class Spring {
  Ball b1, b2;
  float rest;     // rest length
  float k;        // spring constant
  
  // Initialize a Spring between balls b1 and b2, with spring constant k
  Spring(Ball b1, Ball b2, float k)
  {
    this.b1 = b1; this.b2 = b2; this.k = k;
    rest = dist(b1.x,b1.y, b2.x,b2.y);
  }

  void draw()
  {
    line(b1.x,b1.y, b2.x,b2.y);
  }

  // Apply the spring to its masses
  void apply()
  {
    // Restorative force is proportional to difference in length from rest length
    float len = dist(b1.x,b1.y, b2.x,b2.y);
    if (len>0) {
      float f = k*(len-rest);
      // Apply it to x coordinates, moving each toward the other
      float fx = f*(b1.x-b2.x)/len;
      b1.vx -= fx;
      b2.vx += fx;
      // Similarly with y
      float fy = f*(b1.y-b2.y)/len;
      b1.vy -= fy;
      b2.vy += fy;
    }
 }
}