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diff --git a/2024/04-Ceres_Search/second.hs b/2024/04-Ceres_Search/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import qualified Data.Vector          as V
+import qualified Data.Vector.Unboxed  as VU
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 9
+
+type Line = VU.Vector Char
+type Input = V.Vector Line
+
+type Parser = Parsec Void String
+
+parseLine :: Parser Line
+parseLine = do
+  line <- some letterChar <* eol
+  return $ VU.generate (length line) (line !!)
+
+parseInput' :: Parser Input
+parseInput' = do
+  line <- some parseLine <* eof
+  return $ V.generate (length line) (line !!)
+
+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 -> Int
+compute g = length $ filter xmas as
+  where
+    l = V.length g
+    as = [(x, y) | x<-[1..l-2], y<-[1..l-2], g V.! x VU.! y == 'A']
+    xmas :: (Int, Int) -> Bool
+    xmas (x, y) = d1 && d2
+      where
+        tl = g V.! (x-1) VU.! (y-1)
+        tr = g V.! (x+1) VU.! (y-1)
+        bl = g V.! (x-1) VU.! (y+1)
+        br = g V.! (x+1) VU.! (y+1)
+        d1 = tl == 'M' && br == 'S' || tl == 'S' && br == 'M'
+        d2 = bl == 'M' && tr == 'S' || bl == 'S' && tr == 'M'
+
+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