var Game = new function() {                                                                  
    var boards = [];
  
    // Game Initialization
    this.initialize = function(canvasElementId,sprite_data,callback) {
      this.canvas = document.getElementById(canvasElementId)
      this.width = this.canvas.width;
      this.height= this.canvas.height;
  
      this.ctx = this.canvas.getContext && this.canvas.getContext('2d');
      if(!this.ctx) { return alert("Please upgrade your browser to play"); }
  
      this.setupInput();
  
      this.loop(); 
  
      SpriteSheet.load(sprite_data,callback);
    };
  
    // Handle Input
    var KEY_CODES = { 37:'left', 39:'right', 32 :'fire' };
    this.keys = {};
  
    this.setupInput = function() {
      window.addEventListener('keydown',function(e) {
        if(KEY_CODES[event.keyCode]) {
         Game.keys[KEY_CODES[event.keyCode]] = true;
         e.preventDefault();
        }
      },false);
  
      window.addEventListener('keyup',function(e) {
        if(KEY_CODES[event.keyCode]) {
         Game.keys[KEY_CODES[event.keyCode]] = false; 
         e.preventDefault();
        }
      },false);
    }
  
    // Game Loop
    this.loop = function() { 
      var dt = 30 / 1000;
  
      for(var i=0,len = boards.length;i<len;i++) {
        if(boards[i]) { 
          boards[i].step(dt);
          boards[i].draw(Game.ctx);
        }
      }
  
      setTimeout(Game.loop,30);
    };
    
    // Change an active game board
    this.setBoard = function(num,board) { boards[num] = board; };
  };
  
  var SpriteSheet = new function() {
    this.map = { }; 
  
    this.load = function(spriteData,callback) { 
      this.map = spriteData;
      this.image = new Image();
      this.image.onload = callback;
      this.image.src = 'mobile-game-ch02-enemies-images-sprites.png';
    };
  
    this.draw = function(ctx,sprite,x,y,frame) {
      var s = this.map[sprite];
      if(!frame) frame = 0;
      ctx.drawImage(this.image,
                       s.sx + frame * s.w, 
                       s.sy, 
                       s.w, s.h, 
                       Math.floor(x), Math.floor(y),
                       s.w, s.h);
    };
  }
  
  var TitleScreen = function TitleScreen(title,subtitle,callback) {
    var up = false;
    this.step = function(dt) {
      if(!Game.keys['fire']) up = true;
      if(up && Game.keys['fire'] && callback) callback();
    };
  
    this.draw = function(ctx) {
      ctx.fillStyle = "#FFFFFF";
      ctx.textAlign = "center";
  
      ctx.font = "bold 40px bangers";
      ctx.fillText(title,Game.width/2,Game.height/2);
  
      ctx.font = "bold 20px bangers";
      ctx.fillText(subtitle,Game.width/2,Game.height/2 + 40);
    };
  };
  
  var GameBoard = function() {
    var board = this;
  
    // The current list of objects
    this.objects = [];
  
    // Add a new object to the object list
    this.add = function(obj) { 
      obj.board=this; 
      this.objects.push(obj); 
      return obj; 
    };
  
    // Mark an object for removal
    this.remove = function(obj) { 
      this.removed.push(obj); 
    };
  
    // Reset the list of removed objects
    this.resetRemoved = function() { this.removed = []; }
  
    // Removed an objects marked for removal from the list
    this.finalizeRemoved = function() {
      for(var i=0,len=this.removed.length;i<len;i++) {
        var idx = this.objects.indexOf(this.removed[i]);
        if(idx != -1) this.objects.splice(idx,1);
      }
    }
  
    // Call the same method on all current objects 
    this.iterate = function(funcName) {
       var args = Array.prototype.slice.call(arguments,1);
       for(var i=0,len=this.objects.length;i<len;i++) {
         var obj = this.objects[i];
         obj[funcName].apply(obj,args)
       }
    };
  
    // Find the first object for which func is true
    this.detect = function(func) {
      for(var i = 0,val=null, len=this.objects.length; i < len; i++) {
        if(func.call(this.objects[i])) return this.objects[i];
      }
      return false;
    };
  
    // Call step on all objects and them delete
    // any object that have been marked for removal
    this.step = function(dt) { 
      this.resetRemoved();
      this.iterate('step',dt);
      this.finalizeRemoved();
    };
  
    // Draw all the objects
    this.draw= function(ctx) {
      this.iterate('draw',ctx);
    };
  
    // Check for a collision between the 
    // bounding rects of two objects
    this.overlap = function(o1,o2) {
      return !((o1.y+o1.h-1<o2.y) || (o1.y>o2.y+o2.h-1) ||
               (o1.x+o1.w-1<o2.x) || (o1.x>o2.x+o2.w-1));
    };
  
    // Find the first object that collides with obj
    // match against an optional type
    this.collide = function(obj,type) {
      return this.detect(function() {
        if(obj != this) {
         var col = (!type || this.type & type) && board.overlap(obj,this)
         return col ? this : false;
        }
      });
    };
  
  };