/*
 * fosa.java - revised 16 Jan 08 - width 425, height 321
 * identical masses hanging from springs one below the other
 * they are released from rest with the upper spring unstretched and
 *   Case 1: with the lower mass supported by the lower spring
 *   Case 2: with the lower mass raised enough to generate the
 *             low-frequency normal mode
 *   Case 3: with the lower mass pulled down enough to generate the
 *             high-frequency normal mode
 * each spring (length 3*d) stretches d under the weight of one mass
 * @author jackord@kw.igs.net
 */

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

public class fosa extends Applet 
                   implements ActionListener {
  int kk=0; int plt=0; int first=0;                    // Declarations
  double dt=.1; double d, ya, va, aa, yb, vb, ab;
  String b1s="Case1"; Button b1=new Button(b1s);
  String b2s="Case2"; Button b2=new Button(b2s);
  String b3s="Case3"; Button b3=new Button(b3s);
  String b4s="Motion"; Button b4=new Button(b4s);
  Image bim;
  Graphics bgr;

  public void init() {
    setBackground(new Color(211, 211, 211));           // Light Gray Background
    add(b1); add(b2); add(b3); add(b4);
    b1.addActionListener(this);
    b2.addActionListener(this);
    b3.addActionListener(this);
    b4.addActionListener(this);
  }

  double td(double yt) {
    double t, ta, tb, vb1;
    t=0; ya=3*d; yb=yt; ta=0; tb=0;
    aa=d-2*ya+yb; ab=4*d-yb+ya;
    va=aa*dt/2; vb=ab*dt/2; vb1=vb;                    // Feynman half-step
    do {
      t=t+dt; ya=ya+va*dt; yb=yb+vb*dt;                // Step ya, yb
      aa=d-2*ya+yb; ab=4*d-yb+ya;                      // Newton's law
      va=va+aa*dt; vb=vb+ab*dt;                        // Step va, vb
      if ((ta==0)&&(va<0)) { ta=t+dt/2-va/aa; }
      if ((tb==0)&&(vb*vb1<0)) { tb=t+dt/2-vb/ab; }
    } while ((ta==0)||(tb==0));
    return Math.abs(ta-tb);
  }

  public void paint(Graphics g) {                      // Paint
    int ya1, ya2, yb1, yb2, s1, s2, r, del, j;
    double pi, dd, yt, x, xo;
    long tt;
    int [] xs=new int[121];                            // Spring drawing x's
    g.setColor(Color.black); g.drawRect(0, 0, 424, 320);
    r=6; d=25; dd=d/4; pi=Math.PI;
    for (int i=0; i<=120; i=i+1) {
      xs[i]=10+(int)(8*Math.sin(i*pi/10));
    }
    if (first==0) {
      bim=createImage(19, 321);                        // Animation buffer
      bgr=bim.getGraphics();
      first=1;
    }
    if (kk>0) {
      if (kk==1) { yb=7*d; }
      if (kk>1) {
        yt=(4*kk-2)*d; x=td(yt);
        do {
          yt=yt+dd; xo=x; x=td(yt);
          if (x>xo) { dd=-dd/2; }
        } while (Math.abs(dd)>.02);
        yb=yt+2*dd;
        g.drawString("Amplitude Ratio = "+(int)(1000*(9-yb/d))/2000., 145, 50);
      }
      j=20; ya=3*d; ya1=(int)(ya); yb1=(int)(yb);
      aa=d-2*ya+yb; ab=4*d-yb+ya;
      va=aa*dt/2; vb=ab*dt/2;                          // Feynman half-step
      tt=System.currentTimeMillis(); del=20;
      g.drawLine(20, 125, 420, 125); g.drawLine(20, 225, 420, 225);
      do {
        ya2=(int)(ya); yb2=(int)(yb);
        if (j>20) {
          g.setColor(Color.blue); g.drawLine(j-1, ya1, j, ya2);
          g.setColor(Color.red); g.drawLine(j-1, yb1, j, yb2);
        }
        ya1=ya2; yb1=yb2;
        bgr.setColor(getBackground());                 // Clear plot buffer
        bgr.fillRect(1, 1, 18, 319);
        bgr.setColor(Color.black); bgr.drawLine(0, 0, 0, 320);
        bgr.drawLine(0, 0, 18, 0); bgr.drawLine(0, 320, 18, 320); s1=0;
        for (int i=1; i<=120; i=i+1) {                 // Draw upper spring
          s2=(int)(ya*i/120);
          bgr.drawLine(xs[i-1], s1, xs[i], s2); s1=s2;
        }
        for (int i=1; i<=120; i=i+1) {                 // Draw lower spring
          s2=(int)(ya+(yb-ya)*i/120);
          bgr.drawLine(xs[i-1], s1, xs[i], s2); s1=s2;
        }
        bgr.setColor(Color.blue);                      // Draw mass A
        bgr.fillArc(10-r, ya2-r, 2*r, 2*r, 0, 360);
        bgr.setColor(Color.red);                       // Draw mass B
        bgr.fillArc(10-r, yb2-r, 2*r, 2*r, 0, 360);
        g.drawImage(bim, 0, 0, null);                  // Show plot buffer
        ya=ya+va*dt; yb=yb+vb*dt;                      // Step ya, yb
        aa=d-2*ya+yb; ab=4*d-yb+ya;                    // Newton's law
        va=va+aa*dt; vb=vb+ab*dt;                      // Step va, vb
        tt=tt+del; while (tt>System.currentTimeMillis()) {  }
        j=j+1; if (j==421) { kk=0; plt=0; }
      } while (plt==1);
    }
  }

  public void actionPerformed(ActionEvent e) {         // Buttons
    String tst;
    tst=e.getActionCommand();
    if (b1s.equals(tst)) { kk=1; }
    if (b2s.equals(tst)) { kk=2; }
    if (b3s.equals(tst)) { kk=3; }
    if (b4s.equals(tst)) { if (kk>0) { plt=1; } }
    repaint();
  }
}