(function() {
      var lastTime = 0;
      var vendors = ['ms', 'moz', 'webkit', 'o'];
      for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
          window.requestAnimationFrame = window[vendors[x]+'RequestAnimationFrame'];
          window.cancelAnimationFrame = 
            window[vendors[x]+'CancelAnimationFrame'] || window[vendors[x]+'CancelRequestAnimationFrame'];
      }
   
      if (!window.requestAnimationFrame)
          window.requestAnimationFrame = function(callback, element) {
              var currTime = new Date().getTime();
              var timeToCall = Math.max(0, 16 - (currTime - lastTime));
              var id = window.setTimeout(function() { callback(currTime + timeToCall); }, 
                timeToCall);
              lastTime = currTime + timeToCall;
              return id;
          };
   
      if (!window.cancelAnimationFrame)
          window.cancelAnimationFrame = function(id) {
              clearTimeout(id);
          };
  }());
    
  
  var Game = new function() {                                                                  
    var boards = [];
  
    // Game Initialization
    this.initialize = function(canvasElementId,sprite_data,callback) {
  
      var ctx = this.ctx = AppMobi.canvas.getContext("2d");
  
      //wrap in try/catch because XDK does not currently support this property
      try {
        ctx.HTML5CompatibilityMode = true;
      } catch(e){}
      this.ctx.width = 320;
      this.ctx.height = 480;
  
      this.playerOffset = 10;
      this.canvasMultiplier= 1;
      this.mobile = true;
  
      this.width = 320;
      this.height= 480;
  
      this.loop(); 
  
      if(this.mobile) {
        this.setBoard(4,new TouchControls());
      }
  
      SpriteSheet.load(sprite_data,callback);
    };
    
    this.setKeys = function(l,r,fire) {
      Game.keys['left'] = l;
      Game.keys['right'] = r;
      Game.keys['fire'] = fire;
    };
  
   
    var lastTime = new Date().getTime();
    var maxTime = 1/30;
    // Game Loop
    this.loop = function() { 
      var curTime = new Date().getTime();
      requestAnimationFrame(Game.loop);
  
      var dt = (curTime - lastTime)/1000;
      if(dt > maxTime) { dt = maxTime; }
  
      for(var i=0,len = boards.length;i<len;i++) {
        if(boards[i]) { 
          boards[i].step(dt);
          boards[i].draw(Game.ctx);
        }
      }
      Game.ctx.present();
      lastTime = curTime;
    };
    
  
    
    // Change an active game board
    this.setBoard = function(num,board) { boards[num] = board; };
  
    return this;
  };
  
  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-ch27-appmobi-invasion-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);
    };
    return this;
  };
  
  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) {
        Game.keys['fire'] = false;
        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 = [];
    this.cnt = {};
  
    // Add a new object to the object list
    this.add = function(obj) { 
      obj.board=this; 
      this.objects.push(obj); 
      this.cnt[obj.type] = (this.cnt[obj.type] || 0) + 1;
      return obj; 
    };
  
    // Mark an object for removal
    this.remove = function(obj) { 
      var idx = this.removed.indexOf(obj);
      if(idx == -1) {
        this.removed.push(obj); 
        return true;
      } else {
        return false;
      }
    };
  
    // 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.cnt[this.removed[i].type]--;
          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;
        }
      });
    };
  
  };
  
  var Sprite = function() { };
  
  Sprite.prototype.setup = function(sprite,props) {
    this.sprite = sprite;
    this.merge(props);
    this.frame = this.frame || 0;
    this.w =  SpriteSheet.map[sprite].w;
    this.h =  SpriteSheet.map[sprite].h;
  };
  
  Sprite.prototype.merge = function(props) {
    if(props) {
      for (var prop in props) {
        this[prop] = props[prop];
      }
    }
  };
  
  Sprite.prototype.draw = function(ctx) {
    SpriteSheet.draw(ctx,this.sprite,this.x,this.y,this.frame);
  };
  
  Sprite.prototype.hit = function(damage) {
    this.board.remove(this);
  };
  
  var Level = function(levelData,callback) {
    this.levelData = [];
    for(var i =0; i<levelData.length; i++) {
      this.levelData.push(Object.create(levelData[i]));
    }
    this.t = 0;
    this.callback = callback;
  };
  
  Level.prototype.step = function(dt) {
    var idx = 0, remove = [], curShip = null;
  
    // Update the current time offset
    this.t += dt * 1000;
  
    //   Start, End,  Gap, Type,   Override
    // [ 0,     4000, 500, 'step', { x: 100 } ]
    while((curShip = this.levelData[idx]) && 
          (curShip[0] < this.t + 2000)) {
      // Check if we've passed the end time 
      if(this.t > curShip[1]) {
        remove.push(curShip);
      } else if(curShip[0] < this.t) {
        // Get the enemy definition blueprint
        var enemy = enemies[curShip[3]],
            override = curShip[4];
  
        // Add a new enemy with the blueprint and override
        this.board.add(new Enemy(enemy,override));
  
        // Increment the start time by the gap
        curShip[0] += curShip[2];
      }
      idx++;
    }
  
    // Remove any objects from the levelData that have passed
    for(var i=0,len=remove.length;i<len;i++) {
      var remIdx = this.levelData.indexOf(remove[i]);
      if(remIdx != -1) this.levelData.splice(remIdx,1);
    }
  
    // If there are no more enemies on the board or in 
    // levelData, this level is done
    if(this.levelData.length === 0 && this.board.cnt[OBJECT_ENEMY] === 0) {
      if(this.callback) this.callback();
    }
  
  };
  
  Level.prototype.draw = function(ctx) { };
  
  var TouchControls = function() {
  
    var gutterWidth = 10;
    var unitWidth = Game.width/5;
    var blockWidth = unitWidth-gutterWidth;
  
    this.drawSquare = function(ctx,x,y,txt,on) {
      ctx.globalAlpha = on ? 0.9 : 0.6;
      ctx.fillStyle =  "#CCC";
      ctx.fillRect(x,y,blockWidth,blockWidth);
  
      ctx.fillStyle = "#FFF";
      ctx.textAlign = "center";
      ctx.globalAlpha = 1.0;
      ctx.font = "bold " + (3*unitWidth/4) + "px arial";
  
      ctx.fillText(txt, 
                   x+blockWidth/2,
                   y+3*blockWidth/4+5);
    };
  
    this.draw = function(ctx) {
      ctx.save();
  
      var yLoc = Game.height - unitWidth;
      this.drawSquare(ctx,gutterWidth,yLoc,"◀");
      this.drawSquare(ctx,unitWidth + gutterWidth,yLoc,"▶");
      this.drawSquare(ctx,4*unitWidth,yLoc,"A");
  
      ctx.restore();
    };
  
    this.step = function(dt) {
    };
  
    Game.playerOffset = unitWidth + 20;
  };
  
  var GamePoints = function() {
    Game.points = 0;
  
    var pointsLength = 8;
  
    this.draw = function(ctx) {
      ctx.save();
      ctx.font = "bold 18px arial";
      ctx.fillStyle= "#FFFFFF";
  
      var txt = "" + Game.points;
      var i = pointsLength - txt.length, zeros = "";
      while(i-- > 0) { zeros += "0"; }
  
      ctx.fillText(zeros + txt,10,20);
      ctx.restore();
  
    };
  
    this.step = function(dt) { };
  };