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diff --git a/2020/22-Crab_Combat/second.hs b/2020/22-Crab_Combat/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe (catMaybes)
+import Data.Set qualified as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 291
+
+type Card = Int
+type Deck = [Card]
+type Input = (Deck, Deck)
+type History = [Input]
+
+type Parser = Parsec Void String
+
+parseCard :: Parser Card
+parseCard = do
+  n <- some digitChar
+  void $ optional (char '\n')
+  return $ read n
+
+parseDeck :: Parser Deck
+parseDeck = string "Player " *> digitChar *> string ":\n" *> some parseCard
+
+parseInput' :: Parser Input
+parseInput' = do
+  p1 <- parseDeck
+  void $ char '\n'
+  p2 <- parseDeck
+  void $ eof
+  return (p1, p2)
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right input' -> return input'
+
+computeScore :: Deck -> Int
+computeScore d = sum $ zipWith (*) d (reverse $ take (length d) [1..])
+
+recursing :: Input -> History -> Bool
+recursing i = or . map (== i)
+
+computeRound :: Input -> Bool
+computeRound (a:as, b:bs)
+  | (length as >= a) && (length bs >= b) = computeSubGame (take a as, take b bs) []
+  | otherwise = a > b
+
+computeSubGame :: Input -> History -> Bool
+computeSubGame ([], d) _ = False
+computeSubGame (d, []) _ = True
+computeSubGame (a:as, b:bs) h
+  | recursing (a:as, b:bs) h = True
+  | computeRound (a:as, b:bs) = computeSubGame (as ++ [a, b], bs) newH
+  | otherwise = computeSubGame (as, bs ++ [b, a]) newH
+  where
+    newH = (a:as, b:bs) : h
+
+compute' :: Input -> History -> Int
+compute' ([], d) _ = computeScore d
+compute' (d, []) _ = computeScore d
+compute' (a:as, b:bs) h
+  | recursing (a:as, b:bs) h = computeScore (a:as)
+  | computeRound (a:as, b:bs) = compute' (as ++ [a, b], bs) newH
+  | otherwise = compute' (as, bs ++ [b, a]) newH
+  where
+    newH = (a:as, b:bs) : h
+
+compute :: Input -> Int
+compute input = compute' input []
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input