class Spring
{
  Vector3D loc;
  Vector3D vel;
  Vector3D acc;

  Vector3D origin;
  Vector3D shift;
  Vector3D x;

  float diam;




  boolean dragging = false;

  Spring(Vector3D l)
  {
    loc = l.copy();
    vel = new Vector3D(0.0,0.0);
    acc = new Vector3D(0.0,0.0);


    origin = loc.copy();         // save origin

    //shift = shift_.copy(); 

    diam = 40.0;               // set diameter
  }

  void go(Vector3D shift)
    //void go()
  {
    //Hooke's Law : F = -kx


    float k = 1.0f;   // spring constant
    float c = -0.2f;                           // Drag coefficient

    //shift.mult(-1);

    if (shift.magnitude() > 40.0) {
      loc = origin.copy();
      loc.add(shift);   // add vector from MEL ... should do some math on it first...
    } 
    else {

      Vector3D disp = Vector3D.sub(origin,loc);    // "-x"
      disp.mult(k);    // "-kx" spring force
      acc = disp.copy();  // make accel. spring force




    }
    
          // drag force
      Vector3D thingVel = vel.copy();             // Velocity of our thing
      Vector3D dragF = Vector3D.mult(thingVel,c); // Following the formula above
      acc.add(dragF);  // add dragforce
    
      vel.add(acc);

      loc.add(vel);
  }

  void render()
  {
    fill(255,200);            
    noStroke();
    ellipse(loc.x,loc.y,diam,diam);
  }



  // This checks to see if we clicked on the pendulum ball
  void clicked(int mx, int my) {
    float d = dist(mx,my,loc.x,loc.y);
    if (d < diam) {
      dragging = true;
    }
  }

  // This tells us we are not longer clicking on the ball
  void stopDragging() {
    dragging = false;
  }

  void drag() {
    // If we are draging the ball, we calculate the angle between the 
    // pendulum origin and mouse location
    // we assign that angle to the pendulum
    if (dragging) {
      loc = new Vector3D(mouseX,mouseY);   // Difference between 2 points
      //theta = atan2(-1*diff.y,diff.x) - radians(90);                      // Angle relative to vertical axis
    }
  }



}