/*
 * fplw.java - revised 14 Nov 2010- width 401, height 401
 * central-force motion in a power-law force field  F=-k*r^n
 *   for n = 1, 0, -1, -2, -3, and -5
 * @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;                                            // Declarations
  String b1s="Plot"; Button b1=new Button(b1s);
  Choice ch=new Choice();

  public void init() {
    ch.addItem("n=1"); ch.addItem("n=0");
    ch.addItem("n=-1"); ch.addItem("n=-2");
    ch.addItem("n=-3"); ch.addItem("n=-5");
    add(b1); add(ch); 
    b1.addActionListener(this);
    setBackground(new Color(211, 211, 211));
    ch.setBackground(getBackground());
  }

  public void paint(Graphics g) {
    double t, dt, x, dx, y, dy, vx, vy, xo, yo, r, rm, rmin;
    int n, x1, y1, x2, y2, test;
    g.setFont(new Font("Arial", Font.BOLD, 14));
    g.drawString("F = - k*r^n", 6, 18);
    g.setFont(new Font("Arial", Font.PLAIN, 12));
    ch.setBounds(6, 24, 50, 20);                        // screen layout
    b1.setBounds(60, 24, 30, 20);
    g.drawRect(0, 0, 400, 400);
    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);
    n=1-ch.getSelectedIndex(); if (n==-4) { n=-5; }
    dt=.0001; test=1; rmin=0; xo=0; yo=0;
    t=0; x=1; y=0; vx=0; vy=.488; x1=380; y1=200;
    if (n==-3) { vy=.999; }
    if (n==-5) { vy=1/Math.sqrt(2); }
    if (kk==1) {                                      
      r=Math.sqrt(x*x+y*y); vx=vx-dt/2;                // ax=-1, ay=0 at t=0
      do {                                             // Feynman loop
        if (test==1) {
          xo=x; yo=y; rmin=r;
        }
        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;
        r=Math.sqrt(x*x+y*y); rm=Math.pow(r, n-1);
        vx=vx-x*rm*dt; vy=vy-y*rm*dt;
        if (r>rmin) { test=0; }
      } while ((t<47.3)&&(180*r>18));
      if (n>-2.5) {
        g.setColor(Color.magenta); g.fillArc(x2-5, y2-5, 11, 11, 0, 360);
        g.setColor(Color.black);
        g.drawString("Apsidal Angle = "+(float)(180*
          (1+Math.atan(yo/xo)/Math.PI))+" degrees", 4, 394);
        g.drawString("Rmin = "+(float)(rmin), 250, 394);
      }
      if (n==-3) {
        g.setColor(Color.black);
        g.drawString("Vinit = 0.999", 4, 394);
      }
      if (n==-5) {
        t=0; x=1; y=0; vx=-dt/2; vy=-1/Math.sqrt(2); x1=380; y1=200;
        do {
          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;
          r=Math.sqrt(x*x+y*y); rm=Math.pow(r, n-1);
          vx=vx-x*rm*dt; vy=vy-y*rm*dt;
        } while (180*r>18);
        g.setColor(Color.black);
        g.drawString("Vinit = 1/sqrt(2)", 4, 394); 
      }
    }
    kk=0; 
  }

  public void actionPerformed(ActionEvent e) {         // button
    String tst;
    tst=e.getActionCommand();
    if (b1s.equals(tst)) { kk=1; }
    repaint();
  }
}