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 = 0.1;       // spring constant
  float d = 0.95;      // damping
  
  // Initialize a Spring at rest position (x0,y0)
  Spring(float x0, float y0)
  {
    cx = x0; cy = x0;
    x = x0; y = y0;
  }

  void draw()
  {
    // Set color from stretchiness
    float col = 128+128*dist(x,y, cx,cy)/max(width/2,height/2); 
    fill(col); stroke(col);  
    // Attach to wall 
    line(width/2,0,x,y);
    line(width/2,height,x,y);
    ellipse(x,y,r*2,r*2);
  }

  void update()
  {
    if (mousePressed && dist(mouseX,mouseY,x,y) < r) {
      x = mouseX; y = mouseY;
    }
    else {
      // Spring equations for both x and y
      vx -= k * (x-cx);
      vx *= d;
      x += vx;
      vy -= k * (y-cy);
      vy *= d;
      y += vy;
    }
  }
}