Center rectangle with random registration point

/**
 * Copyright milchreis ( http://wonderfl.net/user/milchreis )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/9lprq
 */

package 
{
    import flash.display.Sprite;
    import flash.display.Shape;
    public class FlashTest extends Sprite 
    {
        public function FlashTest() 
        {
            // write as3 code here..
            var w:int = 150, h:int = 200;
            
            var s:Shape = new Shape();
            s.graphics.beginFill(0x00FF00);
            s.graphics.drawRect(-w * Math.random(), -h * Math.random(), w, h);
            s.graphics.drawCircle(0, 0, 5);
            s.graphics.endFill();
            
            addChild(s);
            
            var left:Number = s.getRect(stage).left, top:Number = s.getRect(stage).top;
            
            s.x = (stage.stageWidth  - w)/2 
                    + (s.x - left); // offset origin - left edge
            s.y = (stage.stageHeight - h)/2 
                    + (s.y - top ); // offset origin - top edge  
        }
    }
}