2021: d23: ex1: add solution

2021/d23/ex1/ex1.py

@@ -0,0 +1,200 @@
+#!/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()