function EventEmitter() {
  this._listeners = {};
}


EventEmitter.prototype = {
  on: function(event, callback, context, once) {
    var index = this._indexOf(event, callback, context);
    if (index != -1) {
      if (arguments.length > 3) {
        this._listeners[event][index].once = once;
      }
      return;
    }
    if (!{}.hasOwnProperty.call(this._listeners, event)) {
      this._listeners[event] = [];
    }
    this._listeners[event].push(
      {callback: callback, context: context, once: once});
  },

  once: function(event, callback, context) {
    this.on(event, callback, context, true);
  },

  /*
   * off()                            remove all listeners
   * off('foo')                       remove all listeners for event 'foo'
   * off('foo', function[, context])  remove listener for event 'foo' in a
   *                                  certain context
   */
  off: function(event, callback, context) {
    if (!arguments.length) {
      this._listeners = {};
      return;
    }
    if (arguments.length == 1) {
      if ({}.hasOwnProperty.call(this._listeners, event)) {
        delete this._listeners[event];
      }
      return;
    }
    var index = this._indexOf(event, callback, context);
    if (index == -1) {
      return;
    }
    this._listeners[event].splice(index, 1);
    if (!this._listeners[event].length) {
      delete this._listeners[event];
    }
  },

  emit: function(event) {
    if (!arguments.length ||
          !{}.hasOwnProperty.call(this._listeners, event)) {
      return;
    }
    var args = [].slice.call(arguments, 1);
    var listeners = this._listeners[event];
    var i = 0;
    while (i < listeners.length) {
      listeners[i].callback.apply(listeners[i].context || this, args);
      if (listeners[i].once) {
        listeners.splice(i, 1);
        continue;
      }
      ++i;
    }
    if (!listeners.length) {
      delete this._listeners[event];
    }
  },

  _indexOf: function(event, callback, context) {
    if ({}.hasOwnProperty.call(this._listeners, event)) {
      var listeners = this._listeners[event];
      var i = listeners.length;
      while (i--) {
        if (callback == listeners[i].callback &&
              ((!context && !listeners[i].context)
                || listeners[i].context === context)) {
          return i;
        }
      }
    }
    return -1;
  },
};