function KeyboardInputManager() {
  this.events = {};
 
  this.listen();
}
 
KeyboardInputManager.prototype.on = function (event, callback) {
  if (!this.events[event]) {
    this.events[event] = [];
  }
  this.events[event].push(callback);
};
 
KeyboardInputManager.prototype.emit = function (event, data) {
  var callbacks = this.events[event];
  if (callbacks) {
    callbacks.forEach(function (callback) {
      callback(data);
    });
  }
};
 
KeyboardInputManager.prototype.listen = function () {
  var self = this;
 
  var map = {
    38: 0, // Up
    39: 1, // Right
    40: 2, // Down
    37: 3, // Left
    75: 0, // vim keybindings
    76: 1,
    74: 2,
    72: 3
  };
 
  document.addEventListener("keydown", function (event) {
    var modifiers = event.altKey || event.ctrlKey || event.metaKey ||
                    event.shiftKey;
    var mapped    = map[event.which];
 
    if (!modifiers) {
      if (mapped !== undefined) {
        event.preventDefault();
        self.emit("move", mapped);
      }
 
      if (event.which === 32) self.restart.bind(self)(event);
    }
  });
 
  var retry = document.getElementsByClassName("retry-button")[0];
  retry.addEventListener("click", this.restart.bind(this));
 
  // Listen to swipe events
  var gestures = [Hammer.DIRECTION_UP, Hammer.DIRECTION_RIGHT,
                  Hammer.DIRECTION_DOWN, Hammer.DIRECTION_LEFT];
 
  var gameContainer = document.getElementsByClassName("game-container")[0];
  var handler       = Hammer(gameContainer, {
    drag_block_horizontal: true,
    drag_block_vertical: true
  });
  
  handler.on("swipe", function (event) {
    event.gesture.preventDefault();
    mapped = gestures.indexOf(event.gesture.direction);
 
    if (mapped !== -1) self.emit("move", mapped);
  });
};
 
KeyboardInputManager.prototype.restart = function (event) {
  event.preventDefault();
  this.emit("restart");
};