Advent of Code 2024 day 11 part 2

from functools import cache
from sys import argv

MAX_DEPTH = 75


def parse(src):
    '''Convert the input to a list of integers.'''
    return [int(x) for x in src.split()]


def count_digits(num):
    '''Count the digits of num in base 10'''
    total = 0
    while num > 0:
        total += 1
        num = num // 10
    return total


@cache
def expand(stone, depth=0):
    '''Traverse the 'timeline' depth-first and return the number of stones
       created by blinking at stone from depth to MAX_DEPTH blinks.'''
    while depth < MAX_DEPTH:
        if stone == 0:
            stone = 1
            depth += 1
            continue

        digits = count_digits(stone)
        if digits % 2 == 0:
            mask = 10 ** (digits // 2)
            left = stone // mask
            right = stone % mask

            return expand(left, depth + 1) + expand(right, depth + 1)

        stone *= 2024
        depth += 1

    return 1


def main(stones):
    '''The total number of stones produced after 75 blinks'''
    return sum(expand(stone) for stone in stones)


if __name__ == '__main__':
    # pass the path to the puzzle input as the first argument
    print(main(parse(open(argv[1]).read())))