class Spring {
  float cx, cy;        // rest position
  float x, y;          // current position
  float vx=0, vy=0;    // velocity in the two directions   
  float r;             // radius
  float k;             // spring constant
  float d;             // damping
  
  // Initialize a Spring at rest position (x0,y0), radius r,
  // spring constant k, and damping d
  Spring(float x0, float y0, float r0, float k0, float d0)
  {
    x = x0; y = y0;
    cx = x0; cy = y0;
    r = r0; 
    k = k0; d = d0;
  }

  void draw()
  {
    fill(0); stroke(255); strokeWeight(2);
    ellipse(x,y,r*2,r*2);
  }

  // Update the spring, with a new center
  void update(float ncx, float ncy)
  {
    cx = ncx; cy = ncy;
    
    // usual spring stuff
    vx -= k * (x-cx); vy -= k * (y-cy);
    vx *= d; vy *= d;
    x += vx; y += vy;
  }
}