define(function(require) {

  // Based on https://github.com/diy/intercom.js/blob/master/lib/intercom.js
  // Copyright 2012 DIY Co Apache License, Version 2.0
  // http://www.apache.org/licenses/LICENSE-2.0

  var EventEmitter = require('eventemitter');
  var guid = require('src/shared').guid;

  function throttle(delay, fn) {
    var last = 0;
    return function() {
      var now = Date.now();
      if (now - last > delay) {
        last = now;
        fn.apply(this, arguments);
      }
    };
  }

  function extend(a, b) {
    if (typeof a === 'undefined' || !a) { a = {}; }
    if (typeof b === 'object') {
      for (var key in b) {
        if (b.hasOwnProperty(key)) {
          a[key] = b[key];
        }
      }
    }
    return a;
  }

  var localStorage = (function(window) {
    if (typeof window.localStorage === 'undefined') {
      return {
        getItem : function() {},
        setItem : function() {},
        removeItem : function() {}
      };
    }
    return window.localStorage;
  }(this));

  function Intercom() {
    var self = this;
    var now = Date.now();

    this.origin         = guid();
    this.lastMessage    = now;
    this.receivedIDs    = {};
    this.previousValues = {};

    var storageHandler = function() {
      self._onStorageEvent.apply(self, arguments);
    };
    if (document.attachEvent) {
      document.attachEvent('onstorage', storageHandler);
    } else {
      window.addEventListener('storage', storageHandler, false);
    }
  }

  Intercom.prototype._transaction = function(fn) {
    var TIMEOUT   = 1000;
    var WAIT      = 20;
    var self      = this;
    var executed  = false;
    var listening = false;
    var waitTimer = null;

    function lock() {
      if (executed) {
        return;
      }

      var now = Date.now();
      var activeLock = localStorage.getItem(INDEX_LOCK)|0;
      if (activeLock && now - activeLock < TIMEOUT) {
        if (!listening) {
          self._on('storage', lock);
          listening = true;
        }
        waitTimer = window.setTimeout(lock, WAIT);
        return;
      }
      executed = true;
      localStorage.setItem(INDEX_LOCK, now);

      fn();
      unlock();
    }

    function unlock() {
      if (listening) {
        self._off('storage', lock);
      }
      if (waitTimer) {
        window.clearTimeout(waitTimer);
      }
      localStorage.removeItem(INDEX_LOCK);
    }

    lock();
  };

  Intercom.prototype._cleanup_emit = throttle(100, function() {
    var self = this;

    self._transaction(function() {
      var now = Date.now();
      var threshold = now - THRESHOLD_TTL_EMIT;
      var changed = 0;
      var messages;

      try {
        messages = JSON.parse(localStorage.getItem(INDEX_EMIT) || '[]');
      } catch(e) {
        messages = [];
      }
      for (var i = messages.length - 1; i >= 0; i--) {
        if (messages[i].timestamp < threshold) {
          messages.splice(i, 1);
          changed++;
        }
      }
      if (changed > 0) {
        localStorage.setItem(INDEX_EMIT, JSON.stringify(messages));
      }
    });
  });

  Intercom.prototype._cleanup_once = throttle(100, function() {
    var self = this;

    self._transaction(function() {
      var timestamp, ttl, key;
      var table;
      var now  = Date.now();
      var changed = 0;

      try {
        table = JSON.parse(localStorage.getItem(INDEX_ONCE) || '{}');
      } catch(e) {
        table = {};
      }
      for (key in table) {
        if (self._once_expired(key, table)) {
          delete table[key];
          changed++;
        }
      }

      if (changed > 0) {
        localStorage.setItem(INDEX_ONCE, JSON.stringify(table));
      }
    });
  });

  Intercom.prototype._once_expired = function(key, table) {
    if (!table) {
      return true;
    }
    if (!table.hasOwnProperty(key)) {
      return true;
    }
    if (typeof table[key] !== 'object') {
      return true;
    }

    var ttl = table[key].ttl || THRESHOLD_TTL_ONCE;
    var now = Date.now();
    var timestamp = table[key].timestamp;
    return timestamp < now - ttl;
  };

