class Spring {
  float cx, cy;        // rest position
  float x, y;          // current position
  float vx=0, vy=0;    // velocity in the two directions   
  float r = 10;        // radius
  float k = 0.05;      // spring constant
  float d = 0.75;      // damping
  float g = 1;         // gravity
  
  // Initialize a Spring at rest position (x0,y0)
  Spring(float x0, float y0, float r0)
  {
    x = x0; y = y0;
    cx = x0; cy = y0;
    r = r0;
  }

  void draw()
  {
    stroke(128); strokeWeight(3);
    line(cx,cy,x,y);
    fill(255); stroke(128); strokeWeight(1);
    ellipse(x,y,2*r,2*r);
  }

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