adventofcode/2024/06/main.py

#!/usr/bin/env python
# -*- encoding: utf-8; py-indent-offset: 4 -*-

# Author: Sebastian Mark
# CC-BY-SA (https://creativecommons.org/licenses/by-sa/4.0/deed.de)

# pylint: disable=missing-module-docstring,missing-function-docstring,consider-using-f-string


from copy import deepcopy


def readinput() -> str:
    with open("input", "r", encoding="utf-8") as file:
        lines = [list(line.strip()) for line in file]
    return lines


def find_guard(maze: list) -> tuple:
    for idx, row in enumerate(maze):
        try:
            x = row.index("^")
            y = idx
        except ValueError:
            pass
        else:
            return (x, y)
    return tuple(-1, -1)


def move_guard(maze: list, guard: tuple) -> tuple[int, bool, dict]:
    width = len(maze[0])
    height = len(maze)

    path_history = {}

    # remember the start position as visited
    visited = set()
    visited.add(guard)

    # start upwards
    direction = (0, -1)

    while True:
        # get next position
        nxt = (
            guard[0] + direction[0],
            guard[1] + direction[1],
        )

        # check borders
        if not (0 <= nxt[0] < width and 0 <= nxt[1] < height):
            visited.add(guard)
            break

        # check for obstacle and rotate clockwise
        if maze[nxt[1]][nxt[0]] == "#":
            direction = (-direction[1], direction[0])
            continue

        # loop detection
        # (has the guard been here and walking in the same direction?)
        if nxt not in path_history:
            path_history[nxt] = (guard, direction)
        elif path_history[nxt] == (guard, direction):
            return 0, True, {}

        # mark spot as visited and move guard
        visited.add(guard)
        guard = nxt

    return len(visited), False, path_history


def print_maze(maze: list):
    for row in maze:
        print("".join(row))
    print("-")


def main():
    # part 1
    maze = readinput()
    guard = find_guard(maze)
    count, _, guard_path = move_guard(maze, guard)
    print("Guard positions: %d" % count)

    # part 2
    count = 0
    maze = readinput()
    guard = find_guard(maze)

    # place an obstacle at every position in the guards path
    # then check new maze for loop
    for col, row in guard_path:
        if (col, row) == guard:
            continue
        new_maze = deepcopy(maze)
        new_maze[row][col] = "#"
        _, loop, _ = move_guard(new_maze, guard)
        if loop:
            count += 1

    print("Loop options: %d" % count)


if __name__ == "__main__":
    main()