Day 16: Reindeer Maze

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FAQ

  • @LeixB@lemmy.world
    link
    fedilink
    310 hours ago

    Haskell

    code
    import Control.Arrow
    import Control.Monad
    import Control.Monad.RWS
    import Control.Monad.Trans.Maybe
    import Data.Array.Unboxed
    import Data.List
    import Data.Map qualified as M
    import Data.Maybe
    import Data.Set qualified as S
    
    data Dir = N | S | W | E deriving (Show, Eq, Ord)
    type Maze = UArray Pos Char
    type Pos = (Int, Int)
    type Node = (Pos, Dir)
    type CostNode = (Int, Node)
    type Problem = RWS Maze [(Node, [Node])] (M.Map Node Int, S.Set (CostNode, Maybe Node))
    
    parse = toMaze . lines
    
    toMaze :: [String] -> Maze
    toMaze b = listArray ((0, 0), (n - 1, m - 1)) $ concat b
      where
        n = length b
        m = length $ head b
    
    next :: Int -> (Pos, Dir) -> Problem [CostNode]
    next c (p, d) = do
        m <- ask
    
        let straigth = fmap ((1,) . (,d)) . filter ((/= '#') . (m !)) . return $ move d p
            turn = (1000,) . (p,) <$> rot d
    
        return $ first (+ c) <$> straigth ++ turn
    
    move N = first (subtract 1)
    move S = first (+ 1)
    move W = second (subtract 1)
    move E = second (+ 1)
    
    rot d
        | d `elem` [N, S] = [E, W]
        | otherwise = [N, S]
    
    dijkstra :: MaybeT Problem ()
    dijkstra = do
        m <- ask
        visited <- gets fst
        Just (((cost, vertex@(p, _)), father), queue) <- gets (S.minView . snd)
    
        let (prevCost, visited') = M.insertLookupWithKey (\_ a _ -> a) vertex cost visited
    
        case prevCost of
            Nothing -> do
                queue' <- lift $ foldr S.insert queue <$> (fmap (,Just vertex) <$> next cost vertex)
                put (visited', queue')
                tell [(vertex, maybeToList father)]
            Just c -> do
                if c == cost
                    then tell [(vertex, maybeToList father)]
                    else guard $ m ! p /= 'E'
                put (visited, queue)
        dijkstra
    
    solve b = do
        start <- getStart b
        end <- getEnd b
        let ((m, _), w) = execRWS (runMaybeT dijkstra) b (M.empty, S.singleton (start, Nothing))
            parents = M.fromListWith (++) w
            endDirs = (end,) <$> [N, S, E, W]
            min = minimum $ mapMaybe (`M.lookup` m) endDirs
            ends = filter ((== Just min) . (`M.lookup` m)) endDirs
            part2 =
                S.size . S.fromList . fmap fst . concat . takeWhile (not . null) $
                    iterate (>>= flip (M.findWithDefault []) parents) ends
        return (min, part2)
    
    getStart :: Maze -> Maybe CostNode
    getStart = fmap ((0,) . (,E) . fst) . find ((== 'S') . snd) . assocs
    
    getEnd :: Maze -> Maybe Pos
    getEnd = fmap fst . find ((== 'E') . snd) . assocs
    
    main = getContents >>= print . solve . parse