// Instantons,  Evgeny Demidov 10 May 2004
  import java.awt.*;
  import java.awt.event.*;
  import java.util.StringTokenizer;
  
  public class applet-math-inst extends java.applet.Applet
    implements MouseListener, MouseMotionListener, KeyListener{
  Image buffImage;          Graphics buffGraphics;
  int n = 25, ins=1, w,h,h1,w2,
      max=450, m2=max/2, a=max/n, a2=a/2, ar=a2-1, dx,dy,dz;
  double xo=0,yo=0, xr=0,yr=0, st=.3, al=0;
  Label  lbA;    TextField tfA;
  
  public void init() {
    w = Integer.parseInt(getParameter("width"));
    h = Integer.parseInt(getParameter("height"));  h1 = h-1; w2 = w/2;
    String s=getParameter("XY"); if (s != null) {
      StringTokenizer ST = new StringTokenizer(s);
      xo = Double.valueOf(ST.nextToken()).doubleValue();
      yo = Double.valueOf(ST.nextToken()).doubleValue();
      st = Double.valueOf(ST.nextToken()).doubleValue() / max;}
    s = getParameter("alpha"); if (s != null) al=Double.valueOf(s).doubleValue();
    s=getParameter("ins"); if (s != null) ins = Integer.parseInt(s);
    buffImage = createImage(w, h);
    buffGraphics = buffImage.getGraphics();
  //  setBackground(Color.white);
    addMouseListener(this);
    addMouseMotionListener(this);
    this.setLayout( new FlowLayout(FlowLayout.LEFT, 0, 0) );
    lbA = new Label("a");  add(lbA);
    tfA = new TextField( "" + (float)al, 5);  add(tfA);
    tfA.addKeyListener(this);
    drawField();
  }
  
  public void destroy(){
    removeMouseListener(this);
    removeMouseMotionListener(this);
  }
  public void mouseClicked(MouseEvent e) {}      //1.1 event handling
  public void mousePressed(MouseEvent e) {}
  public void mouseEntered(MouseEvent e) {}
  public void mouseExited(MouseEvent  e) {}
  public void mouseMoved(MouseEvent e){}
  public void keyTyped(KeyEvent e){}
  public void keyPressed(KeyEvent e){}
  public void keyReleased(KeyEvent e){
    final int keyEnter = 10;
    if (e.getKeyCode() == keyEnter){
      try{ al = Double.valueOf(tfA.getText()).doubleValue();
      }catch(NumberFormatException ne){}
      drawField();  repaint();
    }
  }
  
  public void mouseReleased(MouseEvent e) {
    double xt = xo+(e.getX()-m2)*st,
           yt = yo-(e.getY()-m2)*st;
    if ( e.isControlDown() ) {
      if ( e.isShiftDown() )  st *= 3;  else  st *= 2;}
    else if ( e.isAltDown() ) {
      if ( e.isShiftDown() )  st /= 3;  else  st /= 2;}
    else return;
    xo = xt;  yo = yt;
    drawField();  repaint();
  }
  
  public void mouseDragged(MouseEvent e) {
    xr = xo + st*(e.getX() - m2);  
    yr = yo - st*(e.getY() - m2);  
    drawField();  repaint();
  }
  
  public void drawField(){
    buffGraphics.setColor(Color.white);
    buffGraphics.fillRect(0,0, w, h);
    buffGraphics.setColor(Color.lightGray);
    buffGraphics.drawRect(0,0, max,max);
    int d = (int)(1/st),
        ix = m2 + (int)((.5*Math.cos(-al*ins)-xo-.5)/st),
        iy = m2 - (int)((-.5*Math.sin(-al*ins)-yo+.5)/st);
    buffGraphics.drawOval(ix,iy, d,d);
    ix = m2 + (int)((xr-xo)/st);  iy = m2 - (int)((yr-yo)/st);
    buffGraphics.drawLine(ix,0, ix, max );
    buffGraphics.drawLine(0,iy, max,iy );
    double x,y;
    for (iy = max-a2; iy>0; iy-=a){
     y = yo - st*(iy-m2);
     N(xr,y); arrow(max+a,iy, -dz,dy);
     for (ix = max-a2; ix>0; ix-=a){
      x = xo + st*(ix-m2);
      N(x,y); arrow(ix,iy, dx,dy);}}
    for (ix = max-a2; ix>0; ix-=a){
     x = xo + st*(ix-m2);
     N(x,yr); arrow(ix,max+a, dx,-dz);}
  }
  
  public void N(double x, double y){
    double r,f, fi,th, c,s,t;
    r = Math.sqrt(x*x+y*y);  f = ins*Math.atan2(y,x) - al;
    c = Math.cos(f);  s = Math.sin(f);  t=r-c;
    fi = Math.atan2(s,t);  th = 2*Math.atan(r/Math.sqrt(t*t+s*s));
  //System.out.println(""+fi+"  "+th);
    dx = (int)(ar*Math.cos(th));  t = ar*Math.sin(th);
    dy = -(int)(t*Math.sin(fi));  dz = (int)(t*Math.cos(fi));
  //  if (dz > 0) buffGraphics.setColor(Color.red);
    if (Math.abs(fi) < Math.PI/2) buffGraphics.setColor(Color.red);
       else buffGraphics.setColor(Color.blue);
  }
  
  public void arrow(int ix, int iy, int dx, int dy){
    int xa = ix + dx,  ya = iy + dy;
    buffGraphics.drawLine(ix-dx,iy-dy, xa,ya );
    int dx2 = dx/3,  dy2 = dy/3;
    buffGraphics.drawLine(xa,ya, ix+dy2, iy-dx2 );
    buffGraphics.drawLine(xa,ya, ix-dy2, iy+dx2 );
  }
  
  public void paint(Graphics g) {
    g.drawImage(buffImage, 0, 0, this);
    showStatus( "x="+(float)xr+"   y="+(float)yr);
  }
  
  public void update(Graphics g){ paint(g); }
  
  }