/*
 * gc3.java - revised 15 Apr 05 - width 409, height 337
 * @author jackord@kw.igs.net
 * magnetic fieldlines in the x-y plane due to
 *   (1) a circular current-carrying loop centered in the x-z plane
 *   (2) a north/south pole pair centered on the y axis
 * when "Far" is selected, the dimensions of the loop and pole pair
 *   are reduced by a factor of ten below their plotted size
 */

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

public class gc3 extends Applet implements ActionListener {
  double Bx, By, rb; int kk=0;                         // Declarations
  String bs = "Plot"; Button b = new Button(bs);
  Checkbox ch=new Checkbox("Far");

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

  public void pin(Graphics g) {                        // Initialize
    int r=3;
    g.setColor(Color.black);
    g.drawRect(0, 0, 408, 336);
    g.setColor(Color.blue); g.drawLine(204, 156, 204, 180);
    g.fillArc(204-r, 156-r, 2*r, 2*r, 0, 360);
    g.fillArc(204-r, 180-r, 2*r, 2*r, 0, 360);
    g.setColor(Color.red); g.fillRect(192, 167, 24, 3);
  }

  public void paint(Graphics g) {                      // Main Program
    double x, y, d, dx, dy, z;
    int x1, y1, x2, y2, r;
    rb=6; if (ch.getState()==true) { rb=rb/10; }
    d=0.5; r=12;
    pin(g);
    if (kk>0) {
      for (int j=1; j<=2; j=j+1) {
        for (int i=5; i<=175; i=i+10) {
          z=i*Math.PI/180; x=r*Math.cos(z); y=r*Math.sin(z);
          x1=(int)(2*x+204); y1=(int)(168-2*y);
          if (j==1) { bfld(x, y); } else { qfld(x,y); }
          dx=Bx*d; dy=By*d;
          do {
            if (j==1) { bfld(x+dx/2, y+dy/2); } else { qfld(x+dx/2, y+dy/2); }
            dx=Bx*d; dy=By*d; x=x+dx; y=y+dy;
            x2=(int)(2*x+204); y2=(int)(168-2*y);
            g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
          } while (x*x+y*y>=r*r);
        }
        g.setColor(Color.blue);
      }
    }
    kk=0;
  }

  public void bfld(double x, double y) {               // Calculate Bx, By
    double b, dt, r3, sn;                              // using Biot-Savart
    dt=Math.PI/30;                                    // law
    Bx=0; By=0;
    for (int i=1; i<=60; i=i+1) {
      sn=Math.sin(i*dt);
      r3=Math.pow(x*x+y*y+rb*rb-2*x*rb*sn, 1.5);
      Bx=Bx+y*sn/r3; By=By+(rb-x*sn)/r3;
    }
    b=Math.sqrt(Bx*Bx+By*By); Bx=Bx/b; By=By/b;
  }

  public void qfld(double x, double y) {               // Calculate Bx, By
    double b, r;                                       // using inverse square
    r=Math.pow(x*x+(y-rb)*(y-rb), 1.5);                // law
    Bx=x/r; By=(y-rb)/r;
    r=Math.pow(x*x+(y+rb)*(y+rb), 1.5);
    Bx=Bx-x/r; By=By-(y+rb)/r;
    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();
  }

}