/**
    A manager for all the objects in the game
    @author <a href="mailto:matthewcasperson@gmail.com">Matthew Casperson</a>
    @class
*/
function GameObjectManager()
{
    /** An array of game objects 
        @type Arary
    */
    this.gameObjects = new Array();
    /** The time that the last frame was rendered  
        @type Date
    */
    this.lastFrame = new Date().getTime();
    /** The global scrolling value of the x axis  
        @type Number
    */
    this.xScroll = 0;
    /** The global scrolling value of the y axis  
        @type Number
    */
    this.yScroll = 0;
    /** A reference to the ApplicationManager instance  
        @type ApplicationManager
    */
    this.applicationManager = null;
    /** A reference to the canvas element  
        @type HTMLCanvasElement
    */
    this.canvas = null;
    /** A reference to the 2D context of the canvas element
        @type CanvasRenderingContext2D
    */
    this.context2D = null;
    /** A reference to the in-memory canvas used as a back buffer 
        @type HTMLCanvasElement
    */
    this.backBuffer = null;
    /** A reference to the backbuffer 2D context 
        @type CanvasRenderingContext2D
    */
    this.backBufferContext2D = null;

    /**
        Initialises this object
        @return A reference to the initialised object
    */
    this.startupGameObjectManager = function()
    {
        // set the global pointer to reference this object
        g_GameObjectManager = this;
        
        // get references to the canvas elements and their 2D contexts
        this.canvas = document.getElementById('canvas');
        this.context2D = this.canvas.getContext('2d');
        this.backBuffer = document.createElement('canvas');
        this.backBuffer.width = this.canvas.width;
        this.backBuffer.height = this.canvas.height;
        this.backBufferContext2D = this.backBuffer.getContext('2d');
        
        // create a new ApplicationManager
        this.applicationManager = new ApplicationManager().startupApplicationManager();
        
        // use setInterval to call the draw function 
        setInterval(function(){g_GameObjectManager.draw();}, SECONDS_BETWEEN_FRAMES);
        
        return this;        
    }
    
    /**
        The render loop
    */
    this.draw = function ()
    {
        // calculate the time since the last frame
        var thisFrame = new Date().getTime();
        var dt = (thisFrame - this.lastFrame)/1000;
        this.lastFrame = thisFrame;
        
        // clear the drawing contexts
        this.backBufferContext2D.clearRect(0, 0, this.backBuffer.width, this.backBuffer.height);
        this.context2D.clearRect(0, 0, this.canvas.width, this.canvas.height);
        
        // first update all the game objects
        for (x in this.gameObjects)
        {
            if (this.gameObjects[x].update)
            {
                this.gameObjects[x].update(dt, this.backBufferContext2D, this.xScroll, this.yScroll);
            }
        }

        // then draw the game objects
        for (x in this.gameObjects)
        {
            if (this.gameObjects[x].draw)
            {
                this.gameObjects[x].draw(dt, this.backBufferContext2D, this.xScroll, this.yScroll);
            }
        }
        
        // copy the back buffer to the displayed canvas
        this.context2D.drawImage(this.backBuffer, 0, 0);
    };
    
    /**
        Adds a new GameObject to the gameObjects collection
        @param gameObject The object to add
    */
    this.addGameObject = function(gameObject)
    {
        this.gameObjects.push(gameObject);
        this.gameObjects.sort(function(a,b){return a.zOrder - b.zOrder;})
    };
    
    /**
        Removes a GameObject from the gameObjects collection
        @param gameObject The object to remove
    */
    this.removeGameObject = function(gameObject)
    {
        this.gameObjects.removeObject(gameObject);
    }
}