var sprites = {
 ship: { sx: 0, sy: 0, w: 37, h: 42, frames: 1 },
 missile: { sx: 0, sy: 30, w: 2, h: 10, 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.reloadTime = 0.25;  // Quarter second reload
   this.reload = this.reloadTime;

   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.reload-=dt;
     if(Game.keys['fire'] && this.reload < 0) {
       Game.keys['fire'] = false;
       this.reload = this.reloadTime;

       this.board.add(new PlayerMissile(this.x,this.y+this.h/2));
       this.board.add(new PlayerMissile(this.x+this.w,this.y+this.h/2));
     }
   }

   this.draw = function(ctx) {
     SpriteSheet.draw(ctx,'ship',this.x,this.y,0);
   }
}

var PlayerMissile = function(x,y) {
  this.w = SpriteSheet.map['missile'].w;
  this.h = SpriteSheet.map['missile'].h;
  this.x = x - this.w/2; 
  // Use the passed in y as the bottom of the missile
  this.y = y - this.h; 
  this.vy = -700;
};

PlayerMissile.prototype.step = function(dt)  {
  this.y += this.vy * dt;
  if(this.y < -this.h) { this.board.remove(this); }
};

PlayerMissile.prototype.draw = function(ctx)  {
  SpriteSheet.draw(ctx,'missile',this.x,this.y);
};