Advent of code 2020 day 17

def apply_cycles(s, num_cycles=6, dim=3):
  lines = s.strip('\n').split('\n')
  indices = {
      (0,) * (dim - 2) + (y, x)
      for y, line in enumerate(lines) for x, ch in enumerate(line)
      if ch == '#'
  }
  offsets = set(itertools.product(*((-1, 0, 1),) * dim)) - {(0,) * dim}

  def neighbors(index):
    for offset in offsets:
      yield tuple(map(sum, zip(index, offset)))

  def count_neighbors(index):
    return sum(neighbor in indices for neighbor in neighbors(index))

  for _ in range(num_cycles):
    survivors = {index for index in indices if 2 <= count_neighbors(index) <= 3}
    adjacents = {neighbor for index in indices for neighbor in neighbors(index)}
    births = {index for index in adjacents if count_neighbors(index) == 3}
    indices = survivors | births

  return len(indices)