/// Returns cost to destination, plus list of routes to destination.
/// Does Dijkstra/A* search depending on whether heuristic returns 1 or
/// something better.
(num, List<List<T>>) aStarSearch<T>(T start, Map<T, num> Function(T) fNext,
    int Function(T) fHeur, bool Function(T) fAtEnd,
    {multiplePaths = false}) {
  var cameFrom = SetMultimap<T, T>.fromEntries([MapEntry(start, start)]);

  var ends = <T>{};
  var front = PriorityQueue<T>((a, b) => fHeur(a).compareTo(fHeur(b)))
    ..add(start);
  var cost = <T, num>{start: 0};
  while (front.isNotEmpty) {
    var here = front.removeFirst();
    if (fAtEnd(here)) {
      ends.add(here);
      continue;
    }
    var ns = fNext(here);
    for (var n in ns.keys) {
      var nCost = cost[here]! + ns[n]!;
      if (!cost.containsKey(n) || nCost < cost[n]!) {
        cost[n] = nCost;
        front.add(n);
        cameFrom.removeAll(n);
        cameFrom[n].add(here);
      }
      if (multiplePaths && cost[n] == nCost) cameFrom[n].add(here);
    }
  }

  Iterable<List<T>> routes(T h) sync* {
    if (h == start) {
      yield [h];
      return;
    }
    for (var p in cameFrom[h]) {
      yield* routes(p).map((e) => e + [h]);
    }
  }

  if (ends.isEmpty) return (-1, []);
  var minCost = ends.map((e) => cost[e]!).min;
  ends = ends.where((e) => cost[e]! == minCost).toSet();
  return (minCost, ends.fold([], (s, t) => s..addAll(routes(t).toList())));
}

import Control.Monad
import Data.Ix
import Data.List
import Data.Map qualified as Map
import Data.Maybe
import Data.Set (Set)
import Data.Set qualified as Set

readInput :: String -> [(Int, Int)]
readInput = map readCoords . lines
  where
    readCoords l = let (a, _ : b) = break (== ',') l in (read a, read b)

findRoute :: (Int, Int) -> Set (Int, Int) -> Maybe [(Int, Int)]
findRoute goal blocked = go Set.empty (Map.singleton (0, 0) [])
  where
    go seen paths
      | Map.null paths = Nothing
      | otherwise =
          (paths Map.!? goal)
            `mplus` let seen' = Set.union seen (Map.keysSet paths)
                        paths' =
                          (`Map.withoutKeys` seen')
                            . foldl' (flip $ uncurry Map.insert) Map.empty
                            . concatMap (\(p, path) -> (,p : path) <$> step p)
                            $ Map.assocs paths
                     in go seen' paths'
    step (x, y) = do
      (dx, dy) <- [(0, -1), (0, 1), (-1, 0), (1, 0)]
      let p' = (x + dx, y + dy)
      guard $ inRange ((0, 0), goal) p'
      guard $ p' `Set.notMember` blocked
      return p'

dropAndFindRoutes goal skip bytes =
  let drops = drop skip $ zip bytes $ drop 1 $ scanl' (flip Set.insert) Set.empty bytes
   in zip (map fst drops) $ scanl' go (findRoute goal (snd $ head drops)) $ tail drops
  where
    go route (p, blocked) = do
      r <- route
      if p `elem` r then findRoute goal blocked else route

main = do
  input <- readInput <$> readFile "input18"
  let routes = dropAndFindRoutes (70, 70) 1024 input
  print $ length <$> (snd . head) routes
  print $ fst <$> find (isNothing . snd) routes

import Control.Arrow
import Control.Monad
import Data.Bifunctor hiding (first, second)

import Data.Set (Set)
import Data.Map (Map)

import qualified Data.List as List
import qualified Data.Set as Set
import qualified Data.Map as Map
import qualified Data.Maybe as Maybe

parse :: String -> [(Int, Int)]
parse = map (join bimap read) . map (break (== ',') >>> second (drop 1)) . filter (/= "") . lines

lowerBounds = (0, 0)
exitPosition = (70, 70)
initialBytes = 1024

adjacent (py, px) = Set.fromDistinctAscList [(py-1, px), (py, px-1), (py, px+1), (py+1, px)]

data Cost = Wall | Explored Int
        deriving (Show, Eq)

inBounds (py, px)
        | py < 0 = False
        | px < 0 = False
        | py > fst exitPosition = False
        | px > snd exitPosition = False
        | otherwise = True

dijkstra :: Map Int (Set (Int, Int)) -> Map (Int, Int) Cost -> (Int, (Int, Int), Map (Int, Int) Cost)
dijkstra queue walls
        | Map.null queue = (-1, (-1, -1), Map.empty)
        | minPos == exitPosition = (minKey, minPos, walls)
        | Maybe.isJust (walls Map.!? minPos) = dijkstra remainingQueue' walls
        | not . inBounds $ minPos = dijkstra remainingQueue' walls
        | otherwise = dijkstra neighborQueue updatedWalls
        where
                ((minKey, posSet), remainingQueue) = Maybe.fromJust . Map.minViewWithKey $ queue
                (minPos, remainingPosSet) = Maybe.fromJust . Set.minView $ posSet
                remainingQueue' = if not . Set.null $ remainingPosSet then Map.insert minKey remainingPosSet remainingQueue else remainingQueue
                neighborQueue = List.foldl (\ m n -> Map.insertWith (Set.union) neighborKey (Set.singleton n) m) remainingQueue' neighbors
                updatedWalls = Map.insert minPos (Explored minKey) walls
                neighborKey = minKey + 1
                neighbors = adjacent minPos

isExplored :: Cost -> Bool
isExplored Wall = False
isExplored (Explored _) = True

findPath :: Int -> (Int, Int) -> Map (Int, Int) Cost -> [(Int, Int)]
findPath n p ts
        | p == lowerBounds = [lowerBounds]
        | n == 0 = error "Out of steps when tracing backwards"
        | List.null neighbors = error "No matching neighbors when tracing backwards"
        | otherwise = p : findPath (pred n) (fst . head $ neighbors) ts
        where
                neighbors = List.filter ((== Explored (pred n)) . snd) . List.filter (isExplored . snd) . List.map (join (,) >>> second (ts Map.!)) . List.filter inBounds . Set.toList . adjacent $ p

runDijkstra = flip zip (repeat Wall)
        >>> Map.fromList
        >>> dijkstra (Map.singleton 0 (Set.singleton lowerBounds))

fst3 :: (a, b, c) -> a
fst3 (a, _, _) = a

thrd :: (a, b, c) -> c
thrd (_, _, c) = c

part1 = take initialBytes
        >>> runDijkstra
        >>> \ (n, _, _) -> n

firstFailing :: [(Int, Int)] -> [[(Int, Int)]] -> (Int, Int)
firstFailing path (bs:bss)
        | List.last bs `List.notElem` path = firstFailing path bss
        | c == (-1) = List.last bs
        | otherwise = firstFailing (findPath c p ts) bss
        where
                (c, p, ts) = runDijkstra bs

part2 bs = repeat
        >>> zip [initialBytes..length bs]
        >>> map (uncurry take)
        >>> firstFailing path
        $ bs
        where
                (n, p, ts) = runDijkstra . take 1024 $ bs
                path = findPath n p ts

main = getContents
        >>= print
        . (part1 &&& part2)
        . parse