Read Word128 from a bytestring

Question

As a haskell newbie i stuck with the problem: I have a binary data in bytestring and i want to convert it to the list of Word128's (assume that binary data is 16 byte aligned). Actually I have troubles even to convert 16 byte bytestring to the Word128.

Anybody can help ? Thanks!

score 1 · Answer 1 · answered Jun 25 '12 at 19:43

1

Usually you'll write an instance for the binary parsing library of your choice. E.g. for Data.Binary, you'd write something like:

-- | Read a Word64 in big endian format
getWord64be :: Get Word64
getWord64be = do
    s <- readN 8 id
    return $! (fromIntegral (s `B.index` 0) `shiftl_w64` 56) .|.
              (fromIntegral (s `B.index` 1) `shiftl_w64` 48) .|.
              (fromIntegral (s `B.index` 2) `shiftl_w64` 40) .|.
              (fromIntegral (s `B.index` 3) `shiftl_w64` 32) .|.
              (fromIntegral (s `B.index` 4) `shiftl_w64` 24) .|.
              (fromIntegral (s `B.index` 5) `shiftl_w64` 16) .|.
              (fromIntegral (s `B.index` 6) `shiftl_w64`  8) .|.
              (fromIntegral (s `B.index` 7) )
{- INLINE getWord64be -}

that would then be glued into an instance Binary.

So, pick your binary parsing library (either binary or cereal), and write an instance for your type.

answered Jun 25 '12 at 19:43

Don Stewart

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Where do we file a feature request for a `unsafeGetbe :: (Integral a, Bits a) => Int -> a` that would turn this into just `getWord64be = unsafeGetbe 8`? =) – Daniel Wagner Jun 25 '12 at 20:37
Unrolled byte-level loops seem fine to me. Do you want to use TH to generate it? – Don Stewart Jun 25 '12 at 20:49
I was thinking more like a `foldr (\byte val -> shiftl val 8 .|. fromIntegral byte)` or similar. – Daniel Wagner Jun 25 '12 at 22:54
@Don is readN exposed outside the Data.Binary.Get ? or do you really suggest that he should modify the src for Data.Binary.Get? – Jonke Jun 26 '12 at 07:58
@Jonke, @Don: it should be `getBytes` for both the binary (Data.Binary.Get) and cereal (Data.Serialize.Get) packages. – informatikr Jun 26 '12 at 09:07
No, he should implement it in terms of the byte parsing functions that are exposed. – Don Stewart Jun 26 '12 at 12:22

Read Word128 from a bytestring

1 Answers1