public class Spring implements Force {
  
  float springConstant;
  float damping;
  float restLength;
  Particle a, b;
  boolean on;

  public Spring( Particle A, Particle B, float ks, float d, float r ) {
    springConstant = ks;
    damping = d;
    restLength = r;
    a = A;
    b = B;
    on = true;
  }



  public final void turnOff()

  {

    on = false;
  }



  public final void turnOn()

  {

    on = true;
  }



  public final boolean isOn()

  {

    return on;
  }



  public final boolean isOff()

  {

    return !on;
  }



  public final Particle getOneEnd()

  {

    return a;
  }



  public final Particle getTheOtherEnd()

  {

    return b;
  }



  public final float currentLength()

  {

    return a.position().distanceTo( b.position() );
  }



  public final float restLength()

  {

    return restLength;
  }



  public final float strength()

  {

    return springConstant;
  }



  public final void setStrength( float ks )

  {

    springConstant = ks;
  }



  public final float damping()

  {

    return damping;
  }



  public final void setDamping( float d )

  {

    damping = d;
  }



  public final void setRestLength( float l )

  {

    restLength = l;
  }



  public final 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 a2bDistance = (float)Math.sqrt( a2bX*a2bX + a2bY*a2bY + a2bZ*a2bZ );



      if ( a2bDistance == 0 )

      {

        a2bX = 0;

        a2bY = 0;

        a2bZ = 0;
      }

      else

      {

        a2bX /= a2bDistance;

        a2bY /= a2bDistance;

        a2bZ /= a2bDistance;
      }





      // spring force is proportional to how much it stretched 

      float springForce = -( a2bDistance - restLength ) * springConstant; 



      // want velocity along line b/w a & b, damping force is proportional to this

      float Va2bX = a.velocity().x - b.velocity().x;
      float Va2bY = a.velocity().y - b.velocity().y;
      float Va2bZ = a.velocity().z - b.velocity().z;
      

      float dampingForce = -damping * ( a2bX*Va2bX + a2bY*Va2bY + a2bZ*Va2bZ );





      // forceB is same as forceA in opposite direction

      float r = springForce + dampingForce;



      a2bX *= r;
      a2bY *= r;
      a2bZ *= r;


      if ( a.isFree() )
      
        a.force().add( a2bX, a2bY, a2bZ );

      if ( b.isFree() )

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



  protected void setA( Particle p )

  {

    a = p;
  }



  protected void setB( Particle p )

  {

    b = p;
  }
}