Advent of Code 2021 day 12 part 2

AoC_2021_12_2.py

"""
Advent of Code 2021 day 12 part 2.
"""
from collections import defaultdict


def count_paths(curpath):
    path_count = 0
    cave = curpath[-1]
    twice_done = (
        # wether we have crossed any small room twice:
        max(curpath.count(c) for c in curpath if c.islower())
        > 1
    )
    for adj in adjacents[cave]:
        if adj == "start":
            continue
        if adj.islower() and adj in curpath and twice_done:
            continue
        if adj == "end":
            path_count += 1
        else:
            path_count += count_paths(curpath + [adj])
    return path_count


adjacents = defaultdict(set)
with open("input_12.txt") as f:
    for line in f:
        c1, c2 = tuple(line.strip().split("-"))
        adjacents[c1].add(c2)
        adjacents[c2].add(c1)

print(count_paths(["start"]))