/*
 * fosb.java - revised 9 Jan 08 - width 381, height 241
 * an undamped oscillator driven at its natural frequency
 * @author jackord@kw.igs.net
 */

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

public class fosb extends Applet 
                   implements ActionListener {
  int kk=0;                                            // Declarations
  String bs="Plot"; Button b=new Button(bs);
  Image bim;
  Graphics bgr;

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

  public void paint(Graphics g) {                      // Paint
    int x1, x2, y1, y2, z1, z2, s1, s2, r, d, del;
    double m, k, f, ff, w, dt, x, v, pi, q;
    int [] xs=new int[121];                            // Spring drawing x's
    int [] xa=new int[7];                              // Arrow drawing x's
    int [] ya=new int[7];                              // Arrow drawing y's
    m=2; k=8; f=6; x1=20; y1=120; z1=120; s1=0; r=6;
    w=Math.sqrt(k/m); pi=Math.PI; dt=pi/100;
    for (int i=0; i<=120; i=i+1) {
      xs[i]=10+(int)(8*Math.sin(i*pi/10));
    }
    xa[0]=9; xa[1]=6; xa[2]=10; xa[3]=14; xa[4]=11; xa[5]=11; xa[6]=9;
    g.setColor(Color.black);
    g.drawRect(0, 0, 380, 240); g.drawLine(20, 120, 375, 120);
    g.setColor(Color.red);                             // Draw mass
    g.fillArc(10-r, z1-r, 2*r, 2*r, 0, 360);
    g.setColor(Color.black);                           // Draw spring
    for (int i=1; i<=120; i=i+1) {
      g.drawLine(xs[i-1], 241-i, xs[i], 240-i);
    }
    if (kk==1) {
      bim=createImage(19, 241);                        // Animation buffer
      bgr=bim.getGraphics();
      bgr.drawLine(0, 0, 18, 0); bgr.drawLine(0, 240, 18, 240);
      bgr.drawLine(0, 0, 0, 240);
      x=0; v=0;                                        // Feynman algorithm
      long tt=System.currentTimeMillis(); del=12;
      for (int jt=1; jt<=710; jt=jt+1) {
        ff=f*Math.sin(w*jt*dt); x=x+v*dt; v=v+(ff-k*x)/m*dt;
        x2=20+jt/2; d=(int)(5*ff); y2=120-d; z2=120-(int)(5*x);
        g.setColor(Color.blue); g.drawLine(x1, y1, x2, y2);
        g.setColor(Color.red); g.drawLine(x1, z1, x2, z2);
        x1=x2; y1=y2; z1=z2;
        bgr.setColor(getBackground());                 // Clear plot buffer
        bgr.fillRect(1, 1, 18, 239);
        bgr.setColor(Color.red);                       // Draw mass
        bgr.fillArc(10-r, z2-r, 2*r, 2*r, 0, 360);
        bgr.setColor(Color.black); s1=240;             // Draw spring
        for (int i=1; i<=120; i=i+1) {
          s2=240-(int)((120+5*x)*i/120);
          bgr.drawLine(xs[i-1], s1, xs[i], s2); s1=s2;
        }
        if (Math.abs(d)>3) {                           // Draw arrow
          ya[0]=z2-d+4*Math.abs(d)/d; ya[1]=ya[0]; ya[3]=ya[0]; ya[4]=ya[0];
          ya[2]=z2-d;
          ya[5]=z2; ya[6]=ya[5];
          bgr.setColor(Color.blue);
          bgr.drawPolygon(xa, ya, 7);
        }
        g.drawImage(bim, 0, 0, null);                  // Show plot buffer
        tt=tt+del; while (tt>System.currentTimeMillis()) { }
      }
      kk=0;
    }
  }

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