https://twitter.com/taylorfausak/status/1338998520746041344

aoc-2020-15-2.hs

import qualified Data.IntMap as IntMap
import qualified Data.List as List
import qualified Data.Maybe as Maybe
import qualified Data.Ord as Ord
import qualified Data.Text as Text

main = do
  print limit
  interact
    $ show
    . playGame
    . IntMap.fromList
    . fmap (\ ( index, number ) -> ( number, ( index + 1, Nothing ) ))
    . zip [ 0 .. ]
    . fmap (read . Text.unpack)
    . Text.splitOn (Text.singleton ',')
    . Text.strip
    . Text.pack

playGame :: IntMap.IntMap ( Int, Maybe Int ) -> Maybe Int
playGame seen = do
  ( number, ( turn, _ ) ) <- maximumOn snd $ IntMap.toList seen
  pure $ takeTurn number (turn + 1) seen

maximumOn :: Ord b => (a -> b) -> [ a ] -> Maybe a
maximumOn f = Maybe.listToMaybe . List.sortOn (Ord.Down . f)

takeTurn :: Int -> Int -> IntMap.IntMap ( Int, Maybe Int ) -> Int
takeTurn number turn seen =
  let
    nextNumber = case IntMap.lookup number seen of
      Just ( x, Just y ) -> x - y
      _ -> 0
    nextSeen = IntMap.alter (\ m -> Just ( turn, fmap fst m )) nextNumber seen
  in if turn > limit then number else takeTurn nextNumber (turn + 1) nextSeen

limit :: Int
limit = 30000000