package {
          import flash.display.Sprite;
          import flash.display.StageScaleMode;
          import flash.display.StageAlign;
          import flash.events.Event;
          import flash.geom.Point;
  
          public class @ax-animation-ch12-GravityRandom extends Sprite
          {
                  private var particles:Array;
                  private var numParticles:uint = 30;
                  
                  public function @ax-animation-ch12-GravityRandom()
                  {
                          init();
                  }
                  
                  private function init():void
                  {
                          stage.scaleMode = StageScaleMode.NO_SCALE;
                          stage.align = StageAlign.TOP_LEFT;
                          particles = new Array();
                          for(var i:uint = 0; i < numParticles; i++)
                          {
                                  var size:Number = Math.random() * 25 + 5;
                                  var particle:animation_ch12_Ball = new animation_ch12_Ball(size);
                                  particle.x = Math.random() * stage.stageWidth;
                                  particle.y = Math.random() * stage.stageHeight;
                                  particle.mass = size;
                                  addChild(particle);
                                  particles.push(particle);
                          }
                          
                          addEventListener(Event.ENTER_FRAME, onEnterFrame);
                  }
                  
                  private function onEnterFrame(event:Event):void
                  {
                          for(var i:uint = 0; i < numParticles; i++)
                          {
                                  var particle:animation_ch12_Ball = particles[i];
                                  particle.x += particle.vx;
                                  particle.y += particle.vy;
                          }
                          for(i=0; i < numParticles - 1; i++)
                          {
                                  var partA:animation_ch12_Ball = particles[i];
                                  for(var j:uint = i + 1; j < numParticles; j++)
                                  {
                                          var partB:animation_ch12_Ball = particles[j];
                                          checkCollision(partA, partB);
                                          gravitate(partA, partB);
                                  }
                          }
                  }
                  
                  private function gravitate(partA:animation_ch12_Ball, partB:animation_ch12_Ball):void
                  {
                          var dx:Number = partB.x - partA.x;
                          var dy:Number = partB.y - partA.y;
                          var distSQ:Number = dx*dx + dy*dy;
                          var dist:Number = Math.sqrt(distSQ);
                          var force:Number = partA.mass * partB.mass / distSQ;
                          var ax:Number = force * dx / dist;
                          var ay:Number = force * dy / dist;
                          partA.vx += ax / partA.mass;
                          partA.vy += ay / partA.mass;
                          partB.vx -= ax / partB.mass;
                          partB.vy -= ay / partB.mass;
                  }
                  
                  private function checkCollision(ball0:animation_ch12_Ball, ball1:animation_ch12_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
                                  var absV:Number = Math.abs(vel0.x) + Math.abs(vel1.x);
                                  var overlap:Number = (ball0.radius + ball1.radius) 
                                                        - Math.abs(pos0.x - pos1.x);
                                  pos0.x += vel0.x / absV * overlap;
                                  pos1.x += vel1.x / absV * overlap;
                                  
                                  // 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;
                  }                
          }
  }