package {
          import flash.display.Sprite;
          import flash.display.StageAlign;
          import flash.display.StageScaleMode;
          import flash.events.Event;
          import flash.geom.Point;
          import flash.utils.getTimer;
  
          public class @ax-lib-flex-animation-code-06-RK4 extends Sprite
          {
                  private var _ball:Sprite;
                  private var _position:Point;
                  private var _velocity:Point;
                  private var _gravity:Number = 32;
                  private var _bounce:Number = -0.6;
                  private var _oldTime:int;
                  private var _pixelsPerFoot:Number = 10;
                  
                  
                  public function @ax-lib-flex-animation-code-06-RK4()
                  {
                          stage.align = StageAlign.TOP_LEFT;
                          stage.scaleMode = StageScaleMode.NO_SCALE;
                          
                          _ball = new Sprite();
                          _ball.graphics.beginFill(0xff0000);
                          _ball.graphics.drawCircle(0, 0, 20);
                          _ball.graphics.endFill();
                          _ball.x = 50;
                          _ball.y = 50;
                          addChild(_ball);
                          
                          _velocity = new Point(10, 0);
                          _position = new Point(_ball.x / _pixelsPerFoot, _ball.y / _pixelsPerFoot);
                          
                          _oldTime = getTimer();
                          addEventListener(Event.ENTER_FRAME, onEnterFrame);
                  }
                  
                  private function onEnterFrame(event:Event):void
                  {
                          var time:int = getTimer();
                          var elapsed:Number = (time - _oldTime) / 1000;
                          _oldTime = time;
                          
                          var accel1:Point = acceleration(_position, _velocity);
                          
                          var position2:Point = new Point();
                          position2.x = _position.x + _velocity.x / 2 * elapsed;
                          position2.y = _position.y + _velocity.y / 2 * elapsed;
  
                          var velocity2:Point = new Point();
                          velocity2.x = _velocity.x + accel1.x / 2 * elapsed;
                          velocity2.y = _velocity.y + accel1.x / 2 * elapsed;
                          
                          var accel2:Point = acceleration(position2, velocity2);
                          
                          var position3:Point = new Point();
                          position3.x = _position.x + velocity2.x / 2 * elapsed;
                          position3.y = _position.y + velocity2.y / 2 * elapsed;
                          
                          var velocity3:Point = new Point();
                          velocity3.x = _velocity.x + accel2.x / 2 * elapsed;
                          velocity3.y = _velocity.y + accel2.y / 2 * elapsed;
                          
                          var accel3:Point = acceleration(position3, velocity3);
                          
                          var position4:Point = new Point();
                          position4.x = _position.x + velocity3.x * elapsed;
                          position4.y = _position.y + velocity3.y * elapsed;
                          
                          var velocity4:Point = new Point();
                          velocity4.x = _velocity.x + accel3.x * elapsed;
                          velocity4.y = _velocity.y + accel3.y * elapsed;
                          
                          var accel4:Point = acceleration(position4, velocity4);
                          
                          _position.x += (_velocity.x + 2 * velocity2.x + 2 * velocity3.x + velocity4.x) / 6 * elapsed;
                          _position.y += (_velocity.y + 2 * velocity2.y + 2 * velocity3.y + velocity4.y) / 6 * elapsed;
                          
                          _velocity.x += (accel1.x + 2 * accel2.x + 2 * accel3.x + accel4.x) / 6 * elapsed;
                          _velocity.y += (accel1.y + 2 * accel2.y + 2 * accel3.y + accel4.y) / 6 * elapsed;
                          
                          if(_position.y > (stage.stageHeight - 20) / _pixelsPerFoot)
                          {
                                  _position.y = (stage.stageHeight - 20) / _pixelsPerFoot;
                                  _velocity.y *= _bounce;
                          }
                          if(_position.x > (stage.stageWidth - 20) / _pixelsPerFoot)
                          {
                                  _position.x = (stage.stageWidth - 20) / _pixelsPerFoot;
                                  _velocity.x *= _bounce
                          }
                          else if(_position.x < 20 / _pixelsPerFoot)
                          {
                                  _position.x = 20 / _pixelsPerFoot;
                                  _velocity.x *= _bounce;
                          }
                          
                          _ball.x = _position.x * _pixelsPerFoot;
                          _ball.y = _position.y * _pixelsPerFoot;
                  }
                  
                  private function acceleration(p:Point, v:Point):Point
                  {
                          return new Point(0, _gravity);
                  }
          }
  }