World w;
  int numcells = 0;
  int maxcells = 6700;
  Cell[] cells = new Cell[maxcells];
  color spore_color;
  // set lower for smoother animation, higher for faster simulation
  int runs_per_loop = 10000;
  color black = color(0, 0, 0);
    
  void setup()
  {
    size(640, 200, P2D);
    frameRate(24);
    clearscr();  
    w = new World();
    spore_color = color(172, 255, 128);
    seed();
  }
  
  void seed() 
  {
    // Add cells at random places
    for (int i = 0; i < maxcells; i++)
    {
      int cX = (int)random(width);
      int cY = (int)random(height);
      if (w.getpix(cX, cY) == black)
      {
        w.setpix(cX, cY, spore_color);
        cells[numcells] = new Cell(cX, cY);
        numcells++;
      }
    }
  }
  
  void draw()
  {
    // Run cells in random order
    for (int i = 0; i < runs_per_loop; i++) {
      int selected = min((int)random(numcells), numcells - 1);
      cells[selected].run();
    }
  }
  
  void clearscr()
  {
    background(0);
  }
  
  class Cell
  {
    int x, y;
    Cell(int xin, int yin)
    {
      x = xin;
      y = yin;
    }
  
      // Perform action based on surroundings
    void run()
    {
      // Fix cell coordinates
      while(x < 0) {
        x+=width;
      }
      while(x > width - 1) {
        x-=width;
      }
      while(y < 0) {
        y+=height;
      }
      while(y > height - 1) {
        y-=height;
      }
      
      // Cell instructions
      if (w.getpix(x + 1, y) == black) {
        move(0, 1);
      } else if (w.getpix(x, y - 1) != black && w.getpix(x, y + 1) != black) {
        move((int)random(9) - 4, (int)random(9) - 4);
      }
    }
    
    // Will move the cell (dx, dy) units if that space is empty
    void move(int dx, int dy) {
      if (w.getpix(x + dx, y + dy) == black) {
        w.setpix(x + dx, y + dy, w.getpix(x, y));
        w.setpix(x, y, color(0));
        x += dx;
        y += dy;
      }
    }
  }
  
  //  The World class simply provides two functions, get and set, which access the
  //  display in the same way as getPixel and setPixel.  The only difference is that
  //  the World class's get and set do screen wraparound ("toroidal coordinates").
  class World
  {
    void setpix(int x, int y, int c) {
      while(x < 0) x+=width;
      while(x > width - 1) x-=width;
      while(y < 0) y+=height;
      while(y > height - 1) y-=height;
      set(x, y, c);
    }
    
    color getpix(int x, int y) {
      while(x < 0) x+=width;
      while(x > width - 1) x-=width;
      while(y < 0) y+=height;
      while(y > height - 1) y-=height;
      return get(x, y);
    }
  }
  
  void mousePressed()
  {
    numcells = 0;
    setup();
  }