/*
 * BallApplet.java
 * Created on Feb 13, 2005
 */
package ball;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.Timer;

/**
 * Bouncing ball applet.
 */
public final class BallApplet extends Applet implements ActionListener {
    
    private static final int RADIUS = 10;                    // pixels
    private static final int INITIAL_X_VEL = 60;             // pixels / second
    private static final int INITIAL_Y_VEL = 0;              // pixels / second
    private static final int DELAY_MS = 50;                  // milliseconds
    private static final double DELAY_S = DELAY_MS / 1000.0; // seconds
    private static final double GRAVITY = 50;                // pixels / second^2
    private static final double BOUNCE_RATIO = 0.8;
    
    private int height, width;
    private double xPos, yPos;
    private double xVel, yVel;
    
    public void init() {
        height = getHeight();
        width  = getWidth();
        xPos = RADIUS;
        yPos = RADIUS;
        xVel = INITIAL_X_VEL;
        yVel = INITIAL_Y_VEL;
    }
    
    public void start() {
        new Timer(DELAY_MS, this).start();
    }
    
    public void paint(Graphics g) {
        g.setColor(Color.BLUE);
        g.fillOval((int)Math.round(xPos), (int)Math.round(yPos), RADIUS, RADIUS);
        
        // find time till bounce in x direction
        double timeToBounceX = Double.MAX_VALUE;
        if (xVel > 0) {
            timeToBounceX = (width - xPos - RADIUS) / xVel;
        } else if (xVel < 0) {
            timeToBounceX = (xPos - RADIUS) / -xVel;
        }
        
        // update x position and velocity
        if(timeToBounceX > DELAY_S) {
            xPos += DELAY_S * xVel;
        } else {
            xPos += timeToBounceX * xVel;
            xVel = -xVel * BOUNCE_RATIO;
            xPos += (DELAY_S - timeToBounceX) * xVel;
        }
        
        // find time till bounce in y direction
        double timeToBounceY = Double.MAX_VALUE;
        if (yVel > 0) {
            timeToBounceY = (height - yPos - RADIUS) / yVel;
        } else if (yVel < 0) {
            timeToBounceY = (yPos - RADIUS) / -yVel;
        }
        
        // update y position and velocity
        if(timeToBounceY > DELAY_S) {
            yPos += DELAY_S * yVel;
            yVel += DELAY_S * GRAVITY;
        } else {
            yPos += timeToBounceY * yVel;
            yVel = -yVel * BOUNCE_RATIO + (timeToBounceY * GRAVITY);
            yPos += (DELAY_S - timeToBounceY) * yVel;
            yVel += (DELAY_S - timeToBounceY) * GRAVITY;
        }        
    }
    
    public void actionPerformed(ActionEvent arg0) {
        repaint();
    }
    
}