diff --git a/inputs/2024/21.txt b/inputs/2024/21.txt
new file mode 100644
index 0000000..9f0baea
--- /dev/null
+++ b/inputs/2024/21.txt
@@ -0,0 +1,5 @@
+413A
+480A
+682A
+879A
+083A
diff --git a/solutions/solutions.cabal b/solutions/solutions.cabal
index 11d0e9a..3bb8663 100644
--- a/solutions/solutions.cabal
+++ b/solutions/solutions.cabal
@@ -1208,3 +1208,8 @@ executable sln_2024_20
   import:              day
   main-is:             2024/20.hs
   build-depends:       array
+
+executable sln_2024_21
+  import:              day
+  main-is:             2024/21.hs
+  build-depends:       containers
diff --git a/solutions/src/2024/21.hs b/solutions/src/2024/21.hs
new file mode 100644
index 0000000..c659b40
--- /dev/null
+++ b/solutions/src/2024/21.hs
@@ -0,0 +1,261 @@
+{-# Language QuasiQuotes, BlockArguments, LambdaCase, ImportQualifiedPost #-}
+{-|
+Module      : Main
+Description : Day 21 solution
+Copyright   : (c) Eric Mertens, 2024
+License     : ISC
+Maintainer  : emertens@gmail.com
+
+<https://adventofcode.com/2024/day/21>
+
+>>> :{
+:main + "029A
+980A
+179A
+456A
+379A
+"
+:}
+126384
+154115708116294
+
+-}
+module Main (main) where
+
+import Advent (getInputLines)
+import Advent.Coord (above, below, left, origin, right, Coord(..))
+import Advent.Memo (memo2)
+import Control.Monad (guard)
+import Data.Char (isDigit)
+import Data.Map (Map)
+import Data.Map qualified as Map
+import Data.Set (Set)
+import Data.Set qualified as Set
+
+data Pad = Pad (Set Coord) (Map Char Coord)
+
+padFromList :: [(Coord, Char)] -> Pad
+padFromList xs = Pad (Set.fromList [p | (p, _) <- xs]) (Map.fromList [(c,p) | (p,c) <- xs])
+
+padCoord :: Pad -> Char -> Coord
+padCoord (Pad _ m) c = m Map.! c
+
+inPad :: Pad -> Coord -> Bool
+inPad (Pad s _) x = Set.member x s
+
+robotPad :: Pad
+robotPad = padFromList [(C 0 (-1), '^'), (C 0 0, 'A'), (C 1 (-2), '<'), (C 1 (-1), 'v'), (C 1 0, '>')]
+
+doorPad :: Pad
+doorPad = padFromList
+  [ (C (-3) (-2), '7')
+  , (C (-3) (-1), '8')
+  , (C (-3) 0   , '9')
+  , (C (-2) (-2), '4')
+  , (C (-2) (-1), '5')
+  , (C (-2) 0   , '6')
+  , (C (-1) (-2), '1')
+  , (C (-1) (-1), '2')
+  , (C (-1) (0) , '3')
+  , (C 0 (-1)   , '0')
+  , (C 0 0      , 'A')
+  ]
+
+initialStrings :: String -> [String]
+initialStrings str =
+ do let deltas = padDeltas doorPad str
+    keys <- concat <$> traverse deltaToKeys deltas
+    guard (validate doorPad keys)
+    pure keys
+
+
+robotCostN :: Int -> String -> Int
+robotCostN = memo2 \n str ->
+  if n == 0 then length str
+    else
+      minimum $
+     do let deltas = padDeltas robotPad str
+        keys <- concat <$> traverse deltaToKeys deltas
+        guard (validate robotPad keys)
+        pure (sum (map (robotCostN (n-1)) (splitA keys)))
+
+splitA :: [Char] -> [[Char]]
+splitA [] = []
+splitA xs =
+  case break ('A'==) xs of
+    (l,a:r) -> (l ++ [a]) : splitA r
+    _ -> undefined
+
+validate :: Pad -> [Char] -> Bool
+validate pad str = all (inPad pad) posns
+  where
+    posns = scanl move origin str
+    move here 'A' = here
+    move here '>' = right here
+    move here '<' = left here
+    move here '^' = above here
+    move here 'v' = below here
+    move _ _ = undefined
+
+padDeltas :: Pad -> String -> [Coord]
+padDeltas pad str = zipWith (-) (absolutes) (origin:absolutes)
+  where
+    absolutes = map (padCoord pad) str
+
+deltaToKeys :: Coord -> [String]
+deltaToKeys (C y x) =
+  [ keys ++ "A"
+  | let rawKeys =
+          (if y < 0 then (replicate (-y) '^' ++) else id) $
+          (if x > 0 then (replicate x '>' ++) else id) $
+          (if y > 0 then (replicate y 'v' ++) else id) $
+          (if x < 0 then replicate (-x) '<' else "")
+  , keys <- rawKeys : [reverse rawKeys | y /= 0, x /= 0]
+  ]
+
+-- | >>> :main
+-- 177814
+-- 220493992841852
+main :: IO ()
+main =
+ do codes <- getInputLines 2024 21
+    let score n x = (read (takeWhile isDigit x) * answer n x)
+    print (sum (map (score 2) codes))
+    print (sum (map (score 25) codes))
+
