class Ball {
  float x, y;          // position
  float vx=0, vy=0;    // velocity in the two directions   
  float r=5;           // radius
  float gravity=0.1;   // the amount of acceleration)
  float drag=0.99;     // multiplicative factor for velocity
  float frictX=0.75;   // multiplicative factor, only when bounce
  float frictY=0.75;   // multiplicative factor, only when bounce
  color c=shotColor;   // color
  
  // Initial ball is off screen
  Ball()
  {
    x = -2*r; y = -2*r;
  }

  void draw()
  {
    fill(c);
    ellipse(x,y,r*2,r*2);
  }

  void update()
  {
    // Accelerate according to gravity
    vy += gravity;
    // Now damp according to drag
    vx *= drag;
    vy *= drag;
    // Move in the appropriate direction by the step size
    x += vx;
    y += vy;

    // Bounce
    if (x > width-r || x < r) {
      // Frictionless walls
      vx = -vx;
    }
    if (y > height-r) {
      y = height-r;
      vy = -vy;
      vx *= frictX; vy *= frictY;    // damp
    }
  }

  // Shoot the Ball from position (x0,y0), moving with velocity (vx0,vy0),
  // and with the given color 
  void shoot(float x0, float y0, float vx0, float vy0, color c0)
  {
    x = x0; y = y0; 
    vx = vx0; vy = vy0;
    c = c0;
  }
}