var sprites = {
   ship: { sx: 0, sy: 0, w: 37, h: 42, frames: 1 },
   missile: { sx: 0, sy: 30, w: 2, h: 10, frames: 1 },
   enemy_purple: { sx: 37, sy: 0, w: 42, h: 43, frames: 1 },
   enemy_bee: { sx: 79, sy: 0, w: 37, h: 43, frames: 1 },
   enemy_ship: { sx: 116, sy: 0, w: 42, h: 43, frames: 1 },
   enemy_circle: { sx: 158, sy: 0, w: 32, h: 33, frames: 1 },
   explosion: { sx: 0, sy: 64, w: 64, h: 64, frames: 12 }
  };
  
  var enemies = {
    straight: { x: 0,   y: -50, sprite: 'enemy_ship', health: 10, 
                E: 100 },
    ltr:      { x: 0,   y: -100, sprite: 'enemy_purple', health: 10, 
                B: 75, C: 1, E: 100  },
    circle:   { x: 250,   y: -50, sprite: 'enemy_circle', health: 10, 
                A: 0,  B: -100, C: 1, E: 20, F: 100, G: 1, H: Math.PI/2 },
    wiggle:   { x: 100, y: -50, sprite: 'enemy_bee', health: 20, 
                B: 50, C: 4, E: 100 },
    step:     { x: 0,   y: -50, sprite: 'enemy_circle', health: 10,
                B: 150, C: 1.2, E: 75 }
  };
  
  var OBJECT_PLAYER = 1,
      OBJECT_PLAYER_PROJECTILE = 2,
      OBJECT_ENEMY = 4,
      OBJECT_ENEMY_PROJECTILE = 8,
      OBJECT_POWERUP = 16;
  
  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 level1 = [
   // Start,   End, Gap,  Type,   Override
    [ 0,      4000,  500, 'step' ],
    [ 6000,   13000, 800, 'ltr' ],
    [ 10000,  16000, 400, 'circle' ],
    [ 17800,  20000, 500, 'straight', { x: 50 } ],
    [ 18200,  20000, 500, 'straight', { x: 90 } ],
    [ 18200,  20000, 500, 'straight', { x: 10 } ],
    [ 22000,  25000, 400, 'wiggle', { x: 150 }],
    [ 22000,  25000, 400, 'wiggle', { x: 100 }]
  ];
  
  var playGame = function() {
    var board = new GameBoard();
    board.add(new PlayerShip());
    board.add(new Level(level1,winGame));
    Game.setBoard(3,board);
  };
  
  var winGame = function() {
    Game.setBoard(3,new TitleScreen("You win!", 
                                    "Press fire to play again",
                                    playGame));
  };
  
  var loseGame = function() {
    Game.setBoard(3,new TitleScreen("You lose!", 
                                    "Press fire to play again",
                                    playGame));
  };
  
  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.setup('ship', { vx: 0, reloadTime: 0.25, maxVel: 200 });
  
    this.reload = this.reloadTime;
    this.x = Game.width/2 - this.w / 2;
    this.y = Game.height - Game.playerOffset - this.h;
  
    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));
      }
    };
  };
  
  PlayerShip.prototype = new Sprite();
  PlayerShip.prototype.type = OBJECT_PLAYER;
  
  PlayerShip.prototype.hit = function(damage) {
    if(this.board.remove(this)) {
      loseGame();
    }
  };
  
  var PlayerMissile = function(x,y) {
    this.setup('missile',{ vy: -700, damage: 10 });
    this.x = x - this.w/2;
    this.y = y - this.h; 
  };
  
  PlayerMissile.prototype = new Sprite();
  PlayerMissile.prototype.type = OBJECT_PLAYER_PROJECTILE;
  
  PlayerMissile.prototype.step = function(dt)  {
    this.y += this.vy * dt;
    var collision = this.board.collide(this,OBJECT_ENEMY);
    if(collision) {
      collision.hit(this.damage);
      this.board.remove(this);
    } else if(this.y < -this.h) { 
        this.board.remove(this); 
    }
  };
  
  var Enemy = function(blueprint,override) {
    this.merge(this.baseParameters);
    this.setup(blueprint.sprite,blueprint);
    this.merge(override);
  };
  
  Enemy.prototype = new Sprite();
  Enemy.prototype.type = OBJECT_ENEMY;
  
  Enemy.prototype.baseParameters = { A: 0, B: 0, C: 0, D: 0, 
                                     E: 0, F: 0, G: 0, H: 0,
                                     t: 0 };
  
  Enemy.prototype.step = function(dt) {
    this.t += dt;
  
    this.vx = this.A + this.B * Math.sin(this.C * this.t + this.D);
    this.vy = this.E + this.F * Math.sin(this.G * this.t + this.H);
  
    this.x += this.vx * dt;
    this.y += this.vy * dt;
  
    var collision = this.board.collide(this,OBJECT_PLAYER);
    if(collision) {
      collision.hit(this.damage);
      this.board.remove(this);
    }
  
    if(this.y > Game.height ||
       this.x < -this.w ||
       this.x > Game.width) {
         this.board.remove(this);
    }
  };
  
  Enemy.prototype.hit = function(damage) {
    this.health -= damage;
    if(this.health <=0) {
      if(this.board.remove(this)) {
        this.board.add(new Explosion(this.x + this.w/2, 
                                     this.y + this.h/2));
      }
    }
  };
  
  var Explosion = function(centerX,centerY) {
    this.setup('explosion', { frame: 0 });
    this.x = centerX - this.w/2;
    this.y = centerY - this.h/2;
  };
  
  Explosion.prototype = new Sprite();
  
  Explosion.prototype.step = function(dt) {
    this.frame++;
    if(this.frame >= 12) {
      this.board.remove(this);
    }
  };