diff --git a/2024/21-Keypad_Conundrum/example b/2024/21-Keypad_Conundrum/example
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diff --git a/2024/21-Keypad_Conundrum/first.hs b/2024/21-Keypad_Conundrum/first.hs
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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Functor
+import qualified Data.List            as L
+import qualified Data.Map             as M
+import           Data.Maybe
+import           Data.Ord             (comparing)
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+import           Debug.Trace
+
+exampleExpectedOutput = 126384
+
+type Input = [String]
+
+type Parser = Parsec Void String
+
+parseInput' :: Parser Input
+parseInput' = some (some alphaNumChar <* 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'
+
+data Key = U | D | L | R deriving (Eq)
+instance Ord Key where
+  compare R _ = LT
+  compare U _ = LT
+  compare D _ = LT
+  compare _ _ = LT
+type Coord = (Int, Int)
+type Memo = M.Map (Char, Char) String
+
+keyPad :: Char -> Coord
+keyPad '7' = (0, 0)
+keyPad '8' = (1, 0)
+keyPad '9' = (2, 0)
+keyPad '4' = (0, 1)
+keyPad '5' = (1, 1)
+keyPad '6' = (2, 1)
+keyPad '1' = (0, 2)
+keyPad '2' = (1, 2)
+keyPad '3' = (2, 2)
+keyPad '0' = (1, 3)
+keyPad 'A' = (2, 3)
+keyPad '^' = (1, 0)
+keyPad 'B' = (2, 0)
+keyPad '<' = (0, 1)
+keyPad 'v' = (1, 1)
+keyPad '>' = (2, 1)
+
+pairCharacters :: String -> [(Char, Char)]
+pairCharacters []       = []
+pairCharacters [_]      = []
+pairCharacters (x:y:xs) = (x, y) : pairCharacters (y:xs)
+
+paths :: (Char, Char) -> [String]
+paths (a, b) | a == '<' || x1 == 0 && y2 == 3 = [two]
+             | b == '<' || y1 == 3 && x2 == 0 = [one]
+             | otherwise = L.nub [one, two]
+  where
+    one = moves ++ "B"
+    two = reverse moves ++ "B"
+    moves = replicate (abs $ y2 - y1) (if y2 > y1 then 'v' else '^') ++ replicate (abs $ x2 - x1) (if x2 > x1 then '>' else '<')
+    (x1, y1) = keyPad a
+    (x2, y2) = keyPad b
+
+compute :: Input -> Int
+compute codes = sum $ map complexity $ zip (fst $ L.foldl' computeCode ([], M.empty) codes) codes
+  where
+    complexity :: (String, String) -> Int
+    complexity (output, code) = (length output) * (read $ init code)
+    computeCode :: ([String], Memo) -> String -> ([String], Memo)
+    computeCode (acc, memo) code = let (s, memo') = L.foldl' computeMemoPair ("", memo) $ pairCharacters ('A' : code)
+                                   in (acc ++ [s], memo')
+    computeMemoPair :: (String, Memo) -> (Char, Char) -> (String, Memo)
+    computeMemoPair (acc, memo) ab = case M.lookup ab memo of
+      Just s  -> (acc ++ s, memo)
+      Nothing -> let s = computePair ab in (acc ++ s, M.insert ab s memo)
+    computePair :: (Char, Char) -> String
+    computePair ab = head . L.sortBy (comparing L.length) $ iter . iter $ paths ab
+    iter :: [String] -> [String]
+    iter s = concatMap (transition) s
+    transition :: String -> [String]
+    transition s = L.foldl' aggregate [] $ pairCharacters ('B' : s)
+    aggregate :: [String] -> (Char, Char) -> [String]
+    aggregate [] ab  = paths ab
+    aggregate acc ab = concatMap (\x -> map (\a -> a ++ x) acc) $ paths ab
+
+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
diff --git a/2024/21-Keypad_Conundrum/input b/2024/21-Keypad_Conundrum/input
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