Efficient streaming and manipulation of a byte stream in Haskell - haskell

Question

While writing a deserialiser for a large (<bloblength><blob>)* encoded binary file I got stuck with the various Haskell produce-transform-consume libraries. So far I'm aware of four streaming libraries:
Data.Conduit: Widely used, has very careful resource management
Pipes: Similar to conduit (Haskell Cast #6 nicely reveals the differences between conduit and pipes)
Data.Binary.Get: Offers useful functions such as getWord32be, but the streaming example is awkward
System.IO.Streams: Seems to be the easiest one to use
Here's a stripped down example of where things go wrong when I try to do Word32 streaming with conduit. A slightly more realistic example would first read a Word32 that determines the blob length and then yield a lazy ByteString of that length (which is then deserialised further).
But here I just try to extract Word32's in streaming fashion from a binary file:
module Main where
-- build-depends: bytestring, conduit, conduit-extra, resourcet, binary
import Control.Monad.Trans.Resource (MonadResource, runResourceT)
import qualified Data.Binary.Get as G
import qualified Data.ByteString as BS
import qualified Data.ByteString.Char8 as C
import qualified Data.ByteString.Lazy as BL
import Data.Conduit
import qualified Data.Conduit.Binary as CB
import qualified Data.Conduit.List as CL
import Data.Word (Word32)
import System.Environment (getArgs)
-- gets a Word32 from a ByteString.
getWord32 :: C.ByteString -> Word32
getWord32 bs = do
G.runGet G.getWord32be $ BL.fromStrict bs
-- should read BytesString and return Word32
transform :: (Monad m, MonadResource m) => Conduit BS.ByteString m Word32
transform = do
mbs <- await
case mbs of
Just bs -> do
case C.null bs of
False -> do
yield $ getWord32 bs
leftover $ BS.drop 4 bs
transform
True -> return ()
Nothing -> return ()
main :: IO ()
main = do
filename <- fmap (!!0) getArgs -- should check length getArgs
result <- runResourceT $ (CB.sourceFile filename) $$ transform =$ CL.consume
print $ length result -- is always 8188 for files larger than 32752 bytes
The output of the program is just the number of Word32's that were read. It turns out the stream terminates after reading the first chunk (about 32KiB). For some reason mbs is never Nothing, so I must check null bs which stops the stream when the chunk is consumed. Clearly, my conduit transform is faulty. I see two routes to a solution:
The await doesn't want to go to the second chunk of the ByteStream, so is there another function that pulls the next chunk? In examples I've seen (e.g. Conduit 101) this is not how it's done
This is just the wrong way to set up transform.
How is this done properly? Is this the right way to go? (Performance does matter.)
Update: Here's a BAD way to do it using Systems.IO.Streams:
module Main where
import Data.Word (Word32)
import System.Environment (getArgs)
import System.IO (IOMode (ReadMode), openFile)
import qualified System.IO.Streams as S
import System.IO.Streams.Binary (binaryInputStream)
import System.IO.Streams.List (outputToList)
main :: IO ()
main = do
filename : _ <- getArgs
h <- openFile filename ReadMode
s <- S.handleToInputStream h
i <- binaryInputStream s :: IO (S.InputStream Word32)
r <- outputToList $ S.connect i
print $ last r
'Bad' means: Very demanding in time and space, does not handle Decode exception.

Answer 1

Your immediate problem is caused by how you are using leftover. That function is used to "Provide a single piece of leftover input to be consumed by the next component in the current monadic binding", and so when you give it bs before looping with transform you are effectively throwing away the rest of the bytestring (i.e. what is after bs).
A correct solution based on your code would use the incremental input interface of Data.Binary.Get to replace your yield/leftover combination with something that consumes each chunk fully. A more pragmatic approach, though, is using the binary-conduit package, which provides that in the shape of conduitGet (its source gives a good idea of what a "manual" implementation would look like):
import Data.Conduit.Serialization.Binary
-- etc.
transform :: (Monad m, MonadResource m) => Conduit BS.ByteString m Word32
transform = conduitGet G.getWord32be
One caveat is that this will throw a parse error if the total number of bytes is not a multiple of 4 (i.e. the last Word32 is incomplete). In the unlikely case of that not being what you want, a lazy way out would be simply using \bs -> C.take (4 * truncate (C.length bs / 4)) bs on the input bytestring.