+answer :: Int -> String -> Int
+answer n str = minimum (map (robotCostN n) (initialStrings str))
+
+{-
+doorPad :: Map Coord Char
+doorPad = Map.fromList
+  [(C (-3) (-2), '7')
+  ,(C (-3) (-1), '8')
+  ,(C (-3) (0) , '9')
+  ,(C (-2) (-2), '4')
+  ,(C (-2) (-1), '5')
+  ,(C (-2) (0) , '6')
+  ,(C (-1) (-2), '1')
+  ,(C (-1) (-1), '2')
+  ,(C (-1) (0) , '3')
+  , (C 0 (-1)  , '0')
+  , (C 0 0     , 'A')
+  ]
+
+doorMap :: Map Char Coord
+doorMap = Map.fromList [(v,k) | (k,v) <- Map.assocs doorPad  ]
+robotMap :: Map Char Coord
+robotMap = Map.fromList [(v,k) | (k,v) <- Map.assocs robotPad  ]
+
+
+robotPad :: Map Coord Char
+robotPad = Map.fromList
+  [(C 0 (-1), '^'), (C 0 0, 'A'), (C 1 (-2), '<'), (C 1 (-1), 'v'), (C 1 0, '>')]
+
+doorShort :: String -> [(Char, Int)]
+doorShort text = go origin coords
+  where
+    coords = map (doorMap Map.!) text
+
+    go here [] = []
+    go (C y1 x1) (C y2 x2:xs) =
+      (if y2 < y1 then (('^', y1-y2):) else id) $
+      (if x1 < x2 then (('>', x2-x1):) else id) $
+      (if y1 < y2 then (('v', y2-y1):) else id) $
+      (if x2 < x1 then (('<', x1-x2):) else id) $
+      ('A',1) :
+      go (C y2 x2) xs
+
+robotShort :: String -> [(Char, Int)]
+robotShort text = go origin coords
+  where
+    coords = map (robotMap Map.!) text
+
+    go here [] = []
+    go (C y1 x1) (C y2 x2:xs) =
+      (if y2 < y1 then (('^', y1-y2):) else id) $
+      (if x1 < x2 then (('>', x2-x1):) else id) $
+      (if x2 < x1 then (('<', x1-x2):) else id) $
+      (if y1 < y2 then (('v', y2-y1):) else id) $
+      ('A',1) :
+      go (C y2 x2) xs
+
+sequencesD :: String -> [String]
+sequencesD = go (C 0 0)
+  where
+    go here = \case
+      [] -> [""]
+      x:xs ->
+        do let target = stuff Map.! x
+           p <- pd here target
+           q <- go target xs
+           pure (p ++ q)
+
+    stuff = Map.fromList [(v,k) | (k,v) <- Map.assocs doorPad  ]
+
+sequencesR :: String -> [String]
+sequencesR = go (C 0 0)
+  where
+    go here = \case
+      [] -> [""]
+      x:xs ->
+        do let target = stuff Map.! x
+           p <- paths robotPad here target
+           q <- go target xs
+           pure (p ++ q)
+
+    stuff = Map.fromList [(v,k) | (k,v) <- Map.assocs robotPad  ]
+
+paths :: Map Coord a -> Coord -> Coord -> [[Char]]
+paths layout here@(C y1 x1) tgt@(C y2 x2)
+  | Map.notMember here layout = []
+  | here == tgt = ["A"]
+  | otherwise =
+      (if y2 < y1 then map (replicate (y1-y2) '^' ++ ) (paths layout (C y2 x1) tgt) else []) ++
+      (if x1 < x2 then map (replicate (x2-x1) '>' ++ ) (paths layout (C y1 x2) tgt) else []) ++
+      (if y1 < y2 && Map.member (C y2 x1) layout then map (replicate (y2-y1) 'v' ++) (paths layout (C y2 x1) tgt) else []) ++
+      (if x2 < x1 && Map.member (C y1 x2) layout then map (replicate (x1-x2) '<' ++) (paths layout (C y1 x2) tgt) else [])
+
+pd :: Coord -> Coord -> [[Char]]
+pd = memo2 (paths doorPad)
+
+
+main :: IO ()
+main =
+  -- withArgs ["/Users/emertens/Source/advent/example.txt"]
+    do
+    input <- [format|2024 21 (%s%n)*|]
+    -- input <- getInputArray 2024 21
+    print $ sum $ map score input
+
+shortest xs = filter (\x -> length x == n) xs
+   where
+    n = minimum (map length xs)
+
+score x = (read (takeWhile isDigit x) * answer x :: Integer)
+
+answer :: String -> Integer
+answer x = minimum $ map (\m -> sum [toInteger $ length k * v | (k,v)<- Map.assocs m]) $
+  do
+   a <- sequencesD x
+   let a' = counts (splitA a)
+   let go 0 y = y
+       go n y = go (n-1) (Map.fromListWith (+) [(k', v) | (k,v ) <- Map.assocs y, k' <- splitA (pr k)])
+   pure (go 2 a')
+
+shortRobotFast xs = concatMap pr (splitA xs)
+
+splitA [] = []
+splitA xs =
+  case break ('A'==) xs of
+    (l,a:r) -> (l ++ [a]) : splitA r
+
+pr :: String -> [Char]
+pr = memo (\a ->
+  let phase1 =  (sequencesR a)
+  in minimumBy (comparing (minimum . map length . sequencesR)) phase1)
+-}
+
+-- 435231441040286 high
+-- 250698925939238 high
+-- 220493992841852 ???
\ No newline at end of file