day8_part2.py

#!/usr/bin/env python3

import re
from itertools import cycle

map = {}

ROUTE = re.compile(r"^(\w{3}) = \((\w{3}), (\w{3})\)$")
index_map = {
    'L': 0,
    'R': 1,
}

points = []
cycles = []


def gcd(a: int, b: int) -> int:
    while b:
        a, b = b, a % b
    return a


def lcm(a: int, b: int) -> int:
    return a * b // gcd(a, b)


with open("input.txt") as f:
    instructions = f.readline().strip()
    f.readline()

    for line in f:
        line = line.strip()

        route = ROUTE.match(line)
        map[route.group(1)] = (route.group(2), route.group(3))
        if route.group(1)[-1] == 'A':
            points.append(route.group(1))

    for i, route in enumerate(cycle(instructions), start=1):
        idx = index_map[route]

        to_remove = []
        for j in range(len(points)):
            points[j] = map[points[j]][idx]
            if points[j][-1] == 'Z':
                cycles.append(i)
                to_remove.append(j)

        for j in to_remove[::-1]:
            del points[j]
        
        if not points:
            break

for i in range(len(cycles) - 1):
    cycles[i + 1] = lcm(cycles[i], cycles[i + 1])

print(cycles[-1])