/*
 * gab.java - revised 11 Jan 10 - width 409, height 337
 * @author jackord@kw.igs.net
 * Fieldlines, equipotentials, and colour potential plots
 *   for equilateral arrays of masses or charges
 */

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

public class gab extends Applet 
                   implements ActionListener {
  int kk=0; int flg, xo, yo;                           // Global variables
  double d, x, y, xx, yy, gx, gy;
  double [] m=new double[4];
  double [] p=new double[4];
  double [] q=new double[4];
  double [] r=new double[4];
  String b1s = "Lplot"; Button b1 = new Button(b1s);
  String b2s = "Cplot"; Button b2 = new Button(b2s);
  CheckboxGroup chg=new CheckboxGroup();
  Checkbox[] ch=new Checkbox[2];
  Color col[]={ Color.magenta, Color.blue, Color.red, Color.green, Color.yellow, Color.cyan };

  public void init() {                                 // Layout
    add(b1); add(b2);
    add(ch[0]=new Checkbox("Gfield", chg, true));
    add(ch[1]=new Checkbox("Efield", chg, false));
    b1.addActionListener(this);
    b2.addActionListener(this);
    setBackground(new Color(211, 211, 211));
    ch[0].setBackground(getBackground());
    ch[1].setBackground(getBackground());
  }

  public void actionPerformed(ActionEvent e) {         // Buttons
    String tst;
    tst=e.getActionCommand();
    if (b1s.equals(tst)) { kk=1; }
    if (b2s.equals(tst)) { kk=2; }
    repaint();
  }

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

  public void fdln(Graphics g) {                       // Fieldline
    double dx, dy, dxi, dyi;
    int x1, y1, x2, y2; int tst=0;
    xx=x; yy=y; x1=(int)(x); y1=(int)(y);
    field(); dx=-gx*d; dy=-gy*d;
    do {
      dxi=dx; dyi=dy; xx=x+dx/2; yy=y+dy/2;
      field(); 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;
      if (flg==1) {
        if ((x<0)||(y<0)||(x>408)||(y>336)) { tst=1; } // Out of bounds
        if ((dxi*dx<0)&&(dyi*dy<0)) { tst=1; }         // Direction reversal
      }
      else {
        if ((r[1]<1728) || (r[3]<1728)) { tst=1; }
      }
    } while (tst==0);
  }

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

  public void paint(Graphics g) {                      // Main Program
    ch[0].setBounds(293, 313, 55, 20);                 // Screen. .
    ch[1].setBounds(348, 313, 55, 20);
    b1.setBounds(5, 313, 35, 20);
    b2.setBounds(45, 313, 35, 20);                     // . .layout
    double r2, u, t;
    int plt, x1, y1, s;
    if (ch[0].getState()==true) { flg=1; } else { flg=2; }
    g.setFont(new Font("Arial", Font.BOLD, 15));
    d=1; xo=204; yo=168; s=166; 
    p[1]=xo-s/2; q[1]=yo+s/2/Math.sqrt(3);             // Define. . 
    p[2]=xo; q[2]=yo-s/Math.sqrt(3);
    p[3]=xo+s/2; q[3]=q[1];
    if (flg==1) { m[1]=1; m[2]=1; m[3]=1; }
    else { m[1]=-1; m[2]=2; m[3]=-1; }                 // . .arrays
    for (int i=1; i<=3; i=i+1) {                       // Initial. .
      x1=(int)(p[i]); y1=(int)(q[i]);
      g.setColor(Color.white);
      g.fillArc(x1-12, y1-12, 25, 25, 0, 360);
      g.setColor(Color.black);
      g.drawArc(x1-12, y1-12, 24, 24, 0, 360);
      if (flg==1) { g.drawString("M", x1-6, y1+6); }
      else { if (i==2) { g.drawString("2Q", x1-10, y1+6); }
             else { g.drawString("-Q", x1-8, y1+6); }
      }
    }
    g.drawRect(0, 0, 2*xo, 2*yo);                      // . .screen
    if (kk==1) {                                        
      if (flg==1) {                                    // Gravity
        g.setColor(Color.red);                         // Start Fieldlines
        for (int j=1; j<=3; j=j+1) {
          for (int i=1; i<=24; i=i+1) {
            t=(2*i-1)*Math.PI/24;
            x=p[j]+12*Math.cos(t); y=q[j]+12*Math.sin(t);
            fdln(g);
          }
        }
        g.setColor(Color.blue);                        // Start Equipotentials
        for (int i=1; i<=11; i=i+1) {
          x=xo; y=yo-6+14*i;
          eqpt(g);
        }
      }
      else {                                           // Electrostatics
        g.setColor(Color.red);                         // Start Fieldlines
        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(g);
        }
        g.setColor(Color.blue);                        // Start Equipotentials
        for (int i=-11; i<=11; i=i+2) {
          x=xo; y=yo+6*i;
          x1=(int)(x); y1=(int)(y);
          eqpt(g);
        }
      }
    }
    if (kk==2) {                                       // Colour potential plot
      for (int k=1; k<=335; k=k+1) {
        for (int j=1; j<=407; j=j+1) {
          u=0; plt=1;
          for (int i=1; i<=3; i=i+1) {
            r2=(p[i]-j)*(p[i]-j)+(q[i]-k)*(q[i]-k);
            if (r2>=144) { u=u+m[i]/Math.sqrt(r2); }
            else { plt=0; }
          }
          if (plt==1) {
            if (flg==1) { g.setColor(col[(int)(285.65*u)%6]); } // G field
            else { g.setColor(col[(int)(288*(.1+u))%6]); }      // E field
            g.drawLine(j, k, j, k);                    
          }
        }
      }
      g.setColor(Color.black);
      for (int i=1; i<=3; i=i+1) {
        g.drawArc((int)(p[i])-12, (int)(q[i])-12, 24, 24, 0, 360);
      }
    }
    kk=0;
  }

}