Advent of code 2023 day 14

def day14(s, *, part2=False, num=1_000_000_000):
  grid = np.rot90(np.array([list(line) for line in s.splitlines()]))  # North is left.

  def slide_left():
    for row in grid:
      open = 0
      for x, ch in enumerate(row):
        if ch == '.':
          pass
        elif ch == 'O':
          if open < x:
            row[open], row[x] = 'O', '.'
          open += 1
        else:
          open = x + 1

  if not part2:
    slide_left()

  else:
    configs: dict[Any, int] = {}  # hashed_grid -> index.
    period = -1
    index = 0
    while True:
      if period < 0:
        if (prev_index := configs.setdefault(grid.tobytes(), index)) != index:
          period = index - prev_index
          print(f'At {index=}, found cycle with {period=}.')
          index = num - (num - index) % period
      if index == num:
        break
      for _ in range(4):
        slide_left()
        grid = np.rot90(grid, -1)
      index += 1

  return sum(len(grid[0]) - x for _, x in np.argwhere(grid == 'O'))