The wrapper.
   * @return {number|undefined} Wrapper creation time.
   */
  goog.storage.ExpiringStorage.getCreationTime = function(wrapper) {
    return wrapper[goog.storage.ExpiringStorage.CREATION_TIME_KEY];
  };
  
  
  /**
   * Returns the wrapper expiration time.
   *
   * @param {!Object} wrapper The wrapper.
   * @return {number|undefined} Wrapper expiration time.
   */
  goog.storage.ExpiringStorage.getExpirationTime = function(wrapper) {
    return wrapper[goog.storage.ExpiringStorage.EXPIRATION_TIME_KEY];
  };
  
  
  /**
   * Checks if the data item has expired.
   *
   * @param {!Object} wrapper The wrapper.
   * @return {boolean} True if the item has expired.
   */
  goog.storage.ExpiringStorage.isExpired = function(wrapper) {
    var creation = goog.storage.ExpiringStorage.getCreationTime(wrapper);
    var expiration = goog.storage.ExpiringStorage.getExpirationTime(wrapper);
    return !!expiration && expiration < goog.now() ||
           !!creation && creation > goog.now();
  };
  
  
  /**
   * Set an item in the storage.
   *
   * @param {string} key The key to set.
   * @param {*} value The value to serialize to a string and save.
   * @param {number=} opt_expiration The number of miliseconds since epoch
   *     (as in goog.now()) when the value is to expire. If the expiration
   *     time is not provided, the value will persist as long as possible.
   * @override
   */
  goog.storage.ExpiringStorage.prototype.set = function(
      key, value, opt_expiration) {
    var wrapper = goog.storage.RichStorage.Wrapper.wrapIfNecessary(value);
    if (wrapper) {
      if (opt_expiration) {
        if (opt_expiration < goog.now()) {
          goog.storage.ExpiringStorage.prototype.remove.call(this, key);
          return;
        }
        wrapper[goog.storage.ExpiringStorage.EXPIRATION_TIME_KEY] =
            opt_expiration;
      }
      wrapper[goog.storage.ExpiringStorage.CREATION_TIME_KEY] = goog.now();
    }
    goog.storage.ExpiringStorage.base(this, 'set', key, wrapper);
  };
  
  
  /**
   * Get an item wrapper (the item and its metadata) from the storage.
   *
   * @param {string} key The key to get.
   * @param {boolean=} opt_expired If true, return expired wrappers as well.
   * @return {(!Object|undefined)} The wrapper, or undefined if not found.
   * @override
   */
  goog.storage.ExpiringStorage.prototype.getWrapper = function(key, opt_expired) {
    var wrapper = goog.storage.ExpiringStorage.base(this, 'getWrapper', key);
    if (!wrapper) {
      return undefined;
    }
    if (!opt_expired && goog.storage.ExpiringStorage.isExpired(wrapper)) {
      goog.storage.ExpiringStorage.prototype.remove.call(this, key);
      return undefined;
    }
    return wrapper;
  };
  