// CSE 142, Autumn 2009, Marty Stepp
// This program simulates the dropping of three balls from various heights.
// It draws the balls on a DrawingPanel and animates using the sleep method.
// The purpose of the program is to demonstrate return values
// (the displacement method, and Math methods) and type double.
//
// This second version of the program uses Point objects to represent the
// balls' x/y positions.  This makes the code more concise and makes it
// easier to add a 3rd ball.

import java.awt.*;

public class Balls2 {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(600, 600);
        panel.setBackground(Color.CYAN);
        Graphics g = panel.getGraphics();
        
        Point ball1 = new Point(100, (600 - 600));  // height of 600
        Point ball2 = new Point(200, (600 - 400));  // height of 400
        Point ball3 = new Point(300, (600 - 500));  // height of 500
        
        // draw the balls at each time increment
        for (double t = 0; t <= 10.0; t = t + 0.1) {
            drawBall(g, ball1, 25, t);   // initial velocity of 25
            drawBall(g, ball2,  0, t);   // initial velocity of  0
            drawBall(g, ball3, 15, t);   // initial velocity of 15
            panel.sleep(50);   // pause for 50 ms
        }
    }
    
    // Draws the given ball point with the given initial velocity
    // after the given amount of time has elapsed.
    public static void drawBall(Graphics g, Point ball, double v0, double t) {
        double disp = displacement(v0, t, 9.81);
        int newY = ball.y + (int) disp;
        g.fillOval(ball.x, newY, 10, 10);
    }
    
    // Computes the displacement of a moving ball
    // with the given initial velocity, acceleration, and time.
    // displacement = v0 t + 1/2 a t^2
    public static double displacement(double v0, double t, double a) {
        double d = v0 * t + 0.5 * a * Math.pow(t, 2);
        return d;
    }
}