import acm.graphics.*;
import acm.program.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
public class BouncingBalls extends GraphicsProgram {
    public void run() {
        Random rand = new Random();
        for(int i = 0; i < 10; i++) {
            double x = rand.nextDouble() * (this.getWidth() - 20);
            double y = rand.nextDouble() * (this.getHeight() - 20);
            this.add(new Ball(x, y, 20));
        }
        
        while(true) {
            this.pause(40);
            
            for(int i = 0; i < this.getElementCount(); i++) {
                Ball b = (Ball) this.getElement(i);
                b.updateDirection(this);
            }
            
            for(int i = 0; i < this.getElementCount(); i++) {
                Ball b = (Ball) this.getElement(i);
                b.step();
            }
        }
    }
}
class Ball extends GOval {
    private double direction;
    
    public Ball(double x, double y, double diam) {
        super(x, y, diam, diam);
        setFilled(true);
        setFillColor(Color.RED);
        direction = 2 * Math.PI * Math.random();
    }
    
    public void updateDirection(GraphicsProgram win) {
        if(this.getX() < 0 || this.getX() + this.getWidth() > win.getWidth()) {
            direction = Math.PI - direction;
        }
        if(this.getY() < 0 || this.getY() + this.getHeight() > win.getHeight()) {
            direction = -direction;
        }
        double myRad = this.getWidth() / 2;
        for(int i = 0; i < win.getElementCount(); i++) {
            Ball b = (Ball) win.getElement(i);
            if(b != this) {
                double bRad = b.getWidth() / 2;
                double dx = (b.getX() + bRad) - (this.getX() + myRad);
                double dy = (b.getY() + bRad) - (this.getY() + myRad);
                if(dx * dx + dy * dy < (myRad + bRad) * (myRad + bRad)) {
                    direction = Math.PI + direction;
                }
            }
        }
    }
    
    public void step() {
        this.move(5 * Math.cos(direction), 5 * Math.sin(direction));
    }
}