package
  {
          import flash.display.Sprite;
          import flash.events.Event;
          import flash.display.StageScaleMode;
          import flash.display.StageAlign;
  
          public class @ax-animation-ch11-Billiard3 extends Sprite
          {
                  private var ball0:animation_ch11_Ball;
                  private var ball1:animation_ch11_Ball;
                  private var bounce:Number = -1.0;
                  
                  public function @ax-animation-ch11-Billiard3()
                  {
                          init();
                  }
                  
                  private function init():void
                  {
                          stage.scaleMode = StageScaleMode.NO_SCALE;
                          stage.align = StageAlign.TOP_LEFT;
                          ball0 = new animation_ch11_Ball(150);
                          ball0.mass = 2;
                          ball0.x = stage.stageWidth - 200;
                          ball0.y = stage.stageHeight - 200;
                          ball0.vx = Math.random() * 10 - 5;
                          ball0.vy = Math.random() * 10 - 5;
                          addChild(ball0);
  
                          ball1 = new animation_ch11_Ball(90);
                          ball1.mass = 1
                          ball1.x = 100;
                          ball1.y = 100;
                          ball1.vx = Math.random() * 10 - 5;
                          ball1.vy = Math.random() * 10 - 5;
                          addChild(ball1);
  
                          addEventListener(Event.ENTER_FRAME, onEnterFrame);
                  }
                  
                  private function onEnterFrame(event:Event):void
                  {
                          ball0.x += ball0.vx;
                          ball0.y += ball0.vy;
                          ball1.x += ball1.vx;
                          ball1.y += ball1.vy;
                          checkCollision(ball0, ball1);
                          checkWalls(ball0);
                          checkWalls(ball1);
                  }
                  
                  private function checkWalls(ball:animation_ch11_Ball):void
                  {
                          if(ball.x + ball.radius > stage.stageWidth)
                          {
                                  ball.x = stage.stageWidth - ball.radius;
                                  ball.vx *= bounce;
                          }
                          else if(ball.x - ball.radius < 0)
                          {
                                  ball.x = ball.radius;
                                  ball.vx *= bounce;
                          }
                          if(ball.y + ball.radius > stage.stageHeight)
                          {
                                  ball.y = stage.stageHeight - ball.radius;
                                  ball.vy *= bounce;
                          }
                          else if(ball.y - ball.radius < 0)
                          {
                                  ball.y = ball.radius;
                                  ball.vy *= bounce;
                          }
                  }
  
                  private function checkCollision(ball0:animation_ch11_Ball, ball1:animation_ch11_Ball):void
                  {
                          var dx:Number = ball1.x - ball0.x;
                          var dy:Number = ball1.y - ball0.y;
                          var dist:Number = Math.sqrt(dx*dx + dy*dy);
                          if(dist < ball0.radius + ball1.radius)
                          {
                                  // calculate angle, sine and cosine
                                  var angle:Number = Math.atan2(dy, dx);
                                  var sin:Number = Math.sin(angle);
                                  var cos:Number = Math.cos(angle);
                                  
                                  // rotate ball0's position
                                  var x0:Number = 0;
                                  var y0:Number = 0;
                                  
                                  // rotate ball1's position
                                  var x1:Number = dx * cos + dy * sin;
                                  var y1:Number = dy * cos - dx * sin;
                                  
                                  // rotate ball0's velocity
                                  var vx0:Number = ball0.vx * cos + ball0.vy * sin;
                                  var vy0:Number = ball0.vy * cos - ball0.vx * sin;
                                  
                                  // rotate ball1's velocity
                                  var vx1:Number = ball1.vx * cos + ball1.vy * sin;
                                  var vy1:Number = ball1.vy * cos - ball1.vx * sin;
                                  
                                  // collision reaction
                                  var vxTotal:Number = vx0 - vx1;
                                  vx0 = ((ball0.mass - ball1.mass) * vx0 + 2 * ball1.mass * vx1) / (ball0.mass + ball1.mass);
                                  vx1 = vxTotal + vx0;
                                  x0 += vx0;
                                  x1 += vx1;
  
                                  // rotate positions back
                                  var x0Final:Number = x0 * cos - y0 * sin;
                                  var y0Final:Number = y0 * cos + x0 * sin;
                                  var x1Final:Number = x1 * cos - y1 * sin;
                                  var y1Final:Number = y1 * cos + x1 * sin;
  
                                  // adjust positions to actual screen positions
                                  ball1.x = ball0.x + x1Final;
                                  ball1.y = ball0.y + y1Final;
                                  ball0.x = ball0.x + x0Final;
                                  ball0.y = ball0.y + y0Final;
  
                                  // rotate velocities back
                                  ball0.vx = vx0 * cos - vy0 * sin;
                                  ball0.vy = vy0 * cos + vx0 * sin;
                                  ball1.vx = vx1 * cos - vy1 * sin;
                                  ball1.vy = vy1 * cos + vx1 * sin;
                          }
                  }
          }
  }