lwhjp

import Data.Char
import Data.List

readInput :: String -> (Char, Int, String)
readInput s =
  let [d, n, c] = words s
   in (head d, read n, drop 2 $ init c)

boundary :: [(Char, Int)] -> [(Int, Int)]
boundary = scanl' step (0, 0)
  where
    step (x, y) (d, n) =
      let (dx, dy) = case d of
            'U' -> (0, 1)
            'D' -> (0, -1)
            'L' -> (-1, 0)
            'R' -> (1, 0)
       in (x + n * dx, y + n * dy)

area :: [(Char, Int)] -> Int
area steps =
  let a = -- shoelace formula
        (abs . (`quot` 2) . sum)
          . (zipWith (\(x, y) (x', y') -> x * y' - x' * y) <*> tail)
          $ boundary steps
   in a + 1 + sum (map snd steps) `quot` 2 -- Pick's theorem

part1, part2 :: [(Char, Int, String)] -> Int
part1 = area . map (\(d, n, _) -> (d, n))
part2 = area . map (\(_, _, c) -> decode c)
  where
    decode s = ("RDLU" !! digitToInt (last s), read $ "0x" ++ init s)

main = do
  input <- map readInput . lines <$> readFile "input18"
  print $ part1 input
  print $ part2 input

import Control.Monad
import Data.Array.Unboxed (UArray)
import qualified Data.Array.Unboxed as Array
import Data.Char
import qualified Data.HashSet as Set
import qualified Data.PQueue.Prio.Min as PQ

readInput :: String -> UArray (Int, Int) Int
readInput s =
  let rows = lines s
   in Array.amap digitToInt
        . Array.listArray ((1, 1), (length rows, length $ head rows))
        $ concat rows

walk :: (Int, Int) -> UArray (Int, Int) Int -> Int
walk (minStraight, maxStraight) grid = go Set.empty initPaths
  where
    initPaths = PQ.fromList [(0, ((1, 1), (d, 0))) | d <- [(0, 1), (1, 0)]]
    goal = snd $ Array.bounds grid
    go done paths =
      case PQ.minViewWithKey paths of
        Nothing -> error "no route"
        Just ((n, (p@(y, x), hist@((dy, dx), k))), rest)
          | p == goal && k >= minStraight -> n
          | (p, hist) `Set.member` done -> go done rest
          | otherwise ->
              let next = do
                    h'@((dy', dx'), _) <-
                      join
                        [ guard (k >= minStraight) >> [((dx, dy), 1), ((-dx, -dy), 1)],
                          guard (k < maxStraight) >> [((dy, dx), k + 1)]
                        ]
                    let p' = (y + dy', x + dx')
                    guard $ Array.inRange (Array.bounds grid) p'
                    return (n + grid Array.! p', (p', h'))
               in go (Set.insert (p, hist) done) $
                    (PQ.union rest . PQ.fromList) next

main = do
  input <- readInput <$> readFile "input17"
  print $ walk (0, 3) input
  print $ walk (4, 10) input

(edited for readability)

import Control.Monad
import Data.Array.Unboxed (UArray)
import qualified Data.Array.Unboxed as A
import Data.Foldable
import Data.Set (Set)
import qualified Data.Set as Set

type Pos = (Int, Int)

readInput :: String -> UArray Pos Char
readInput s =
  let rows = lines s
   in A.listArray ((1, 1), (length rows, length $ head rows)) $ concat rows

energized :: (Pos, Pos) -> UArray Pos Char -> Set Pos
energized start grid = go Set.empty $ Set.singleton start
  where
    go seen beams
      | Set.null beams = Set.map fst seen
      | otherwise =
          let seen' = seen `Set.union` beams
              beams' = Set.fromList $ do
                ((y, x), (dy, dx)) <- toList beams
                d'@(dy', dx') <- case grid A.! (y, x) of
                  '/' -> [(-dx, -dy)]
                  '\\' -> [(dx, dy)]
                  '|' | dx /= 0 -> [(-1, 0), (1, 0)]
                  '-' | dy /= 0 -> [(0, -1), (0, 1)]
                  _ -> [(dy, dx)]
                let p' = (y + dy', x + dx')
                    beam' = (p', d')
                guard $ A.inRange (A.bounds grid) p'
                guard $ beam' `Set.notMember` seen'
                return beam'
           in go seen' beams'

part1 = Set.size . energized ((1, 1), (0, 1))

part2 input = maximum counts
  where
    (_, (h, w)) = A.bounds input
    starts =
      concat $
        [[((y, 1), (0, 1)), ((y, w), (0, -1))] | y <- [1 .. h]]
          ++ [[((1, x), (1, 0)), ((h, x), (-1, 0))] | x <- [1 .. w]]
    counts = map (\s -> Set.size $ energized s input) starts

main = do
  input <- readInput <$> readFile "input16"
  print $ part1 input
  print $ part2 input

A whopping 130.050 line-seconds!