/*
 * gc2.java - revised 27 Apr 08 - width 409, height 337
 * @author jackord@kw.igs.net
 * magnetic fieldlines in the x-y plane due to a circular
 *   current-carrying loop centered in the x-z plane
 */

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

public class gc2 extends Applet implements ActionListener {

  int phase; int kk=0;                                 // Declarations
  double Bx, By;
  String bs = "Plot"; Button b = new Button(bs);

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

  public void pin(Graphics g) {                        // Initialize
    g.setColor(Color.black);
    g.drawRect(0, 0, 408, 336);
    g.setColor(Color.red); g.fillRect(132, 167, 144, 3);
  }

  public void paint(Graphics g) {                      // Main Program
    pin(g);
    if (kk>0) {                                        
      g.setColor(Color.blue);                          // Plot Fieldlines
      for (int i=-13; i<=13; i=i+2) {
        phase=1; fdln(i, -14, g);
      }
      for (int i=-14; i<=-2; i=i+4) {
        phase=2; fdln(-17, i, g);
      }
      for (int i=-15; i<=-7; i=i+4) {
        phase=4; fdln(i, 0, g);
      }
      for (int i=-14; i<=-2; i=i+4) {
        phase=3; fdln(17, i, g);
      }
      for (int i=7; i<=15; i=i+4) {
        phase=4; fdln(i, 0, g);
      }
    }
    kk=0;
  }

  public void fdln(double x, double y, Graphics g) {   // Fieldline
    double d, dx, dyi, dy, z;
    int x1, y1, x2, y2; int test=0;
    d=0.1; 
    x1=(int)(12*(x+17)); y1=(int)(12*(14-y));
    bfld(x, y); dx=Bx*d; dy=By*d; dyi=dy;
    do {
      bfld(x+dx/2, y+dy/2);
      dx=Bx*d; dy=By*d; z=y; x=x+dx; y=y+dy;
      x2=(int)(12*(x+17)); y2=(int)(12*(14-y));
      g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
      if ((phase==1) && (y>14)) { test=1; }
      if ((phase==2) && (x<-17)) { test=1; }
      if ((phase==3) && (x>17)) { test=1; }
      if ((phase==4) && (z*dyi<0) && (y*dyi>0)) { test=1; }
    } while (test==0);
  }

  public void bfld(double x, double y) {               // Calculate Bx, By
    double b, dt, r3, sn; int r=6;
    dt=Math.PI/100;
    Bx=0; By=0;
    for (int i=1; i<=200; i=i+1) {
      sn=Math.sin(i*dt);
      r3=Math.pow(x*x+y*y+r*r-2*x*r*sn, 1.5);
      Bx=Bx+y*sn/r3; By=By+(r-x*sn)/r3;
    }
    b=Math.sqrt(Bx*Bx+By*By); Bx=Bx/b; By=By/b;
  }

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

}