/*
 * fele.java - revised 9 Jan 08 - width 401, height 201
 * @author jackord@kw.igs.net
 * motion of electrons in a field with cylindrical symmetry
 * field set up by conducting plates in the x-z plane with a narrow gap
 *   between them - positive (attractive) plate in the left half-plane
 *                - negative (repulsive) plate in the right half-plane
 * the source emits monoenergetic electrons in the x-y plane
 */

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

public class fele extends Applet 
                  implements ActionListener {
  int kk=0; String bs="Plot"; Button b=new Button(bs);

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

  public void paint(Graphics g) {
    double dt, x, y, vx, vy, r2, vo, phi;
    int x1, y1, x2, y2, del;
    g.setColor(Color.black); g.drawRect(0, 0, 400, 200);
    g.setColor(Color.magenta); g.drawLine(0, 200, 200, 200);
    g.setColor(Color.green); g.drawLine(200, 200, 400, 200);
    g.setColor(Color.darkGray);
    for (int i=40; i<=160; i=i+40) {                   // Field lines
      g.drawArc(200-i, 200-i, 2*i, 2*i, 0, 180);
    }
    g.setColor(Color.red); g.fillArc(300-5, 100-5, 10, 10, 0, 360);
    g.setColor(Color.blue);
    vo=Math.sqrt(Math.PI/2); dt=1;
    if (kk==1) {
      long tt=System.currentTimeMillis(); del=5;
      for (int jt=0; jt<=11; jt=jt+1) {                                             // Angle loop
        phi=Math.PI*jt/6; vx=vo*Math.cos(phi); vy=vo*Math.sin(phi);
        x=100; y=100; r2=x*x+y*y; x1=200+(int)(x); y1=200-(int)(y);
        vx=vx-y/r2*dt/2; vy=vy+x/r2*dt/2;              // Half-step
        do {                                           // Feynman loop
          x=x+vx*dt; y=y+vy*dt; x2=200+(int)(x); y2=200-(int)(y);
          g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
          r2=x*x+y*y; vx=vx-y/r2*dt; vy=vy+x/r2*dt;
          tt=tt+del; while (tt>System.currentTimeMillis()) { }
        } while ((x1>0)&&(x1<400)&&(y1>0)&&(y1<200)&&(kk>0));  
      }
      kk=0;
    }
  }

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