var sprites = {
   ship: { sx: 0, sy: 0, w: 37, h: 42, frames: 1 }
  };
  
  var startGame = function() {
    Game.setBoard(0,new Starfield(20,0.4,100,true))
    Game.setBoard(1,new Starfield(50,0.6,100))
    Game.setBoard(2,new Starfield(100,1.0,50));
    Game.setBoard(3,new TitleScreen("Alien Invasion", 
                                    "Press fire to start playing",
                                    playGame));
  }
  
  var playGame = function() {
    var board = new GameBoard();
    board.add(new PlayerShip());
    Game.setBoard(3,board);
  }
  
  window.addEventListener("load", function() {
    Game.initialize("game",sprites,startGame);
  
  });
  
  var Starfield = function(speed,opacity,numStars,clear) {
  
    // Set up the offscreen canvas
    var stars = document.createElement("canvas");
    stars.width = Game.width; 
    stars.height = Game.height;
    var starCtx = stars.getContext("2d");
  
    var offset = 0;
  
    // If the clear option is set, 
    // make the background black instead of transparent
    if(clear) {
      starCtx.fillStyle = "#000";
      starCtx.fillRect(0,0,stars.width,stars.height);
    }
  
    // Now draw a bunch of random 2 pixel
    // rectangles onto the offscreen canvas
    starCtx.fillStyle = "#FFF";
    starCtx.globalAlpha = opacity;
    for(var i=0;i<numStars;i++) {
      starCtx.fillRect(Math.floor(Math.random()*stars.width),
                       Math.floor(Math.random()*stars.height),
                       2,
                       2);
    }
  
    // This method is called every frame
    // to draw the starfield onto the canvas
    this.draw = function(ctx) {
      var intOffset = Math.floor(offset);
      var remaining = stars.height - intOffset;
  
      // Draw the top half of the starfield
      if(intOffset > 0) {
        ctx.drawImage(stars,
                  0, remaining,
                  stars.width, intOffset,
                  0, 0,
                  stars.width, intOffset);
      }
  
      // Draw the bottom half of the starfield
      if(remaining > 0) {
        ctx.drawImage(stars,
                0, 0,
                stars.width, remaining,
                0, intOffset,
                stars.width, remaining);
      }
    }
  
    // This method is called to update
    // the starfield
    this.step = function(dt) {
      offset += dt * speed;
      offset = offset % stars.height;
    }
  }
  
  var PlayerShip = function() { 
     this.w =  SpriteSheet.map['ship'].w;
     this.h =  SpriteSheet.map['ship'].h;
     this.x = Game.width/2 - this.w / 2;
     this.y = Game.height - 10 - this.h;
     this.vx = 0;
  
     this.maxVel = 200;
  
     this.step = function(dt) {
       if(Game.keys['left']) { this.vx = -this.maxVel; }
       else if(Game.keys['right']) { this.vx = this.maxVel; }
       else { this.vx = 0; }
  
       this.x += this.vx * dt;
  
       if(this.x < 0) { this.x = 0; }
       else if(this.x > Game.width - this.w) { 
         this.x = Game.width - this.w 
       }
     }
  
     this.draw = function(ctx) {
       SpriteSheet.draw(ctx,'ship',this.x,this.y,0);
     }
  }