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
  
  // Initialize a Spring at rest position (x0,y0), of size w*h
  Spring(float x0, float y0, float r0)
  {
    x = x0; y = y0;
    cx = x0; cy = y0;
    r = r0;
    // Random spring constant and damping
    k = random(0.01,0.02);
    d = random(0.975,1.0);
  }

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

  void update()
  {
    if(dist(mouseX,mouseY,x,y) < r) {
      // Give it a kick of velocity when (while) mouse is over it
      vy = 10;
    }
    
    // usual spring stuff
    vx -= k * (x-cx); vy -= k * (y-cy);
    vx *= d; vy *= d;
    x += vx; y += vy;
  }
}