for each y going from -1 to MAX_Y:
  for each x going from -1 to MAX_X:
    if (x, y) is air:
      if (x, y + 1) is NOT air:
        draw a south facing wall at (x, y + 1)
      if (x + 1, y) is NOT air:
        draw a west facing wall at (x + 1, y)
    else:
      if (x, y + 1) IS air:
        draw a north facing wall at (x, y)
      if (x + 1, y) IS air:
        draw an east facing wall at (x, y)