Advent of Code 2025 - Day 04 - Python

from copy import deepcopy
from dataclasses import dataclass
from typing import Iterable


@dataclass(slots=True, frozen=True)
class Floor:
    paper_locations: frozenset[complex]


def parse_data(fp: str) -> Floor:
    paper_locations = set()
    with open(fp, "r") as f:
        for y, line in enumerate(map(str.strip, f.readlines())):
            for x, v in enumerate(line):
                x_max = x
                y_max = y
                if v == "@":
                    paper_locations.add(complex(x, y))
    return Floor(paper_locations=frozenset(paper_locations))


DIRECTIONS = (
    complex(-1, 1),
    complex(0, 1),
    complex(1, 1),
    complex(-1, 0),
    complex(1, 0),
    complex(-1, -1),
    complex(0, -1),
    complex(1, -1),
)


def count_adjacent(
    position: complex, floor: Floor, directions: Iterable[complex] = DIRECTIONS
) -> int:
    return sum(
        direction + position in floor.paper_locations for direction in directions
    )


def iter_find_accessible_paper_locations(floor: Floor) -> Iterable[complex]:
    for position in floor.paper_locations:
        if count_adjacent(position, floor) < 4:
            yield position


def find_removable_roll_count(floor: Floor) -> int:
    initial_floor = deepcopy(floor)
    current_floor = deepcopy(floor)
    while True:
        removable = set(iter_find_accessible_paper_locations(current_floor))
        next_floor = Floor(frozenset(current_floor.paper_locations - removable))
        if current_floor == next_floor:
            return len(initial_floor.paper_locations - current_floor.paper_locations)
        current_floor = next_floor


def part_a() -> None:
    """
    >>> fp = "day04a.txt"
    >>> floor = parse_data(fp)
    >>> assert len([*iter_find_accessible_paper_locations(floor)]) == 13
    """
    fp = "day04.txt"
    floor = parse_data(fp)
    accessible_paper_locations_count = len(
        [*iter_find_accessible_paper_locations(floor)]
    )
    print(accessible_paper_locations_count)


def part_b():
    """
    >>> fp = "day04a.txt"
    >>> floor = parse_data(fp)
    >>> assert find_removable_roll_count(floor) == 43
    """
    fp = "day04.txt"
    floor = parse_data(fp)
    removable_count = find_removable_roll_count(floor)
    print(removable_count)


if __name__ == "__main__":
    part_a()
    part_b()