/*
 * frds.java - revised 17 Apr 05 - width 301, height 201
 * @author jackord@kw.igs.net
 * gravitational force between identical rods of length D:
 *   rod A lies along the x-axis with its center at (D/2, 0)
 *   rod B is perpendicular to the x-axis with its center at (2*D, D/2)
 * each rod is made up of N spheres of diameter D/N
 * in the program, D=128, and N=8 or N=16
 * the rods are released from rest and their motion is followed
 *   until they collide
 */

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

public class frds extends Applet 
                     implements ActionListener {
  String b1s="Run"; Button b1=new Button(b1s);
  String b2s="8/16"; Button b2=new Button(b2s);
  int kk=0; int n=8; int d=128; int M=128; int first=0;
  int r, m, mm;
  double xac, yac, xbc, ybc, fax, fay, Ta, Tb, rm;
  double [] xa=new double[33]; double [] ya=new double[33];
  double [] xb=new double[33]; double [] yb=new double[33];
  Image bim; Graphics bgr;

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

  public void paint(Graphics g) {
    double Ic, dt, vax, vay, wa, wb, aa, ab;
    int lim;
    g.setColor(Color.black);
    g.drawRect(0, 0, 300, 200);
    if (first==0) {
      bim=createImage(265, 161);                       // Animation buffer
      bgr=bim.getGraphics();
      first=1;
    }
    int r=d/n/2; lim=8*r*r*r; rm=2*lim; m=M/n; mm=m*m; dt=.1;
    aa=0; ab=Math.PI/2;
    if (kk==0) {
      bgr.clearRect(0, 0, 265, 161);
      bgr.setColor(Color.red);
      for (int i=1; i<=n; i=i+1) {                     // Define and plot rods
        xa[i]=r*(2*i-1); ya[i]=128; xa[i+n]=xa[i]; ya[i+n]=ya[i];
        xb[i]=256; yb[i]=r*(2*i-1); xb[i+n]=xb[i]; yb[i+n]=yb[i];
        bgr.fillArc((int)(xa[i])-r, (int)(ya[i])-r, 2*r, 2*r, 0, 360);
        bgr.fillArc((int)(xb[i])-r, (int)(yb[i])-r, 2*r, 2*r, 0, 360);
      }
      bgr.drawLine(155, 96, 166, 96); bgr.drawLine(160, 91, 160, 102);
      g.drawImage(bim, 20, 30, null);
    }
    Ic=.4*M*r*r;                                       // Moment of Inertia
    for (int i=1; i<=n/2; i=i+1) {
      Ic=Ic+2*m*(xa[i]-d/2)*(xa[i]-d/2);
    }
    if (kk==1) {
      force();                                         // Feynman
      vax=fax/M*dt/2; vay=fay/M*dt/2;                  //   half
      wa=Ta/Ic*dt/2; wb=Tb/Ic*dt/2;                    //   step
      while (rm>lim) {                                 // Loop until collision
        xac=xac+vax*dt; yac=yac+vay*dt;                // Translation
        xbc=xbc-vax*dt; ybc=ybc-vay*dt; 
        aa=aa+wa*dt; ab=ab+wb*dt;                      // Rotation
        for (int i=1; i<=n; i=i+1) {                   // New rod positions
          xa[i]=xac+(2*i-1-n)*r*Math.cos(aa);
          ya[i]=yac+(2*i-1-n)*r*Math.sin(aa);
          xb[i]=xbc+(2*i-1-n)*r*Math.cos(ab);
          yb[i]=ybc+(2*i-1-n)*r*Math.sin(ab);
        }
        bgr.clearRect(0, 0, 265, 161);
        bgr.setColor(Color.red);
        bgr.drawLine(155, 96, 166, 96); bgr.drawLine(160, 91, 160, 102);
        for (int i=2*n; i>=1; i=i-1) {                 // Plot rods
          if (i==n) { bgr.setColor(Color.blue); }
          bgr.fillArc((int)(xa[i])-r, (int)(ya[i])-r, 2*r, 2*r, 0, 360);
          bgr.fillArc((int)(xb[i])-r, (int)(yb[i])-r, 2*r, 2*r, 0, 360);
        }
        g.drawImage(bim, 20, 30, null);
        force();                                       // Newton's law
        vax=vax+fax/M*dt; vay=vay+fay/M*dt;
        wa=wa+Ta/Ic*dt; wb=wb+Tb/Ic*dt;
      }
      aa=-(int)(aa*18000/Math.PI); ab=(int)(ab*18000/Math.PI-9000);
      g.drawString("Rotates "+(aa/100)+" degrees", 20, 120);
      g.drawString("Rotates "+(ab/100)+" degrees", 130, 40);
    }
  }

  public void force() {
    double dx, dy, dfx, dfy, fx, fy, rcube;
    xac=(xa[1]+xa[n])/2; yac=(ya[1]+ya[n])/2;          // C of M
    xbc=(xb[1]+xb[n])/2; ybc=(yb[1]+yb[n])/2;
    fax=0; fay=0; Ta=0; Tb=0;
    for (int i=1; i<=n; i=i+1) {                       // Rod A
      fx=0; fy=0;
      for (int j=1; j<=n; j=j+1) {                     // Rod B
        dx=xb[j]-xa[i]; dy=yb[j]-ya[i];
        rcube=Math.pow(dx*dx+dy*dy, 1.5);
        dfx=mm*dx/rcube;                               // Force between spheres
        dfy=mm*dy/rcube;
        fx=fx+dfx; fy=fy+dfy;
        Tb=Tb-(xb[j]-xbc)*dfy+(yb[j]-ybc)*dfx;         // Torque on rod B
        if (rcube<rm) { rm=rcube; } 
      }
      fax=fax+fx; fay=fay+fy;                          // Force on rod A
      Ta=Ta+(xa[i]-xac)*fy-(ya[i]-yac)*fx;             // Torque on rod A
    }
  }

  public void actionPerformed(ActionEvent e) {         // Buttons
    String tst;
    tst=e.getActionCommand();
    if (b1s.equals(tst)) { kk=kk+1; }                  // Run if initialized
    if (b2s.equals(tst)) { kk=0; n=128/n; }            // Initialize & set n
    repaint();
  }
}