// Imports / Requirements
var Geometry = Geometry || global.Geometry;
var MovingObject = MovingObject || global.MovingObject;
var RenderablePolygon = RenderablePolygon || global.RenderablePolygon;
var ObjectTypes = ObjectTypes || global.ObjectTypes;
var Bullet = Bullet || global.Bullet;
var Asteroid = Asteroid || global.Asteroid;
var Explosion = Explosion || global.Explosion;
var SpaceShip = SpaceShip || global.SpaceShip;



var KeyCodes = {LEFT: 37, UP: 38, RIGHT: 39, DOWN: 40, CTRL: 17};

/**
* An object to look after game animation/movement.
*/
function GameEngine(canvas){
  var GE = {};

  var context;
  var keysPressed = [];
  var ships = [];
  var movable = [];
  var renderable = [];
  var player = null;
  var gamePaused = true;



  /*
   Sets up the GameEngine with the document and supplied canvas.
  */
  var init = function(){
    setCanvas(canvas);
    
    document.onkeyup = keyup;
    document.onkeydown = keydown;
  };



  var setCanvas = function(c){
    canvas = c;
    context = c.getContext("2d");
  };



  var clearWorld = function(){
    player =  null;
    ships = [];
    movable = [];
    renderable = [];
  }



  // Sets up a game world.
  var newGame = function(){
    clearWorld();
    
    var ship = new SpaceShip(null, {x: canvas.width / 2, y: canvas.height / 2})
    ships[0] = ship;
    player = ship;
    movable.push(player);
    renderable.push(player);
    
    addAsteroids(4);
  };



  //Adds n asteroids to the game
  var addAsteroids = function(n){
    for(var i = 0; i < n; i++){
      var args = {};
      
      do {
        //Set x and y, but not too close to centre
        args.x = Math.random() * canvas.width;
        args.y = Math.random() * canvas.width;
      } while(Math.abs(canvas.width / 2 - args.x) < 20 &&
              Math.abs(canvas.height / 2 - args.y) < 20);
      
      args.rockSize = Math.floor(Math.random() * 3) + 8;
      var aster = new Asteroid(null, args);
      
      movable.push(aster);
      renderable.push(aster);
    }
  };




  // Start/Resume game animation.
  var start = function(){
    if(gamePaused){
      gamePaused = false;
      var date = new Date();
      var time = date.getTime();
      tick(time);
    }
  };



  var tick = function tick(lastTime){
    var date = new Date();
    var time = date.getTime();
    var delta = time - lastTime;
    
    move(delta);
    render();
    
    //Call function again
    if(!gamePaused){
      window.setTimeout(tick, 1000/30, time);
    }
  };



  /*
   Move game entities, and check collisions.
   O(n^2)
  */
  var move = function(delta){
    handlePlayerInput();
    
    for(var i = 0; i < movable.length; i++){
      if(movable[i]){
        var mover = movable[i];
        
        mover.move(delta);
        ensureWithinBounds(mover);
        
        if(mover.collidesWith){
          for(var j = 0; j < movable.length; j++){
            if(i === j) continue;
            
            var other = movable[j];
            
            if(mover.collidesWith(other)){
              handleCollision(mover, other);
            }
          }
        }
        
        //Check if items have expired.
        if(mover.hasExpired && mover.hasExpired()){
          removeObject(mover);
        }
      }
    }
  };



  var handleCollision = function(obj1, obj2){
    if((obj1.getObjectType() === ObjectTypes.SHIP && obj2.getObjectType() === ObjectTypes.ASTEROID) ||
       (obj1.getObjectType() === ObjectTypes.ASTEROID && obj2.getObjectType() === ObjectTypes.SHIP)){
      destroyObject(obj1);
      destroyObject(obj2);
    }
    
    if((obj1.getObjectType() === ObjectTypes.BULLET && obj2.getObjectType() === ObjectTypes.ASTEROID) ||
       (obj1.getObjectType() === ObjectTypes.ASTEROID && obj2.getObjectType() === ObjectTypes.BULLET)){
      destroyObject(obj1);
      destroyObject(obj2);
    }
  };



