/*
 * fcom.java - revised 21 Mar 08 - width 361, height 241
 * @author jackord@kw.igs.net
 *   motion of two particles attracted to each other
 *   by an inverse square-law force
 * orbits plotted in either LAB or CofM frame
 */

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

public class fcom extends Applet
                   implements ActionListener { 
  int kk=0;                          
  String b1s="LAB"; Button b1=new Button(b1s);
  String b2s="C M"; Button b2=new Button(b2s);
  Image aim, bim;
  Graphics agr, bgr;

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

  public void paint(Graphics g) {
    double k, ma, mb, t, dt, dx, dy, r3;
    double xa, ya, aax, aay;
    double xb, yb, abx, aby;
    double vax, vay, vbx, vby, vcx, vcy;
    int xa1, ya1, xa2, ya2, xb1, yb1, xb2, yb2, xo, yo, del;
    xo=1; yo=1; 
    k=60000; ma=2; mb=3; dt=.01;         
    xa=12; ya=20; xb=72; yb=20; t=0;               
    vax=0; vay=-10.8; vbx=0; vby=7.2; vcx=3.6; vcy=-2.4;
    g.setColor(Color.black); g.drawRect(0, 0, 360, 240);
    g.setColor(Color.red);
    g.fillArc(9, 207, 9, 9, 0, 360);
    g.fillArc(69, 207, 9, 9, 0, 360);
    g.drawArc(9, 207, 8, 8, 0, 360);
    g.drawArc(69, 207, 8, 8, 0, 360);
    g.setColor(Color.blue);
    g.drawLine(13, 211, 31, 145); g.drawLine(31, 145, 33, 151);
    g.drawLine(33, 151, 26, 149); g.drawLine(26, 149, 31, 145);
    g.drawLine(73, 211, 91, 235); g.drawLine(91, 235, 85, 234);
    g.drawLine(85, 234, 91, 229); g.drawLine(91, 229, 91, 235);
    if (kk>0) {
      long tt=System.currentTimeMillis()+1000; del=10;
      while (System.currentTimeMillis()<tt) { }
      if (kk==1) {
        aim=createImage(358, 238);              
        agr=aim.getGraphics();
        bim=createImage(358, 238);             
        bgr=bim.getGraphics();
        ya=210; yb=210;
        vax=vax+vcx; vay=vay+vcy; vbx=vbx+vcx; vby=vby+vcy;
      }
      if (kk==2) {
        aim=createImage(80, 48);              
        agr=aim.getGraphics();
        bim=createImage(80, 48);                 
        bgr=bim.getGraphics();
        g.setColor(new Color(211, 211, 211)); g.fillRect(12, 143, 80, 93);
        g.clearRect(12, 143, 80, 93);
      }
      agr.setColor(Color.red);
      bgr.setColor(Color.blue);
      dx=xb-xa; dy=yb-ya; r3=Math.pow(dx*dx+dy*dy, 1.5);
      aax=k/ma*dx/r3; aay=k/ma*dy/r3;            
      abx=-k/mb*dx/r3; aby=-k/mb*dy/r3;
      xa1=(int)(xa); ya1=(int)(ya);           
      xb1=(int)(xb); yb1=(int)(yb);
      vax=vax+aax*dt/2; vay=vay+aay*dt/2;           
      vbx=vbx+abx*dt/2; vby=vby+aby*dt/2;
      tt=System.currentTimeMillis(); 
      for (int i=1; i<=1250; i=i+1) {
        for (int j=1; j<=6; j=j+1) { 
          xa=xa+vax*dt; ya=ya+vay*dt;              
          xb=xb+vbx*dt; yb=yb+vby*dt; t=t+dt;
          dx=xb-xa; dy=yb-ya; r3=Math.pow(dx*dx+dy*dy, 1.5);
          aax=k/ma*dx/r3; aay=k/ma*dy/r3;          
          abx=-k/mb*dx/r3; aby=-k/mb*dy/r3;
          vax=vax+aax*dt; vay=vay+aay*dt;          
          vbx=vbx+abx*dt; vby=vby+aby*dt;
        }
        xa2=(int)(xa); ya2=(int)(ya);
        xb2=(int)(xb); yb2=(int)(yb);
        bgr.drawLine(xa1, ya1, xa2, ya2);
        bgr.drawLine(xb1, yb1, xb2, yb2);
        agr.drawImage(bim, 0, 0, null);
        agr.fillArc(xa2-4, ya2-4, 9, 9, 0, 360);
        agr.fillArc(xb2-4, yb2-4, 9, 9, 0, 360);
        agr.drawArc(xa2-4, ya2-4, 8, 8, 0, 360);
        agr.drawArc(xb2-4, yb2-4, 8, 8, 0, 360);
        if (kk==2) {
          xo=1+(int)(t*vcx);
          yo=191+(int)(t*vcy);
        }
        g.drawImage(aim, xo, yo, null);
        xa1=xa2; ya1=ya2; xb1=xb2; yb1=yb2;
        tt=tt+del;
        while (System.currentTimeMillis()<tt) { }
      }
    }
    kk=0;
  }

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