package {
          import flash.display.Sprite;
          import flash.display.StageScaleMode;
          import flash.display.StageAlign;
          import flash.events.Event;
  
          public class @ax-animation-ch12-Gravity extends Sprite
          {
                  private var particles:Array;
                  private var numParticles:uint = 30;
                  
                  public function @ax-animation-ch12-Gravity()
                  {
                          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 particle:animation_ch12_Ball = new animation_ch12_Ball(5);
                                  particle.x = Math.random() * stage.stageWidth;
                                  particle.y = Math.random() * stage.stageHeight;
                                  particle.mass = 1;
                                  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];
                                          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;
                  }
          }
  }