text: 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
    };
  
    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));
  };
  
  KeyboardInputManager.prototype.restart = function (event) {
    event.preventDefault();
    this.emit("restart");
  };
  