/*
 BouncingBall.java
 
 Implementation of a 2-d bouncing ball in the box from (-1, -1) to (1, 1).
 
 OS X / Linux
 ------------
 % javac -classpath .:stdlib.jar BouncingBall.java
 % java  -classpath .:stdlib.jar BouncingBall
 
 Windows
 -------
 % javac -classpath .;stdlib.jar BouncingBall.java
 % java  -classpath .;stdlib.jar BouncingBall

*/

public class BouncingBall
{ 
  public static void main(String[] args)
  {
    // set the scale of the coordinate system
    StdDraw.setXscale(-1.0, 1.0);
    StdDraw.setYscale(-1.0, 1.0);
    StdDraw.enableDoubleBuffering();

    // initial values
    double rx = 0.480, ry = 0.860; // position
    double vx = 0.015, vy = 0.023; // velocity
    double radius = 0.05; // radius

    // main animation loop
    while (true) { 

      // bounce off wall according to law of elastic collision
      if (Math.abs(rx + vx) > 1.0 - radius) vx = -vx;
      if (Math.abs(ry + vy) > 1.0 - radius) vy = -vy;

      // update position
      rx = rx + vx; 
      ry = ry + vy; 

      // clear the background
      StdDraw.clear(StdDraw.LIGHT_GRAY);

      // draw ball on the screen
      StdDraw.setPenColor(StdDraw.BLACK); 
      StdDraw.filledCircle(rx, ry, radius); 

      // copy offscreen buffer to onscreen
      StdDraw.show();

      // pause for 20 ms
      StdDraw.pause(20);
    } 
  } // end of main()

} // end of BouncingBall class