/*
 * File: MyBouncingBall.java
 * -------------------------
 * This program animates a bouncing ball
 */

import acm.graphics.*;
import acm.program.*;

public class MyBouncingBall extends GraphicsProgram {

   public void run() {
      /* center of the ball */
      int ballX = getWidth() / 2;
      int ballY = getHeight() / 2;
      /* draw a ball at the center */
      GOval ball = new GOval(ballX - BALL_RADIUS, ballY - BALL_RADIUS,
                             2*BALL_RADIUS, 2*BALL_RADIUS);
      ball.setFilled(true);
      add(ball);

      /* initial velocity */
      int dx = 1;
      int dy = 1;

      /* animation */
      for (int i = 0; i < N_STEPS; i++) {
         if ((ballX + dx < BALL_RADIUS) ||
             (ballX + dx > getWidth() - BALL_RADIUS))
            /* ball has hit the left or right wall */
            dx = -dx;
         if ((ballY + dy < BALL_RADIUS) ||
             (ballY + dy > getHeight() - BALL_RADIUS))
            /* ball has hit the top or bottom wall */
            dy = -dy;
         ball.move(dx, dy);
         /* update ball center position */
         ballX += dx;
         ballY += dy;
         pause(PAUSE_TIME);
      } /* for i */
   }

/* private constants */
   private static final int BALL_RADIUS = 20;
   private static final int N_STEPS = 2000;
   private static final int PAUSE_TIME = 20;
}