  Intercom.prototype._localStorageChanged = function(event, field) {
    if (event && event.key) {
      return event.key === field;
    }

    var currentValue = localStorage.getItem(field);
    if (currentValue === this.previousValues[field]) {
      return false;
    }
    this.previousValues[field] = currentValue;
    return true;
  };

  Intercom.prototype._onStorageEvent = function(event) {
    event = event || window.event;
    var self = this;

    if (this._localStorageChanged(event, INDEX_EMIT)) {
      this._transaction(function() {
        var now = Date.now();
        var data = localStorage.getItem(INDEX_EMIT);
        var messages;

        try {
          messages = JSON.parse(data || '[]');
        } catch(e) {
          messages = [];
        }
        for (var i = 0; i < messages.length; i++) {
          if (messages[i].origin === self.origin) continue;
          if (messages[i].timestamp < self.lastMessage) continue;
          if (messages[i].id) {
            if (self.receivedIDs.hasOwnProperty(messages[i].id)) continue;
            self.receivedIDs[messages[i].id] = true;
          }
          self.trigger(messages[i].name, messages[i].payload);
        }
        self.lastMessage = now;
      });
    }

    this._trigger('storage', event);
  };

  Intercom.prototype._emit = function(name, message, id) {
    id = (typeof id === 'string' || typeof id === 'number') ? String(id) : null;
    if (id && id.length) {
      if (this.receivedIDs.hasOwnProperty(id)) return;
      this.receivedIDs[id] = true;
    }

    var packet = {
      id        : id,
      name      : name,
      origin    : this.origin,
      timestamp : Date.now(),
      payload   : message
    };

    var self = this;
    this._transaction(function() {
      var data = localStorage.getItem(INDEX_EMIT) || '[]';
      var delimiter = (data === '[]') ? '' : ',';
      data = [data.substring(0, data.length - 1), delimiter, JSON.stringify(packet), ']'].join('');
      localStorage.setItem(INDEX_EMIT, data);
      self.trigger(name, message);

      window.setTimeout(function() {
        self._cleanup_emit();
      }, 50);
    });
  };

  Intercom.prototype.emit = function(name, message) {
    this._emit.apply(this, arguments);
    this._trigger('emit', name, message);
  };

  Intercom.prototype.once = function(key, fn, ttl) {
    if (!Intercom.supported) {
      return;
    }

    var self = this;
    this._transaction(function() {
      var data;
      try {
        data = JSON.parse(localStorage.getItem(INDEX_ONCE) || '{}');
      } catch(e) {
        data = {};
      }
      if (!self._once_expired(key, data)) {
        return;
      }

      data[key] = {};
      data[key].timestamp = Date.now();
      if (typeof ttl === 'number') {
        data[key].ttl = ttl * 1000;
      }

      localStorage.setItem(INDEX_ONCE, JSON.stringify(data));
      fn();

      window.setTimeout(function() {
        self._cleanup_once();
      }, 50);
    });
  };

  extend(Intercom.prototype, EventEmitter.prototype);

  Intercom.supported = (typeof localStorage !== 'undefined');

  var INDEX_EMIT = 'intercom';
  var INDEX_ONCE = 'intercom_once';
  var INDEX_LOCK = 'intercom_lock';

  var THRESHOLD_TTL_EMIT = 50000;
  var THRESHOLD_TTL_ONCE = 1000 * 3600;

  Intercom.destroy = function() {
    localStorage.removeItem(INDEX_LOCK);
    localStorage.removeItem(INDEX_EMIT);
    localStorage.removeItem(INDEX_ONCE);
  };

  Intercom.getInstance = (function() {
    var intercom;
    return function() {
      if (!intercom) {
        intercom = new Intercom();
      }
      return intercom;
    };
  })();

  return Intercom;
});