AoC/2020/day10/day10.hs

import Control.Monad
import Data.List

main :: IO ()
main = do
  input <- map read . lines <$> readFile "input"
  print $ (product . map length . group . sort . jolts) input
  print $
    (product . map (arrangements . sum) . filter (elem 1) . group . jolts) input

arrangements :: Int -> Int
arrangements =
  (\x -> length . filter (== x)) <*>
  (map sum . nub . map (filter (/= 0)) . flip replicateM [0 .. 3])

jolts :: [Int] -> [Int]
jolts = ap (zipWith subtract) tail . (\x -> 0 : x ++ [last x + 3]) . sort