Answer 2

With pipes (and pipes-group and pipes-bytestring) the demo problem reduces to combinators. First we resolve the incoming undifferentiated byte stream into little 4 byte chunks:
chunksOfStrict :: (Monad m) => Int -> Producer ByteString m r -> Producer ByteString m r
chunksOfStrict n = folds mappend mempty id . view (Bytes.chunksOf n)
then we map these to Word32s and (here) count them.
main :: IO ()
main = do
filename:_ <- getArgs
IO.withFile filename IO.ReadMode $ \h -> do
n <- P.length $ chunksOfStrict 4 (Bytes.fromHandle h) >-> P.map getWord32
print n
This will fail if we have less than 4 bytes or otherwise fail to parse but we can as well map with
getMaybeWord32 :: ByteString -> Maybe Word32
getMaybeWord32 bs = case G.runGetOrFail G.getWord32be $ BL.fromStrict bs of
Left r -> Nothing
Right (_, off, w32) -> Just w32
The following program will then print the parses for the valid 4 byte sequences
main :: IO ()
main = do
filename:_ <- getArgs
IO.withFile filename IO.ReadMode $ \h -> do
runEffect $ chunksOfStrict 4 (Bytes.fromHandle h)
>-> P.map getMaybeWord32
>-> P.concat -- here `concat` eliminates maybes
>-> P.print
There are other ways of dealing with failed parses, of course.
Here, though, is something closer to the program you asked for. It takes a four byte segment from a byte stream (Producer ByteString m r) and reads it as a Word32 if it is long enough; it then takes that many of the incoming bytes and accumulates them into a lazy bytestring, yielding it. It just repeats this until it runs out of bytes. In main below, I print each yielded lazy bytestring that is produced:
module Main (main) where
import Pipes
import qualified Pipes.Prelude as P
import Pipes.Group (folds)
import qualified Pipes.ByteString as Bytes ( splitAt, fromHandle, chunksOf )
import Control.Lens ( view ) -- or Lens.Simple (view) -- or Lens.Micro ((.^))
import qualified System.IO as IO ( IOMode(ReadMode), withFile )
import qualified Data.Binary.Get as G ( runGet, getWord32be )
import Data.ByteString ( ByteString )
import qualified Data.ByteString.Lazy.Char8 as BL
import System.Environment ( getArgs )
splitLazy :: (Monad m, Integral n) =>
n -> Producer ByteString m r -> m (BL.ByteString, Producer ByteString m r)
splitLazy n bs = do
(bss, rest) <- P.toListM' $ view (Bytes.splitAt n) bs
return (BL.fromChunks bss, rest)
measureChunks :: Monad m => Producer ByteString m r -> Producer BL.ByteString m r
measureChunks bs = do
(lbs, rest) <- lift $ splitLazy 4 bs
if BL.length lbs /= 4
then rest >-> P.drain -- in fact it will be empty
else do
let w32 = G.runGet G.getWord32be lbs
(lbs', rest') <- lift $ splitLazy w32 bs
yield lbs
measureChunks rest
main :: IO ()
main = do
filename:_ <- getArgs
IO.withFile filename IO.ReadMode $ \h -> do
runEffect $ measureChunks (Bytes.fromHandle h) >-> P.print
This is again crude in that it uses runGet not runGetOrFail, but this is easily repaired. The pipes standard procedure would be to stop the stream transformation on a failed parse and return the unparsed bytestream.
If you were anticipating that the Word32s were for large numbers, so that you did not want to accumulate the corresponding stream of bytes as a lazy bytestring, but say write them to different files without accumulating, we could change the program pretty easily to do that. This would require a sophisticated use of conduit but is the preferred approach with pipes and streaming.

Answer 3

Here's a relatively straightforward solution that I want to throw into the ring. It's a repeated use of splitAt wrapped into a State monad that gives an interface identical to (a subset of) Data.Binary.Get. The resulting [ByteString] is obtained in main with a whileJust over getBlob.
module Main (main) where
import Control.Monad.Loops
import Control.Monad.State
import qualified Data.Binary.Get as G (getWord32be, runGet)
import qualified Data.ByteString.Lazy as BL
import Data.Int (Int64)
import Data.Word (Word32)
import System.Environment (getArgs)
-- this is going to mimic the Data.Binary.Get.Get Monad
type Get = State BL.ByteString
getWord32be :: Get (Maybe Word32)
getWord32be = state $ \bs -> do
let (w, rest) = BL.splitAt 4 bs
case BL.length w of
4 -> (Just w', rest) where
w' = G.runGet G.getWord32be w
_ -> (Nothing, BL.empty)
getLazyByteString :: Int64 -> Get BL.ByteString
getLazyByteString n = state $ \bs -> BL.splitAt n bs
getBlob :: Get (Maybe BL.ByteString)
getBlob = do
ml <- getWord32be
case ml of
Nothing -> return Nothing
Just l -> do
blob <- getLazyByteString (fromIntegral l :: Int64)
return $ Just blob
runGet :: Get a -> BL.ByteString -> a
runGet g bs = fst $ runState g bs
main :: IO ()
main = do
fname <- head <$> getArgs
bs <- BL.readFile fname
let ls = runGet loop bs where
loop = whileJust getBlob return
print $ length ls
There's no error handling in getBlob, but it's easy to extend. Time and space complexity is quite good, as long as the resulting list is used carefully. (The python script that creates some random data for consumption by the above is here).