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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+
+import Control.Applicative.Permutations
+import Control.Monad (void, when)
+import Data.Char qualified as C
+import Data.Either
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe
+import Data.Set qualified as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+import Debug.Trace
+
+exampleExpectedOutput = 30
+
+data Card = Card Int [Int] [Int] deriving Show
+type Input = [Card]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar <* many (char ' ')
+
+parseCard :: Parser Card
+parseCard = do
+ id <- string "Card" *> many (char ' ') *> parseNumber <* char ':' <* many (char ' ')
+ winnings <- some parseNumber
+ void $ char '|' <* many (char ' ')
+ numbers <- some parseNumber
+ return $ Card id winnings numbers
+
+parseInput' :: Parser Input
+parseInput' = some (parseCard <* eol) <* eof
+
+parseInput :: String -> IO Input
+parseInput filename = do
+ input <- readFile filename
+ case runParser parseInput' filename input of
+ Left bundle -> error $ errorBundlePretty bundle
+ Right input' -> return input'
+
+compute :: Input -> Integer
+compute = fst . L.foldl' compute' (0, L.repeat 1) . map score'
+ where
+ compute' :: (Integer, [Integer]) -> Int -> (Integer, [Integer])
+ compute' (acc, stack) score = (acc', stack')
+ where
+ copies = head stack
+ acc' = acc + copies
+ (scored, remain) = L.splitAt score $ tail stack
+ stack' = map (+ copies) scored ++ remain
+ score' :: Card -> Int
+ score' (Card _ winnings numbers) | wins == 0 = 0
+ | otherwise = wins
+ where
+ wins = length $ winnings `L.intersect` numbers
+
+main :: IO ()
+main = do
+ example <- parseInput "example"
+ let exampleOutput = compute example
+ when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+ input <- parseInput "input"
+ print $ compute input
+