Advent of code 2020 day 24

def process(s, num_days=0):
  offsets = dict(e=(0, 1), w=(0, -1), sw=(1, 0), se=(1, 1),
                 nw=(-1, -1), ne=(-1, 0))
  indices = set()
  for line in s.strip('\n').splitlines():
    index = (0, 0)
    while line:
      for prefix, offset in offsets.items():
        if line.startswith(prefix):
          index = tuple(map(sum, zip(index, offset)))
          line = line[len(prefix):]
    indices ^= {index}

  def neighbors(index):
    for offset in offsets.values():
      # yield tuple(map(sum, zip(index, offset)))  # 2x slower
      yield index[0] + offset[0], index[1] + offset[1]

  for _ in range(num_days):
    neighbor_counts = collections.Counter(
        itertools.chain.from_iterable(neighbors(index) for index in indices))
    indices = {index for index, count in neighbor_counts.items()
               if count == 2 or count == 1 and index in indices}

  return len(indices)