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diff --git a/2023/12-Hot_Springs/second.hs b/2023/12-Hot_Springs/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators MemoTrie
+{-# LANGUAGE DeriveGeneric, TypeOperators, TypeFamilies #-}
+module Main (main) where
+
+import Control.Applicative.Permutations
+import Control.Monad (void, when)
+import Data.Char qualified as C
+import Data.Either
+import Data.Functor
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe
+import Data.MemoTrie
+import Data.Set qualified as S
+import Data.Vector qualified as V
+import Data.Void (Void)
+import GHC.Generics (Generic)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+import Debug.Trace
+
+exampleExpectedOutput = 525152
+
+data Tile = Broken | Operational | Unknown deriving (Eq, Generic, Ord)
+instance HasTrie Tile where
+  newtype (Tile :->: b) = TileTrie { unTileTrie :: Reg Tile :->: b }
+  trie = trieGeneric TileTrie 
+  untrie = untrieGeneric unTileTrie
+  enumerate = enumerateGeneric unTileTrie
+instance Show Tile where
+  show Broken = "#"
+  show Operational = "."
+  show Unknown = "?"
+data Row = Row [Tile] [Int] deriving Show
+type Input = [Row]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar <* optional (char ',')
+
+parseTile :: Parser Tile
+parseTile = char '#' $> Broken
+        <|> char '.' $> Operational
+        <|> char '?' $> Unknown
+
+parseRow :: Parser Row
+parseRow = Row <$> some parseTile <* space
+               <*> some parseNumber <* eol
+
+parseInput' :: Parser Input
+parseInput' = some parseRow <* 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'
+
+--type Memo = M.Map [Tile] Int
+--
+--permutations :: Memo -> [Int] -> [Tile] -> (Memo, Int)
+--permutations m [] ts | Broken `L.notElem` ts = (m, 1)
+--                     | otherwise = (m, 0)
+--permutations m rr@(r:rs) ts | isJust cached = (m, fromJust cached)
+--                            | (length rr - 1) + sum rr < length ts' = (m, 0)
+--                            | Operational `L.elem` take r ts' = (m'', operational)
+--                            | null ts'' = (m'', operational + 1)
+--                            | head ts'' == Broken = (m'', operational)
+--                            | otherwise = (m'''', operational + broken)
+--  where
+--    cached = M.lookup ts m
+--    ts' = L.dropWhile (== Operational) ts
+--    ts'' = drop r ts'
+--    (m', operational) = permutations m rr ts'
+--    m'' = if length ts == 5 then M.insert ts operational m' else m'
+--    (m''', broken) = permutations m' rs $ tail ts''
+--    m'''' = if length ts == 5 then M.insert ts broken m''' else m'''
+--
+-- permutations rs (Operational:xs) = map (Operational:) $ permutations rs xs
+-- permutations [r] (Broken:xs) = map (\n -> Broken:Operational:n) $ permutations rs xs
+-- permutations (r:rs) (Broken:xs) = map (replicate l' Broken ++) (if null xs || head xs == Operational then permutations rs xs' else [])
+--   where
+--     l = length (L.takeWhile (/= Operational) xs)
+--     l' = min l (r-1)
+--     xs' = drop l' xs
+-- permutations rr@(r:rs) (Unknown:xs) = (if possible rs xs then map (Operational:) (permutations rr (Operational:xs)) else [])
+--                                    ++ (if l' == l && possible rs xs' then map (replicate l' Broken ++) (permutations rs xs') else [])
+--   where
+--     l = length (L.takeWhile (/= Operational) xs)
+--     l' = min l (r-1)
+--     xs' = drop l' xs
+-- permutations _ _ = []
+permutations = memo2 permutations'
+  where
+    permutations' :: [Int] -> [Tile] -> Int
+    permutations' [] ts | Broken `L.notElem` ts = 1
+                        | otherwise = 0
+    permutations' _ [] = 0
+    permutations' rs (Operational:ts) = permutations rs ts
+    permutations' (r:rs) (Broken:ts) = case splitAt (r-1) ts of
+      (a, t:b) | length a == r-1, Operational `L.notElem` a, t /= Broken -> permutations rs b
+      (a, [])  | length a == r-1, Operational `L.notElem` a -> permutations rs []
+      _ -> 0
+    permutations' rs (Unknown:ts) = permutations rs (Operational:ts) + permutations rs (Broken:ts)
+
+compute :: Input -> Int
+compute input = sum $ map compute' expand
+  where
+    compute' :: Row -> Int
+    compute' (Row tiles record) = permutations record tiles
+    expand = map expandOne input
+    expandOne (Row tiles record) = Row (L.intercalate [Unknown] $ replicate 5 tiles) (concat $ replicate 5 record)
+
+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