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diff --git a/2024/09-Disk_Fragmenter/second.hs b/2024/09-Disk_Fragmenter/second.hs
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+++ b/2024/09-Disk_Fragmenter/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+-- very slow with runghc, use ghc -O3 -o second second.hs and get the result in seconds
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import qualified Data.List            as L
+import           Data.Maybe
+import qualified Data.Vector          as V
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 2858
+
+type Input = [Int]
+
+type Parser = Parsec Void String
+
+parseBlockSize :: Parser Int
+parseBlockSize = read . pure <$> digitChar
+
+parseInput' :: Parser Input
+parseInput' = some parseBlockSize <* 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'
+
+type Disk = V.Vector (Maybe Int) -- Maybe fileId
+type File = (Int, Int, Int) -- fileId, fileIndex, fileSize
+type Free = (Int, Int) -- index, size
+
+compute :: Input -> Int
+compute input = V.sum $ V.imap checksum defragmentedDisk
+  where
+    checksum _ Nothing  = 0
+    checksum i (Just n) = i * n
+    defragmentedDisk :: Disk
+    (_, defragmentedDisk) = L.foldr defragment (frees, startingDisk) files
+    startingDisk :: Disk
+    startingDisk = V.replicate (sum input) Nothing
+    (files, frees, _, _) = computeFileIndexAndSize input ([], [], 0, 0)
+    computeFileIndexAndSize :: Input -> ([File], [Free], Int, Int) -> ([File], [Free], Int, Int)
+    computeFileIndexAndSize [] acc = acc
+    computeFileIndexAndSize (fileSize:[]) (files, frees, fileId, fileIndex) = (files ++ [(fileId, fileIndex, fileSize)], frees, 0, 0)
+    computeFileIndexAndSize (fileSize:freeSize:fs) (files, frees, fileId, fileIndex) = computeFileIndexAndSize fs
+                                                                                       ( files ++ [(fileId, fileIndex, fileSize)]
+                                                                                       , frees ++ [(fileIndex + fileSize, freeSize)]
+                                                                                       , fileId + 1
+                                                                                       , fileIndex + fileSize + freeSize )
+    defragment :: File -> ([Free], Disk) -> ([Free], Disk)
+    defragment (fileId, fileIndex, fileSize) (frees, disk) = (frees', disk V.// [(i, Just fileId)|i<-[fileIndex'..fileIndex'+fileSize-1]])
+      where
+        (fileIndex', frees') = findHole frees []
+        findHole :: [Free] -> [Free] -> (Int, [Free])
+        findHole [] _ = (fileIndex, frees)
+        findHole (f@(freeIndex, freeSize):fs) acc | freeIndex > fileIndex = (fileIndex, frees)
+                                                  | freeSize == fileSize = (freeIndex, acc ++ fs)
+                                                  | freeSize > fileSize = (freeIndex, acc ++ ((freeIndex + fileSize, freeSize - fileSize):fs))
+                                                  | freeSize < fileSize = findHole fs (acc ++ [f])
+
+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