Advent of code 2021 day 14

def process1(s, part2=False):
  lines = s.strip('\n').split('\n')
  start = lines[0]
  pairs = collections.Counter(start[i:i+2] for i in range(len(start) - 1))
  rules = dict(rule.split(' -> ') for rule in lines[2:])

  for step in range(40 if part2 else 10):
    new_pairs = collections.Counter()
    for pair, count in pairs.items():
      new_pairs[pair[0] + rules[pair]] += count
      new_pairs[rules[pair] + pair[1]] += count
    pairs = new_pairs

  letters_in_pairs = collections.Counter()
  for pair, count in pairs.items():
    for letter in pair:
      letters_in_pairs[letter] += count

  letters_in_pairs[start[0]] += 1
  letters_in_pairs[start[-1]] += 1
  return (max(letters_in_pairs.values()) - min(letters_in_pairs.values())) // 2