Advent of code 2023 day 10

def day10(s, *, part2=False):
  grid = np.array([list(line) for line in s.splitlines()])
  ((y, x),) = np.argwhere(grid == 'S')
  loop = []
  dy, dx = 1, 0  # The start had a downward loop arm in all examples I saw.
  while (ch := grid[y, x]) != 'S' or not loop:
    loop.append((y, x))
    dy, dx = (dx, dy) if ch in 'L7' else (-dx, -dy) if ch in 'JF' else (dy, dx)
    y, x = y + dy, x + dx

  if not part2:
    return len(loop) // 2

  grid2 = np.full(np.array(grid.shape) * 2 + 1, 1)
  for (y0, x0), (y1, x1) in itertools.pairwise(loop + [loop[0]]):
    grid2[y0 * 2 + 1, x0 * 2 + 1] = grid2[y0 + y1 + 1, x0 + x1 + 1] = 0

  skimage.segmentation.flood_fill(grid2, (0, 0), 0, in_place=True)
  return np.count_nonzero(grid2[1::2, 1::2])