/*
 * gb1.java - revised 27 Feb 08 - width 409, height 337
 * @author jackord@kw.igs.net
 * fieldlines and equipotentials for a -Q, 2Q, -Q charge array
 */

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

public class gb1 extends Applet implements ActionListener {
  int d, x1, y1, x2, y2; int kk=0;                        // Declarations
  double gx, gy;
  int [] m=new int[4];
  int [] p=new int[4];
  int [] q=new int[4];
  double [] r=new double[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
    m[1]=-1; m[2]=2; m[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("Arial", Font.PLAIN, 12));
    g.setColor(Color.gray);
    for (int i=1; i<=3; i=i+1) {
      g.fillArc(p[i]-12, q[i]-12, 25, 25, 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("2Q", p[2]-7, q[2]+5);
    g.drawString("-Q", p[1]-7, q[1]+5);
    g.drawString("-Q", p[3]-7, q[3]+5);
    g.drawRect(0, 0, 408, 336);
  }

  public void paint(Graphics g) {                      // Main Program
    double x, y, t;
    pin(g);
    if (kk>0) {                                        // Start Fieldlines
      g.setColor(Color.blue);
      for (int i=1; i<=24; i=i+1) {
        t=(2*i-1)*Math.PI/24;
        x=p[2]+12*Math.cos(t); y=q[2]+12*Math.sin(t);
        x1=(int)(x); y1=(int)(y);
        fdln(x, y, g);
      }
      g.setColor(Color.red);                           // Start Equipotentials
      for (int i=-11; i<=11; i=i+2) {
        x=204; y=168+6*i;
        x1=(int)(x); y1=(int)(y);
        eqpt(x, y, g);
      }
    }
    kk=0;
  }

  public void fdln(double x, double y, Graphics g) {   // Fieldline
    double dx, dy;
    grav(x, y); dx=-gx*d; dy=-gy*d;
    do {
      grav(x+dx/2, y+dy/2);
      dx=-gx*d; dy=-gy*d; x=x+dx; y=y+dy;
      x2=(int)(x); y2=(int)(y);
      g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
    } while ((r[1]>1728) && (r[3]>1728));
  }

  public void eqpt(double x, double y, Graphics g) {   // Equipotential
    double dx, dy, xold, dxi;
    grav(x,y); dx=gy*d; dy=-gx*d; dxi=dx;
    do {
      grav(x+dx/2, y+dy/2);
      dx=gy*d; dy=-gx*d; xold=x; x=x+dx; y=y+dy;
      x2=(int)(x); y2=(int)(y);
      g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
    } while (((xold-204)*dxi>=0) || ((x-204)*dxi<=0));
  }

  public void grav(double x, double y) {               // Calculate gx, gy
    double gg;
    gx=0; gy=0;
    for (int i=1; i<=3; i=i+1) {
      r[i]=Math.pow((x-p[i])*(x-p[i])+(y-q[i])*(y-q[i]), 1.5);
      gx=gx+m[i]*(p[i]-x)/r[i]; gy=gy+m[i]*(q[i]-y)/r[i];
    }
    gg=Math.sqrt(gx*gx+gy*gy); gx=gx/gg; gy=gy/gg;
  }

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

}