/*
 * fplw.java - revised 26 Mar 08 - width 401, height 401
 * central-force motion in a power-law force field
 * @author jackord@kw.igs.net
 */

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

public class fplw extends Applet
                   implements ActionListener { 
  int kk=0; int first=0;                               // Declarations
  String b1s="-2"; Button b1=new Button(b1s);
  String b2s="-1"; Button b2=new Button(b2s);
  String b3s="+1"; Button b3=new Button(b3s);
  Thread tmr;

  public void init() {
    add(b1); add(b2); add(b3);
    b1.addActionListener(this);
    b2.addActionListener(this);
    b3.addActionListener(this);
    setBackground(new Color(211, 211, 211));
  }

  public void paint(Graphics g) {
    double t, dt, x, dx, y, dy, vx, vy, xo, yo, rm, rmin, rc;
    int x1, y1, x2, y2, test, loop;
    if (first==0) {
      b1.setBounds(6, 4, 26, 18);                      // screen layout
      b2.setBounds(42, 4, 26, 18);
      b3.setBounds(78, 4, 26, 18);
      g.setFont(new Font("TimesRoman", Font.PLAIN, 14));
      first=1;
    }
    pin(g);
    dt=.001; test=1; loop=1; rmin=0; rc=0; xo=0; yo=0;
    t=0; x=1; y=0; vx=0; vy=.488; x1=380; y1=200;
    long tt=System.currentTimeMillis();
    if (kk==-2) {                                      // gravity
      rm=Math.pow(x*x+y*y, 1.5); vx=vx-x/rm*dt/2; vy=vy-y/rm*dt/2;
      do {                                             // Feynman loop
        xo=x; yo=y; t=t+dt; x=x+vx*dt; y=y+vy*dt;
        if ((test==1)&&(y<=0)&&(yo>0)) {
          rmin=Math.abs(x-vx*y/vy); test=0;
        }
        x2=200+(int)(180*x+.5); y2=200-(int)(180*y+.5);
        g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
        rm=Math.pow(x*x+y*y, 1.5); vx=vx-x/rm*dt; vy=vy-y/rm*dt;
        if ((y>=0)&&(yo<0)) { loop=0; }
        while ((System.currentTimeMillis()-tt)<3000*t) { }
      } while (loop==1);
      t=t-y/vy; rc=x-vx*y/vy;
      g.setColor(Color.black);
      g.drawString("Gravity: n=-2", 4, 314);
      g.drawString("Closure Test: Rmax = "+(float)(rc), 4, 330);
      g.drawString("Closest Approach: Rmin = "+(float)(rmin), 4, 346);
      g.drawString("Apsidal Angle = 180 degrees", 4, 362);
      g.drawString("Torbit/Tcircle = "+(float)(t/2/Math.PI), 4, 378);
      g.drawString("Kepler's Law a^1.5 = "+(float)(Math.pow((rc+rmin)/2,
        1.5))+" where a = (Rmin+Rmax)/2", 4, 394);
    }
    if (kk==-1) {                                      // inverse R
      rm=x*x+y*y; vx=vx-x/rm*dt/2; vy=vy-y/rm*dt/2;
      do {                                             // Feynman loop
        if (test==1) {
          xo=x; yo=y; rmin=rm;
        }
        t=t+dt; x=x+vx*dt; y=y+vy*dt;
        x2=200+(int)(180*x+.5); y2=200-(int)(180*y+.5);
        g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
        rm=x*x+y*y; vx=vx-x/rm*dt; vy=vy-y/rm*dt;
        if (rm>rmin) { test=0; }
        while ((System.currentTimeMillis()-tt)<500*t) { }
      } while (t<47.5);
      rmin=Math.sqrt(rmin);
      g.setColor(Color.black);
      g.drawString("Inverse R: n=-1", 4, 362);
      g.drawString("Rmin = "+(float)(rmin), 4, 378);
      g.drawString("Apsidal Angle = "+(float)(180*
        (1+Math.atan(yo/xo)/Math.PI))+" = 360*11/32 degrees", 4, 394);
    }
    if (kk==1) {                                       // spring
      vx=vx-x*dt/2; vy=vy-y*dt/2;
      do {                                             // Feynman loop
        xo=x; yo=y; t=t+dt; x=x+vx*dt; y=y+vy*dt;
        if ((test==1)&&(x<=0)&&(xo>0)) {
          rmin=y-vy*x/vx; test=0;
        }
        x2=200+(int)(180*x+.5); y2=200-(int)(180*y+.5);
        g.drawLine(x1,y1,x2,y2); x1=x2; y1=y2;
        vx=vx-x*dt; vy=vy-y*dt;
        if ((y>=0)&&(yo<0)) { loop=0; }
        while ((System.currentTimeMillis()-tt)<1300*t) { }
      } while (loop==1);
      t=t-y/vy; rc=x-vx*y/vy;
      g.setColor(Color.black);
      g.drawString("Spring: n=+1", 4, 330);
      g.drawString("Closure Test: Rmax = "+(float)(rc), 4, 346);
      g.drawString("Closest Approach : Rmin = "+(float)(rmin), 4, 362);
      g.drawString("Apsidal Angle = 90 degrees", 4, 378);
      g.drawString("Torbit/Tcircle = "+(float)(t/2/Math.PI), 4, 394);
    }
    kk=0; 
  }

  public void pin(Graphics g) {
    g.drawRect(0, 0, 400, 400); g.drawString("Select Power", 16, 36);
    g.setColor(Color.green); g.fillArc(182, 182, 37, 37, 0, 360);
    g.setColor(Color.red); g.fillArc(375, 195, 11, 11, 0, 360);
    g.setColor(Color.black);
    g.drawLine(377, 176, 380, 170);
    g.drawLine(380, 200, 380, 170);
    g.drawLine(383, 176, 380, 170);
    g.setColor(Color.blue);
  }

  public void actionPerformed(ActionEvent e) {         // buttons
    String tst;
    tst=e.getActionCommand();
    if (b1s.equals(tst)) { kk=-2; }
    if (b2s.equals(tst)) { kk=-1; }
    if (b3s.equals(tst)) { kk=1; }
    repaint();
  }
}