Advent of code 2023 day 19 part2 old

def day19_part2(s):
  s_rules, _ = s.split('\n\n')
  rules = {'R': []}
  for line in s_rules.splitlines():
    name, s_rules = line[:-1].split('{')
    rules[name] = s_rules.split(',')

  total = 0
  initial_intervals = {category: (1, 4001) for category in 'xmas'}  # (start, stop).
  stack = [('in', initial_intervals)]

  while stack:
    name, intervals = stack.pop()
    if name == 'A':
      total += math.prod(stop - start for start, stop in intervals.values())
      continue

    for rule in rules[name]:
      if rule.isalpha():
        stack.append((rule, intervals))
        break

      category, op, s_num, label = re.match(r'^(\w)([<>])(\d+):(\w+)$', rule).groups()
      num = int(s_num)
      start, stop = intervals[category]

      if op == '<':
        if start < num:
          stack.append((label, intervals | {category: (start, min(stop, num))}))
        start = max(start, num)
      else:
        if stop > num + 1:
          stack.append((label, intervals | {category: (max(start, num + 1), stop)}))
        stop = min(stop, num + 1)

      if start < stop:
        intervals[category] = start, stop
      else:
        break

  return total