Advent of Code 2021 day 12 part 1 iterator version

AoC_2021_12_1.iter.py

"""
My solution for Advent of Code 2021 day 12 part 1.
This version uses an iterator to implement depth-first-search.
The algorithm is based on networkx.all_simple_paths().
"""
from collections import defaultdict
DEAD_END = object()


def route(adjacents):
    curpath = ["start"]
    stack = [iter(adjacents["start"])]
    while stack:
        adjs = stack[-1]
        cave = next(adjs, DEAD_END)
        if cave is DEAD_END:
            curpath.pop()
            stack.pop()
        elif cave in curpath and cave.islower():
            continue
        elif cave == "end":
            yield 1
        else:
            curpath.append(cave)
            stack.append(iter(adjacents[cave]))


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(sum(route(adjacents)))