/** Henon's Strange Attractor Generator
    by Robert J Morton UK-YE572246C
    30 October 1997 */

import java.awt.*;

public class henon extends java.applet.Applet implements Runnable {
  int
    XBIAS = 150,  // pixel co-ordinates of real point 0,0
    YBIAS = 150,
    XMAX  = 300,  // dimensions of the display area (pixels) 
    YMAX  = 300,
    i,            // pixel co-ordinates of the chaotic body
    j;            // within the visible xy plane

  long TF = 150, T;  // inter-plot time frame (milliseconds)

  double
    Scale = 100,       //pixels per unit real eg 100 pixels = 1.00d
    x = 0, y = 0, z;   //real co-ordinates of 'chaotic body'

  Color
    A[][],  // colour array and current plot colour
    colour,
    bgc = Color.black,  // background colour
    fgc = Color.white,  // foreground colour
    axc = Color.gray;   // axes colour

  Thread HT;  // declare a thread reference variable


  public void init() {

    // Create a colour array for the display area.
    A = new Color[XMAX][YMAX];

    // Clear the whole colour array to the background colour.
    for(i = 0 ; i < XMAX ; i++)
      for(j = 0 ; j < YMAX ; j++)
        A[i][j] = bgc;

    // Create the x and y axis graticules.
    for(i = 0 ; i < XMAX ; i++)
      A[i][YBIAS] = axc;
    for(j = 0 ; j < YMAX ; j++)
      A[XBIAS][j] = axc;

    // Set the system time at which the first time frame will finish.
    T = System.currentTimeMillis()  + TF;
  }                                       


  public void paint(Graphics g) {  // PAINT/RE-PAINT THE PICTURE SO FAR

    // For each of the YMAX lines in the picture ...
    for(j = 0 ; j < YMAX ; j++) {

      int I = 0;   //starting dot of next single-colour stretch of line

      /* To find next pixel in current line with a different colour,
      make the colour of first dot the reference colour, then while
      we have not yet reached the end of the current line ... */

      i = 0;
      colour = A[I][j];
      while(++i < XMAX) {

        /* If this dot is a different colour to previous dot, set the drawing
        colour to the colour of previous dot, draw a line from the start dot
        to the previous dot and make the reference colour that of this dot. */

        if(colour != A[i][j]) {
          g.setColor(colour);
          g.drawLine(I,j,i - 1,j);
          colour = A[I = i][j];
        }
      }
      /* Set the colour for the final stretch of the current
      line and draw the last stretch of the current line. */

      g.setColor(colour);
      g.drawLine(I,j,i - 1,j);
    }
  }


  // called in response a request for a repaint()
  public void update(Graphics g) {
    z = x;                          // z inherits last time's value of x
    x = 1.4 * x * x + 0.3 * y - 1;  // advance x
    y = z;                          // advance y

    /* Compute the pixel co-ordinates cor-
    responding to the new values of x and y */
    int
      i = XBIAS + (int)(x * Scale),
      j = YBIAS - (int)(y * Scale);

    A[i][j] = fgc;           // put the plot on the 'canvas'
    g.setColor(fgc);         // set colour for plot
    g.drawLine(i, j, i, j);  // draw the plot in the applet window
  }


  public void run() {  // run the Henon Attractor's painting thread

    while(true) {  // permanent loop
      repaint();   // paint in the next plot

      /* Get the time left in this plot's time frame and
      ensure that it is at lease 5 milliseconds. Then put
      the thread to sleep for this remaining amount of time. */

      long s = T - System.currentTimeMillis(); if(s < 5) s = 5;
      try { Thread.currentThread().sleep(s); }

      /* Catch any external events here during the thread's sleep
      time but do nothing about them because they are normal events. */

      catch (InterruptedException e) { }

      // Set the system time at which the next plots time frame will end.
      T = System.currentTimeMillis() + TF;
    }
  }


  /* Start the program thread by creating the thread object
  and starting it running by requesting a call to run(). */

  public void start() { HT = new Thread(this); HT.start(); }

  public void stop() { HT = null; }  // Stop this a program thread
}