// PIMC oscillator,  Evgeny Demidov, 1 July 2004
  import java.awt.*;
  import java.awt.event.*;
  public class applet-math-pimc extends java.applet.Applet
   implements ActionListener{
    int  nt = 100, nt2=nt-2, np = 200, np2 = np/2, k = 1,
      w, h, h2, y0, lbSize = 50, it = 0, io, P[], stP=200;
    double x[], stX=.01;
    long generTime;
    Button btStep;
  
  public void init() {
    w = getSize().width;  h = getSize().height - lbSize;
    h2 = h/2; y0 = h2 + lbSize;
    x = new double[nt];
    for (int i = 0; i < nt; i++) x[i] = 0;
    P = new int[np];
    for (int i = 0; i < np; i++) P[i] = 0;
    String s=getParameter("k"); if (s != null) k = Integer.parseInt(s);
    btStep = new Button("Step"); btStep.addActionListener(this); add(btStep);
    mc();
  }
  
  public void mc(){
    double dx, xnew, xr, ds;
    generTime = System.currentTimeMillis();
    io = 0;
    for (int i = 0; i < 1000000; i++){
     int r = (int)(nt*Math.random());
     if (r<1) r=1;   if (r>nt2) r=nt2;
     xr = x[r];
     dx = 1*(Math.random()-.5);   xnew = xr + dx;
     ds = dx*(2*(xnew+xr-x[r+1]-x[r-1]) + xnew+xr );
     if ( (ds<0) || (Math.random() < Math.exp(-ds)) ){
       x[r] = xr = xnew;  io++;}
     int l = (int)(30*xr) + np2;
     if (l>0 && l<np) P[l]++;}
    it++;
    generTime = System.currentTimeMillis()-generTime;
  //   System.out.println(""+ds+"  "+Math.exp(-ds)+"  "+r+"   "+i);
  }
  
  public void paint(Graphics g) {
    g.setColor(Color.white);   g.fillRect(0, 0, w, h+lbSize);
    g.setColor(Color.black);   g.drawLine(0,y0 ,w,y0);
    int xt, x0=(int)(x[0]/stX)+y0;
    g.setColor(Color.red);
    for (int t = 1; t < nt; t++){
      xt = (int)(x[t]/stX) + y0;
      g.drawLine(3*(t-1),x0 ,3*t,xt);
      x0 = xt;}
    g.setColor(Color.blue);
    int pt, p0 = (int)(P[0]/(it*stP)) + 400, t0=y0-np2, t1=t0+1;
    for (int t = 1; t < np; t++){
      pt = (int)(P[t]/(it*stP)) + 400;
      g.drawLine(p0,t+t0, pt,t+t1);
      p0 = pt;}
    showStatus( "T=" + generTime +"   "+io );
  }
  
  public void actionPerformed(ActionEvent e){
    mc();  repaint();
  }
  
  public void update(Graphics g){ paint(g); }
  }