  // Creates an explosion as well as removing it
  var destroyObject = function(movingObj){
    function makeExplosion(movingObj, numParticles, maxRadius){
      var explArgs = {x: movingObj.getX(),
                      y: movingObj.getY(),
                      angle: movingObj.getViewAngle(),
                      speed: 0,
                      numParticles: numParticles,
                      maxRadius: maxRadius};
      
      return new Explosion(null, explArgs);
    }
    
    removeObject(movingObj);
    
    if(movingObj.getObjectType() === ObjectTypes.ASTEROID){
      // Create Explosion
      var expl = makeExplosion(movingObj, 12, 75);
      movable.push(expl);
      renderable.push(expl);
      
      // Create more asteroids
      if(movingObj.getRockSize() > 7){
        createAsteroidsFromAsteroid(movingObj);
      }
    } else if(movingObj.getObjectType() === ObjectTypes.SHIP){
      var expl = makeExplosion(movingObj, 24, 100);
      movable.push(expl);
      renderable.push(expl);
    }
  }



  var createAsteroidsFromAsteroid = function(movingObj){
    var x = movingObj.getX();
    var y = movingObj.getY();
    
    var n = Math.floor(Math.random() * 3) + 1;
    for(var i = 0; i < n; i++){
      args = {};
      args.x = x;
      args.y = y;
      args.angle = 2 * Math.PI * Math.random();
      args.speed = (1 + 2 * Math.random()) * movingObj.getSpeed() / 3;
      args.rockSize = Math.floor(Math.random() * 2) + 5;
      
      var aster = new Asteroid(null, args);
      movable.push(aster);
      renderable.push(aster);
    }
  };



  var removeObject = function(movingObj){
    if(!movingObj.equals){
      throw "Expects object to have equals functionality.";
    }
    
    function removeFromList(obj, list){
      for(var i = 0; i < list.length; i++){
        if(obj.equals(list[i])){
          list.splice(i, 1);
          return;
        }
      }
    }
    
    removeFromList(movingObj, movable);
    removeFromList(movingObj, renderable);
  }



  // Wrap around world
  var ensureWithinBounds = function(obj){
    var x = obj.getX();
    var y = obj.getY();
    var prevX = obj.getPreviousX();
    var prevY = obj.getPreviousY();
    
    if(x < 0){
      obj.setX(prevX + canvas.width);
      obj.setX(x + canvas.width);
    }
    if(x > canvas.width){
      obj.setX(prevX - canvas.width);
      obj.setX(x - canvas.width);
    }
    if(y < 0){
      obj.setY(prevY + canvas.height);
      obj.setY(y + canvas.height);
    }
    if(y > canvas.height){
      obj.setY(prevY - canvas.height);
      obj.setY(y - canvas.height);
    }
  };



  var handlePlayerInput = function(){
    if(!player) player = ships[0];
    
    if(isPressed(KeyCodes.LEFT)){ player.turnLeft(); }
    if(isPressed(KeyCodes.RIGHT)){ player.turnRight(); }
    if(isPressed(KeyCodes.UP)){ player.accelerate(); }
    if(isPressed(KeyCodes.DOWN)){ player.decelerate(); }
    
    if(isPressed(KeyCodes.CTRL) && player.canShoot()){
      var shot = player.shoot();
      
      // add shots to game world
      movable.push(shot);
      renderable.push(shot);
    }
  };



  /*
   Render objects to canvas' context.
  */
  var render = function(){
    //Fill Background
    context.fillStyle = "#00FFFF";
    context.fillRect(0, 0, canvas.width, canvas.height);
    
    for(var i = 0; i < renderable.length; i++){
      if(renderable[i]){
        renderable[i].render(context);
      }
    }
    
    context.stroke();
  };



  // Record key press
  var keydown = function(evt){
    var keyCode = (window.event) ? event.keyCode : evt.keyCode;
    keysPressed[keyCode] = true;
  }



  // Record key release
  var keyup = function(evt){
    var keyCode = (window.event) ? event.keyCode : evt.keyCode;
    keysPressed[keyCode] = false;
  }



  // check whether the given keyCode is pressed
  var isPressed = function(keyCode){
    return keysPressed[keyCode] === true;
  }



  GE.init = init;
  GE.newGame = newGame;
  GE.start = start;
  GE.pause = function(){ gamePaused = true; };
  GE.setCanvas = setCanvas;
  GE.isPressed = isPressed;

  return GE;
}






global.KeyCodes = KeyCodes;

global.GameEngine = GameEngine;