package
  {
          import flash.display.Sprite;
          import flash.events.Event;
          import flash.display.StageScaleMode;
          import flash.display.StageAlign;
          import flash.geom.Point;
  
          public class @ax-animation-ch11-Billiard4 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-Billiard4()
                  {
                          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 pos0:Point = new Point(0, 0);
                                  
                                  // rotate ball1's position
                                  var pos1:Point = rotate(dx, dy, sin, cos, true);
                                  
                                  // rotate ball0's velocity
                                  var vel0:Point = rotate(ball0.vx,
                                                                                  ball0.vy,
                                                                                  sin,
                                                                                  cos,
                                                                                  true);
                                  
                                  // rotate ball1's velocity
                                  var vel1:Point = rotate(ball1.vx,
                                                                                  ball1.vy,
                                                                                  sin,
                                                                                  cos,
                                                                                  true);
                                  
                                  // collision reaction
                                  var vxTotal:Number = vel0.x - vel1.x;
                                  vel0.x = ((ball0.mass - ball1.mass) * vel0.x + 
                                            2 * ball1.mass * vel1.x) / 
                                            (ball0.mass + ball1.mass);
                                  vel1.x = vxTotal + vel0.x;
  
                                  // update position
                                  pos0.x += vel0.x;
                                  pos1.x += vel1.x;
                                  
                                  // rotate positions back
                                  var pos0F:Object = rotate(pos0.x,
                                                                                    pos0.y,
                                                                                    sin,
                                                                                    cos,
                                                                                    false);
                                                                                    
                                  var pos1F:Object = rotate(pos1.x,
                                                                                    pos1.y,
                                                                                    sin,
                                                                                    cos,
                                                                                    false);
  
                                  // adjust positions to actual screen positions
                                  ball1.x = ball0.x + pos1F.x;
                                  ball1.y = ball0.y + pos1F.y;
                                  ball0.x = ball0.x + pos0F.x;
                                  ball0.y = ball0.y + pos0F.y;
                                  
                                  // rotate velocities back
                                  var vel0F:Object = rotate(vel0.x,
                                                                                    vel0.y,
                                                                                    sin,
                                                                                    cos,
                                                                                    false);
                                  var vel1F:Object = rotate(vel1.x,
                                                                                    vel1.y,
                                                                                    sin,
                                                                                    cos,
                                                                                    false);
                                  ball0.vx = vel0F.x;
                                  ball0.vy = vel0F.y;
                                  ball1.vx = vel1F.x;
                                  ball1.vy = vel1F.y;
                          }
                  }
                  
                  private function rotate(x:Number,
                                                                  y:Number,
                                                                  sin:Number,
                                                                  cos:Number,
                                                                  reverse:Boolean):Point
                  {
                          var result:Point = new Point();
                          if(reverse)
                          {
                                  result.x = x * cos + y * sin;
                                  result.y = y * cos - x * sin;
                          }
                          else
                          {
                                  result.x = x * cos - y * sin;
                                  result.y = y * cos + x * sin;
                          }
                          return result;
                  }                
          }
  }