#!/usr/bin/env python import enum import functools import sys from typing import Iterator, List, NamedTuple, Optional, Tuple class Point(NamedTuple): x: int y: int class Amphipod(enum.IntEnum): A = 0 B = 1 C = 2 D = 3 class Direction(enum.IntEnum): ALLEY = 0 ROOM = 1 # 7-length tuple, but easier for mypy in a variadic type Alley = Tuple[Optional[Amphipod], ...] # Actually variadic tuple, instead of a list, for memoization purposes Room = Tuple[Amphipod, ...] # 4-length tuple, but easier for mypy in a variadic type Rooms = Tuple[Room, ...] class Board(NamedTuple): alley: Alley rooms: Rooms class Move(NamedTuple): cost: int new_board: Board FUEL_COST = { Amphipod.A: 1, Amphipod.B: 10, Amphipod.C: 100, Amphipod.D: 1000, } DISTANCE = [ # From room 1 (2, 1, 1, 3, 5, 7, 8), # From room 2 (4, 3, 1, 1, 3, 5, 6), # From room 3 (6, 5, 3, 1, 1, 3, 4), # From room 4 (8, 7, 5, 3, 1, 1, 2), ] AMPHIPOD_FROM_STRING = { "A": Amphipod.A, "B": Amphipod.B, "C": Amphipod.C, "D": Amphipod.D, } ROOM_SIZE = 2 def solve(input: List[str]) -> int: def parse() -> Board: alley: Alley = (None,) * 7 rooms: Rooms = () for i in (3, 5, 7, 9): room: Room = tuple( AMPHIPOD_FROM_STRING[input[j][i]] for j in range(2, 3 + 1) ) rooms = rooms + (room,) return Board(alley, rooms) def room_is_solved(board: Board, amphipod: Amphipod) -> bool: room = board.rooms[amphipod] return len(room) == ROOM_SIZE and all(a == amphipod for a in room) def board_is_solved(board: Board) -> bool: return all(room_is_solved(board, Amphipod(i)) for i in range(len(board.rooms))) def move_cost( board: Board, room: int, alley_spot: int, direction: Direction ) -> Optional[int]: # Going left-to-right, or right-to-left if room < (alley_spot - 1): alley_start = room + 2 # Look at the end spot if we're going to the alley, not if we come from there alley_end = alley_spot + (1 - direction) else: # Look at the first spot if we're going to the alley, not if we come from there alley_start = alley_spot + direction alley_end = room + 2 # Is there any obstacle in the way if any(spot is not None for spot in board.alley[alley_start:alley_end]): return None amphipod = ( board.alley[alley_spot] if direction == Direction.ROOM else board.rooms[room][0] ) assert amphipod is not None # Sanity check return FUEL_COST[amphipod] * ( DISTANCE[room][alley_spot] + direction + ROOM_SIZE - len(board.rooms[room]) ) # Yes this returns a 0-or-1 length iterator, but it's practical for `moves` def alley_moves_for(board: Board, i: int) -> Iterator[Move]: # Return early if we're trying to move out of an empty spot spot = board.alley[i] if spot is None: return # Can't yet move to the target room if any amphipod is out of place there if any(other != spot for other in board.rooms[spot]): return cost = move_cost(board, spot, i, Direction.ROOM) # Can't move there yet, there's an obstacle in the way if cost is None: return # Update the board state alley, rooms = board rooms = rooms[:spot] + ((spot,) + rooms[spot],) + rooms[spot + 1 :] alley = alley[:i] + (None,) + alley[i + 1 :] yield Move(cost, Board(alley, rooms)) def rooms_moves_for(board: Board, i: int) -> Iterator[Move]: room = board.rooms[i] # No need to move out of a solved room if all(a == i for a in room): return for dest in range(len(board.alley)): cost = move_cost(board, i, dest, Direction.ALLEY) # Can't move there yet, there's an obstacle in the way if cost is None: continue # Update the board state alley, rooms = board rooms = rooms[:i] + (room[1:],) + rooms[i + 1 :] alley = alley[:dest] + (room[0],) + alley[dest + 1 :] yield Move(cost, Board(alley, rooms)) def moves(board: Board) -> Iterator[Move]: for i in range(len(board.alley)): yield from alley_moves_for(board, i) for i in range(len(board.rooms)): yield from rooms_moves_for(board, i) @functools.cache def total_cost(board: Board) -> Optional[int]: if board_is_solved(board): return 0 best = None for cost, new_board in moves(board): if (end_cost := total_cost(new_board)) is None: continue cost += end_cost if best is None or cost < best: best = cost return best board = parse() cost = total_cost(board) assert cost is not None # Sanity check return cost def main() -> None: input = [line.rstrip("\n") for line in sys.stdin.readlines()] print(solve(input)) if __name__ == "__main__": main()