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!