class InCircle extends UnaryForce{

  Vector3D position;
  float diameter;
  float SQRT_2 = sqrt(2);

  InCircle(ParticleSystem s, Vector3D p, float d){
    super(s,1);
    position = p;
    diameter = d;
  }

  void apply(){
    float maxDist = (diameter - a.size())*0.5;
    
    float bounded = diameter/(2*SQRT_2);
    float maxDistSq = bounded - a.size()*0.5;
    
    //test if bounding box of point is outside of bounded box of circle
    if( Math.abs( a.position().x() - position.x() ) < maxDistSq && Math.abs( a.position().y() - position.y() ) < maxDistSq )
      return;

    Vector3D a2b = a.position().copy().subtract(position);
    float a2bDistance = a2b.length();

    if( a2bDistance < maxDist )
      return;

    if( a2bDistance == 0 ){
      a2b.set( (new Vector3D((float)Math.random(), (float)Math.random(), 0)).unit() );
    }else{
      a2b.divide(a2bDistance);
    }

    float force = (maxDist - a2bDistance);
    a2b.multiply(force);

    // add on the forces
    addForce( a2b );
  }

}