Advent of code 2021 day 18

def parse_snail(line):
  return eval(line)  # Unsafe.

def explode_one(snail) -> bool:

  def add_to_middle(snail, side, value):
    side1 = 1 - side
    while not isinstance(snail[side], int):
      snail = snail[side]
      side = side1
    snail[side] += value

  def recurse(snail, depth=0):
    for i, child in enumerate(snail):
      if not isinstance(child, int):
        if depth == 3:
          snail[i] = 0
          add_to_middle(snail, 1 - i, child[1 - i])
          child[1 - i] = None
          return True, child
        done, left_right = recurse(child, depth + 1)
        if done:
          if left_right[1 - i] is not None:
            add_to_middle(snail, 1 - i, left_right[1 - i])
            left_right[1 - i] = None
          return done, left_right
    return False, [None, None]

  return recurse(snail)[0]

def split_one(snail) -> bool:
  for i, child in enumerate(snail):
    if isinstance(child, int):
      if child >= 10:
        snail[i] = [child // 2, (child + 1) // 2]
        return True
    elif split_one(snail[i]):
      return True
  return False

def reduce_snail(snail):
  while explode_one(snail) or split_one(snail):
    pass
  return snail

def magnitude(snail):
  return (snail if isinstance(snail, int) else
          3 * magnitude(snail[0]) + 2 * magnitude(snail[1]))

def process1(s, part2=False):
  lines = s.strip('\n').split('\n')
  snails = [parse_snail(line) for line in lines]

  if not part2:
    snail = snails[0]
    for i in range (1, len(snails)):
      snail = reduce_snail([snail, snails[i]])
    return magnitude(snail)

  return max(magnitude(reduce_snail(copy.deepcopy([snail0, snail1])))
             for snail0, snail1 in itertools.permutations(snails, 2))