adler32 :: String -> Int
adler32 = helper 1 0
  where
    helper a b (x:xs) = helper a' b' xs
      where a' = (a + (ord x .&. 0xff)) `mod` base
            b' = (a' + b) `mod` base
            base = 65521

    helper a b _     = (b `shiftL` 16) .|. a