Advent of code 2023 day 10 fast

def day10(s, *, part2=False):  # Interior test using row intersection counting.
  grid = np.array([list(line) for line in s.splitlines()])
  ((y, x),) = np.argwhere(grid == 'S')
  loop: list[tuple[int, int]] = []
  dy, dx = 1, 0  # The start has a downward loop arm in all the examples I see.
  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

  loop_mask = np.full(grid.shape, False)
  loop_mask[tuple(np.array(loop).T)] = True
  lookup = np.array([chr(i) in '|LJ' for i in range(256)])
  is_wall = lookup[grid.view(np.uint32)]  # ('U1' and uint32 have the same bitwidth.)
  is_wall[~loop_mask] = 0
  count = is_wall.cumsum(1) % 2
  count[loop_mask] = 0
  return count.sum()