import java.awt.*;
import java.lang.Math;
import java.applet.Applet;
import java.util.Vector;

public class Caustic extends Applet {
    int w, h;
    public void init() {
        w = size().width;
        h = size().height;
        if(h > (3*w)/5) h = (3*w)/5;
        resize(w, h);
        setLayout(new BorderLayout());
        DrawPanel dp = new DrawPanel(w, h);
        add("Center", dp);
    }

    public boolean handleEvent(Event e) {
        switch (e.id) {
          case Event.WINDOW_DESTROY:
            System.exit(0);
            return true;
          default:
            return false;
        }
    }
}

class DrawPanel extends Panel {
    Vector lines = new Vector();
    double h2, radius;
    int hh, w1, w2, w3, w4;

    public DrawPanel(int w, int h) {
        setBackground(Color.black);
        radius = 0.4*h;
        h2     = 0.5*h;
        w1 = w - (4*h)/10;
        w2 = w - (2*h)/10;
        w3 = w - h/10;
        w4 = w;
        hh = h;
    }

    public boolean handleEvent(Event e) {
        double y;
        switch (e.id) {
          case Event.MOUSE_DOWN:
            if(e.x >= w1 && e.x <= w2){
                y = (h2 - e.y)/radius;
                if(y < 1.0 && y > -1.0)
                    lines.addElement(new Double(y));
            }
            if(e.x > w3){
                lines.removeAllElements();
            }
            repaint();
            return true;
          case Event.WINDOW_DESTROY:
            System.exit(0);
            return true;
          default:
            return false;
        }
    }

    public void paint(Graphics g) {
        int np = lines.size();
        double x, y, a, b, p, q;
    /* draw the current lines */
        g.setPaintMode();

        g.setColor(Color.white);
        g.drawOval(
            xscale(-1.0), yscale(1.0), 
            (int)(2.0*radius), (int)(2.0*radius));

        g.setColor(Color.yellow);
        g.fillRect(
            w1, yscale(1.0), w2-w1, (int)(2.0*radius));

        g.setColor(Color.red);
        g.fillRect( w3, 0, w4-w3, hh);

        g.setColor(Color.gray);
        for (int i=0; i < np; i++) {
            y = ((Double)lines.elementAt(i)).doubleValue();
            x = -Math.sqrt(1.0 - y*y);
            g.drawLine( xscale(x), yscale(y), w1,  yscale(y));
        }

        g.setColor(Color.yellow);
        for (int i=0; i < np; i++) {
            y = ((Double)lines.elementAt(i)).doubleValue();
            x = -Math.sqrt(1.0 - y*y);
            a = x*x - y*y;
            b = 2.0*x*y;
            p = -a*x + b*y;
            q = -b*x - a*y;
            g.drawLine( xscale(x), yscale(y), xscale(p), yscale(q));
        }
    }

    public int xscale(double x) {
        return (int)(x*radius + h2);
    }

    public int yscale(double y) {
        return (int)(-y*radius + h2);
    }
}