int[][] grid;
int row;
int col;
int dir = 0;
final int DIR_UP = 0;
final int DIR_RIGHT = 1;
final int DIR_DOWN = 2;
final int DIR_LEFT = 3;

void setup()
{
  size(500, 500);
  background(255);

  grid = new int[width][height];
  col = width / 2;
  row = height / 2;
}

void draw()
{
  go();
}

void go()
{
  int pix = col + row * width;
  int state = grid[row][col];

  loadPixels();

  if(state == 0)
  {
    turn(1);
    grid[row][col] = 1;

    pixels[pix] = color(255);
  }
  else
  {
    turn(-1);
    grid[row][col] = 0;

    pixels[pix] = color(0);
  }

  updatePixels();

  move();
}

void turn(int rotation)
{
  dir += rotation;

  if(dir < 0)
  {
    dir = 3;
  }
  else if(dir > 3)
  {
    dir = 0;
  }

  // Does not work, can return negative values
  //dir = (dir + rotation) % 4;
}

void move()
{
  switch(dir)
  {
    case DIR_UP:
      row--;
    case DIR_RIGHT:
      col++;
    case DIR_LEFT:
      col--;
    case DIR_DOWN:
      row++;
  }

  if(col < 0)
    col = width - 1;
  else if(col >= width)
    col = 0;

  if(row < 0)
    row = height - 1;
  else if(row >= height)
    row = 0;
}