advent-of-code/2023/day16.py

import os
import sys
from typing import Literal, TypeAlias, cast

VERBOSE = os.getenv("AOC_VERBOSE") == "True"

CellType: TypeAlias = Literal[".", "|", "-", "\\", "/"]
Direction: TypeAlias = Literal["R", "L", "U", "D"]

Mappings: dict[
    CellType,
    dict[
        Direction,
        tuple[tuple[tuple[int, int, Direction], ...], tuple[Direction, ...]],
    ],
] = {
    ".": {
        "R": (((0, +1, "R"),), ("R", "L")),
        "L": (((0, -1, "L"),), ("R", "L")),
        "U": (((-1, 0, "U"),), ("U", "D")),
        "D": (((+1, 0, "D"),), ("U", "D")),
    },
    "-": {
        "R": (((0, +1, "R"),), ("R", "L")),
        "L": (((0, -1, "L"),), ("R", "L")),
        "U": (((0, +1, "R"), (0, -1, "L")), ("U", "D")),
        "D": (((0, +1, "R"), (0, -1, "L")), ("U", "D")),
    },
    "|": {
        "U": (((-1, 0, "U"),), ("U", "D")),
        "D": (((+1, 0, "D"),), ("U", "D")),
        "R": (((-1, 0, "U"), (+1, 0, "D")), ("R", "L")),
        "L": (((-1, 0, "U"), (+1, 0, "D")), ("R", "L")),
    },
    "/": {
        "R": (((-1, 0, "U"),), ("R", "D")),
        "L": (((+1, 0, "D"),), ("L", "U")),
        "U": (((0, +1, "R"),), ("U", "L")),
        "D": (((0, -1, "L"),), ("R", "D")),
    },
    "\\": {
        "R": (((+1, 0, "D"),), ("R", "U")),
        "L": (((-1, 0, "U"),), ("L", "D")),
        "U": (((0, -1, "L"),), ("U", "R")),
        "D": (((0, +1, "R"),), ("L", "D")),
    },
}


def propagate(
    layout: list[list[CellType]], start: tuple[int, int], direction: Direction
) -> list[list[tuple[Direction, ...]]]:
    n_rows, n_cols = len(layout), len(layout[0])

    beams: list[list[tuple[Direction, ...]]] = [
        [() for _ in range(len(layout[0]))] for _ in range(len(layout))
    ]

    queue = [(start, direction)]

    while queue:
        (row, col), direction = queue.pop()

        if (
            row not in range(0, n_rows)
            or col not in range(0, n_cols)
            or direction in beams[row][col]
        ):
            continue

        moves, update = Mappings[layout[row][col]][direction]

        beams[row][col] += update

        for move in moves:
            queue.append(((row + move[0], col + move[1]), move[2]))

    return beams


layout: list[list[CellType]] = [
    [cast(CellType, col) for col in row] for row in sys.stdin.read().splitlines()
]


beams = propagate(layout, (0, 0), "R")

if VERBOSE:
    print("\n".join(["".join("#" if col else "." for col in row) for row in beams]))

# part 1
answer_1 = sum(sum(map(bool, row)) for row in beams)
print(f"answer 1 is {answer_1}")

# part 2
n_rows, n_cols = len(layout), len(layout[0])
cases: list[tuple[tuple[int, int], Direction]] = []

for row in range(n_rows):
    cases.append(((row, 0), "R"))
    cases.append(((row, n_cols - 1), "L"))
for col in range(n_cols):
    cases.append(((0, col), "D"))
    cases.append(((n_rows - 1, col), "U"))

answer_2 = max(
    sum(sum(map(bool, row)) for row in propagate(layout, start, direction))
    for start, direction in cases
)
print(f"answer 2 is {answer_2}")