/*
 * gc1.java - 27 Apr 08 - width 409, height 337
 * @author jackord@kw.igs.net
 * magnetic fieldlines about parallel wires carrying
 *   currents of -I, 2I, and -I
 */

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

public class gc1 extends Applet implements ActionListener {
  int d, x1, y1, x2, y2; int kk=0;                        // Declarations
  double bx, by;
  int [] c=new int[4];
  int [] p=new int[4];
  int [] q=new int[4];
  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
    c[1]=-1; c[2]=2; c[3]=-1; d=1;
    p[1]=121; q[1]=216; p[2]=204; q[2]=72; p[3]=287; q[3]=216;
    g.setFont(new Font("TimesRoman", Font.PLAIN, 14));
    g.setColor(Color.gray);
    for (int i=1; i<=3; i=i+1) {
      g.fillArc(p[i]-12, q[i]-12, 24, 24, 0, 360);
    }
    g.setColor(Color.black);
    for (int i=1; i<=3; i=i+1) {
      g.drawArc(p[i]-12, q[i]-12, 24, 24, 0, 360);
    }
    g.drawString("2I", p[2]-6, q[2]+6);
    g.drawString("-I", p[1]-6, q[1]+6);
    g.drawString("-I", p[3]-6, q[3]+6);
    g.drawRect(0, 0, 408, 336);
  }

  public void paint(Graphics g) {                      // Main Program
    double x, y;
    pin(g);
    if (kk>0) {                                        // Plot Fieldlines
      g.setColor(Color.blue);
      for (int i=-15; i<=11; i=i+1) {
        x=204; y=170-6*i;
        x1=(int)(x); y1=(int)(y);
        fdln(x, y, g);
      }
    }
    kk=0;
  }

  public void fdln(double x, double y, Graphics g) {   // Fieldline
    double dx, dxi, dy, z;
    mfld(x, y); dx=-bx*d; dy=-by*d; dxi=dx;
    do {
      mfld(x+dx/2, y+dy/2);
      dx=-bx*d; dy=-by*d; z=x; x=x+dx; y=y+dy;
      x2=(int)(x+.5); y2=(int)(y+.5);
      g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
    } while (((z-204)*dxi>=0)||((x-204)*dxi<0));
  }

  public void mfld(double x, double y) {               // Calculate bx, by
    double b, r2;
    bx=0; by=0;
    for (int i=1; i<=3; i=i+1) {
      r2=(x-p[i])*(x-p[i])+(y-q[i])*(y-q[i]);
      bx=bx+c[i]*(q[i]-y)/r2; by=by+c[i]*(x-p[i])/r2;
    }
    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();
  }

}