gistfile1.hs

csum16 :: BS.ByteString -> Maybe Word16
csum16 s = words >>= return . rotate . trunc . (foldl' (\acc w -> acc + fromIntegral w) 0)
  where withResult (Left _, _)  = Nothing
        withResult (Right s, _) = Just s
        words = withResult $ flip runGet s (replicateM (BS.length s `div` 2) $ getWord16le)
        trunc :: Word32 -> Word16
        trunc w = fromIntegral $ complement $ (w .&. 0xFFFF) + (w `shiftR` 16)

        rotate :: Word16 -> Word16
        rotate x = x `rotateL` 8
        {-# INLINE rotate #-}

{-# INLINE csum16 #-}