#!/usr/bin/env python import enum import heapq import sys from collections import defaultdict from typing import Iterator, NamedTuple class Point(NamedTuple): x: int y: int class ParsedMaze(NamedTuple): start: Point end: Point blocks: set[Point] class Direction(enum.IntEnum): EAST = enum.auto() WEST = enum.auto() NORTH = enum.auto() SOUTH = enum.auto() def rotations(self) -> tuple["Direction", "Direction"]: match self: case Direction.EAST | Direction.WEST: return (Direction.NORTH, Direction.SOUTH) case Direction.NORTH | Direction.SOUTH: return (Direction.EAST, Direction.WEST) def step(self, p: Point) -> Point: dx: int dy: int match self: case Direction.EAST: dx, dy = 0, 1 case Direction.WEST: dx, dy = 0, -1 case Direction.NORTH: dx, dy = -1, 0 case Direction.SOUTH: dx, dy = 1, 0 return Point(p.x + dx, p.y + dy) Node = tuple[Point, Direction] def solve(input: str) -> int: def parse(input: list[str]) -> ParsedMaze: start: Point | None = None end: Point | None = None blocks: set[Point] = set() for x, line in enumerate(input): for y, c in enumerate(line): if c == ".": continue p = Point(x, y) if c == "S": start = p elif c == "E": end = p elif c == "#": blocks.add(p) else: assert False # Sanity check assert start is not None # Sanity check assert end is not None # Sanity check return ParsedMaze(start, end, blocks) def count_path_cells(start: Point, end: Point, blocks: set[Point]) -> int: def next_moves( pos: Point, dir: Direction, ) -> Iterator[tuple[int, Point, Direction]]: transitions = [(1, dir.step(pos), dir)] for new_dir in dir.rotations(): transitions.append((1000, pos, new_dir)) for cost, pos, dir in transitions: if pos in blocks: continue yield cost, pos, dir def get_all_predecessors( predecessors: dict[Node, set[Node]], ) -> set[Point]: queue = {(end, dir) for dir in Direction} visited: set[Node] = set() while queue: cur = queue.pop() visited.add(cur) for pred in predecessors[cur]: if pred in visited: continue queue.add(pred) return set(p for p, _ in visited) # Priority queue of (distance, point) queue = [(0, start, Direction.EAST)] seen: set[Node] = set() predecessors: dict[Node, set[Node]] = defaultdict(set) predecessor_cost: dict[Node, int] = {} # Use an invalid maximum cost to simplify the loop max_cost: int = -1 while len(queue) > 0: cost, p, dir = heapq.heappop(queue) # Did we go past the optimal cost, if so stop the loop if max_cost > 0 and cost > max_cost: break # Otherwise, record the minimum cost if p == end: max_cost = cost # We must have seen (p, dir) with a smaller distance before if (p, dir) in seen: continue # First time encountering (p, dir), must be the smallest distance to it seen.add((p, dir)) # Add all neighbours to be visited for n_cost, n, n_dir in next_moves(p, dir): n_cost += cost # Record predecessors if predecessor_cost.setdefault((n, n_dir), n_cost) > n_cost: predecessor_cost[(n, n_dir)] = n_cost predecessors[(n, n_dir)] = {(p, dir)} elif predecessor_cost[(n, n_dir)] == n_cost: predecessors[(n, n_dir)].add((p, dir)) heapq.heappush(queue, (n_cost, n, n_dir)) # Run back up the tree of predecessors to count all cells return len(get_all_predecessors(predecessors)) start, end, blocks = parse(input.splitlines()) return count_path_cells(start, end, blocks) def main() -> None: input = sys.stdin.read() print(solve(input)) if __name__ == "__main__": main()