Advent of code 2023 day 21 obscure

def day21_part2(s):
  empty = np.tile(np.array([list(line) for line in s.splitlines()]) != '#', (5, 5))
  a = np.zeros((657, 657), bool)
  a[328, 328] = 1
  for _ in range(327):
    a[1:-1, 1:-1] = empty & (a[:-2, 1:-1] | a[2:, 1:-1] | a[1:-1, :-2] | a[1:-1, 2:])

  counts = a[1:-1, 1:-1].reshape(5, 131, 5, 131).sum((1, 3)).flat
  return (
      counts[12] * 40924885401
      + counts[7] * 40925290000
      + counts[[1, 3, 21, 23]].sum() * 202300
      + counts[[6, 8, 16, 18]].sum() * 202299
      + counts[[2, 10, 14, 22]].sum()
  )