public class Attraction implements Force

{

  Particle a;

  Particle b;

  float k;

  boolean on;

  float distanceMin;

  float distanceMinSquared;



  public Attraction( Particle a, Particle b, float k, float distanceMin )

  {

    this.a = a;

    this.b = b;

    this.k = k;

    on = true;

    this.distanceMin = distanceMin;

    this.distanceMinSquared = distanceMin*distanceMin;
  }



  protected void setA( Particle p )

  {

    a = p;
  }



  protected void setB( Particle p )

  {

    b = p;
  }



  public final float getMinimumDistance()

  {

    return distanceMin;
  }



  public final void setMinimumDistance( float d )

  {

    distanceMin = d;

    distanceMinSquared = d*d;
  }



  public final void turnOff()

  {

    on = false;
  }



  public final void turnOn()

  {

    on = true;
  }



  public final void setStrength( float k )

  {

    this.k = k;
  }



  public final Particle getOneEnd()

  {

    return a;
  }



  public final Particle getTheOtherEnd()

  {

    return b;
  }



  public void apply()

  {

    if ( on && ( a.isFree() || b.isFree() ) )

    {

      float a2bX = a.position().x() - b.position().x();

      float a2bY = a.position().y() - b.position().y();

      float a2bZ = a.position().z() - b.position().z();



      float a2bDistanceSquared = a2bX*a2bX + a2bY*a2bY + a2bZ*a2bZ;



      if ( a2bDistanceSquared < distanceMinSquared )

        a2bDistanceSquared = distanceMinSquared;



      float force = k * a.mass * b.mass / a2bDistanceSquared;



      float length = (float)Math.sqrt( a2bDistanceSquared );



      // make unit vector



      a2bX /= length;

      a2bY /= length;

      a2bZ /= length;



      // multiply by force 



      a2bX *= force;

      a2bY *= force;

      a2bZ *= force;



      // apply



      if ( a.isFree() )

        a.force().add( -a2bX, -a2bY, -a2bZ );

      if ( b.isFree() )

        b.force().add( a2bX, a2bY, a2bZ );
    }
  }



  public final float getStrength()

  {

    return k;
  }



  public final boolean isOn()

  {

    return on;
  }



  public final boolean isOff()

  {

    return !on;
  }
}