/*
 * forb.java - revised 8 Apr 05 - width 401, height 301
 * @author jackord@kw.igs.net
 */

import java.applet.Applet;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class forb extends Applet 
                    implements ActionListener {
  int kk=0;                                            // Declarations
  String bs="Plot"; Button b=new Button(bs);

  public void init() {
    setLayout(new FlowLayout(FlowLayout.LEFT));
    add(b);
    b.addActionListener(this);
    setBackground(new Color(211, 211, 211));
  }

  public void paint(Graphics g) {                      // Paint
    double G, M, GM, R, d, dcube, dmax, phi, phiv;
    double dt, v, t, x, y, vx, vy, xmin, ymax, sc;
    int x1, y1, x2, y2;
    xmin=-2E+07; ymax=1.5E+07; sc=1E-05;
    g.setColor(Color.black);
    g.drawRect(0, 0, 400, 300);
    g.setFont(new Font("TimesRoman", Font.PLAIN, 14));
    R=6370000;
    x1=(int)(sc*(-R-xmin)); y1=(int)(sc*(ymax-R));
    y2=(int)(2*sc*R);
    g.setColor(Color.green); g.fillArc(x1, y1, y2+1, y2+1, 0, 360);
    g.setColor(Color.black); g.drawArc(x1, y1, y2, y2, 0, 360);
    g.setColor(Color.blue);
    if (kk==1) {
      String ns;
      M=5.98E+24; G=6.672E-11; GM=G*M; d=R; dmax=R;
      dt=.1; v=9000; t=0; x=0; y=R;
      x1=(int)(sc*(x-xmin)); y1=(int)(sc*(ymax-y));
      vx=v/Math.sqrt(2); vy=vx; vy=vy-GM/(R*R)*dt/2;
      do {
        t=t+dt; x=x+vx*dt; y=y+vy*dt;
        x2=(int)(sc*(x-xmin)); y2=(int)(sc*(ymax-y));
        g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
        d=Math.sqrt(x*x+y*y); dcube=d*d*d;
        if (d>dmax) { dmax=d; }
        vx=vx-GM*x/dcube*dt; vy=vy-GM*y/dcube*dt;
      } while (d>R);
      v=Math.sqrt(vx*vx+vy*vy);
      phi=Math.atan(y/x)*180/Math.PI; phiv=Math.atan(vy/vx)*180/Math.PI-phi;
      if (phi<0) { ns="south"; } else { ns="north"; }
      g.setColor(Color.black);
      g.drawString("Lands at "+ns+" latitude "+(float)(Math.abs(phi))+" deg", 10, 45);
      g.drawString("with velocity "+(float)v+" m/s", 10, 60);
      g.drawString("at "+(float)phiv+" deg to the vertical", 10, 75);
      g.drawString("Flight time "+(float)(t/60)+" min", 10, 90);
      g.drawString("dmax = "+(float)(dmax/R)+" R", 10, 105);
    }
    kk=0;
  }

  public void actionPerformed(ActionEvent e) {
    String tst;
    tst=e.getActionCommand();
    if (bs.equals(tst)) { kk=1; }
    repaint();
  }
}