Advent of code 2024 day 18 faster

def day18(s, *, size=71, n_step=1024, part2=False):  # Recompute only if shortest path is broken.
  blocks = np.array([row.split(',') for row in s.splitlines()], int)[:, ::-1] + 1
  grid = np.pad(np.full((size, size), '.'), 1, constant_values='#')
  grid[tuple(blocks[:n_step].T)] = '#'

  def get_shortest_path() -> list[tuple[int, int]]:
    yx, yxl = (1, 1), (size, size)
    parent = {yx: (-1, -1)}
    lst = [yx]
    while lst:
      lst2 = []
      for yx in lst:
        if yx == yxl:
          path = []
          while yx[0] >= 0:
            path.append(yx)
            yx = parent[yx]
          return path
        y, x = yx
        for dy, dx in ((0, -1), (0, 1), (-1, 0), (1, 0)):
          yx2 = y + dy, x + dx
          if grid[yx2] == '.' and yx2 not in parent:
            parent[yx2] = yx
            lst2.append(yx2)
      lst = lst2
    return []

  if not part2:
    return len(get_shortest_path()) - 1

  shortest_path = set(get_shortest_path())
  for y, x in blocks[n_step:]:
    grid[y, x] = '#'
    if (y, x) in shortest_path:
      shortest_path = set(get_shortest_path())
      if not shortest_path:
        return f'{x - 1},{y - 1}'