conditionalBitParsing.hs

import Data.Binary.Put (runPut)
import Data.Binary.Get (runGet)
import Data.Binary.Bits.Put
import Data.Binary.Bits.Get

-- helper function to create a bytestring from a list of [Int]
toByteString = runPut . runBitPut . mapM_ (putBool . toBool)
  where
    toBool n = n /= 0 || False
 
-- test packet
packet = toByteString [1, 0, 0, 1, 0, 1, 0, 0, 1, 0, 1, 1, 1]

-- the parser. reads the first 4 bits as a length, 
-- then length more bits as a 16-bit word
parse = runGet . runBitGet $ do
  len <- getWord8 4
  getWord16be (fromIntegral len)

-- > parse packet
-- > 151