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module LengthPrefixedBE
( LengthPrefixedBE(..)
, encode_bigendian
, decode_bigendian
) where
import Debug.Trace
import qualified Data.ByteString.Lazy as L
import Control.Monad (when)
import Data.Bits
import Data.Binary
import Data.Binary.Get
import Data.Int
{-
From RFC4251...
string
Arbitrary length binary string. Strings are allowed to contain
arbitrary binary data, including null characters and 8-bit
characters. They are stored as a uint32 containing its length
(number of bytes that follow) and zero (= empty string) or more
bytes that are the value of the string. Terminating null
characters are not used.
mpint ( LengthPrefixedBE )
Represents multiple precision integers in two's complement format,
stored as a string, 8 bits per byte, MSB first. Negative numbers
have the value 1 as the most significant bit of the first byte of
the data partition. If the most significant bit would be set for
a positive number, the number MUST be preceded by a zero byte.
Unnecessary leading bytes with the value 0 or 255 MUST NOT be
included. The value zero MUST be stored as a string with zero
bytes of data.
-}
newtype LengthPrefixedBE = LengthPrefixedBE Integer
instance Binary LengthPrefixedBE where
put (LengthPrefixedBE n) = do
put len
put bytes
where
bytes = encode_bigendian n
len = fromIntegral (L.length bytes) :: Word32
get = do
len <- get
bs <- getLazyByteString (word32_to_int64 len)
return . LengthPrefixedBE $ decode_bigendian bs
where
word32_to_int64 :: Word32 -> Int64
word32_to_int64 = fromIntegral
encode_bigendian :: (Integral a, Bits a) => a -> L.ByteString
encode_bigendian n =
if (bit /= sbyte)
then sbyte `L.cons` bytes
else bytes
where
bytes = L.reverse $ unroll n
sbyte :: Word8
sbyte = if n<0 then 0xFF else 0
bit = if L.null bytes
then 0x00
else fromIntegral ((fromIntegral (L.head bytes) :: Int8) `shiftR` 7)
unroll :: (Integral a, Bits a) => a -> L.ByteString
unroll = L.unfoldr step
-- TODO: Is reversing L.unfoldr more or less efficient
-- than using Data.List.unfoldr ?
-- Probably Data.ByteString.Lazy should export an unfoldrEnd
-- function that efficiently unfolds reversed bytestrings.
where
step 0 = Nothing
step (-1) = Nothing
step i = Just (fromIntegral i, i `shiftR` 8)
decode_bigendian :: (Num a, Bits a) => L.ByteString -> a
decode_bigendian bs = if isneg then n - 256^(L.length bs)
else n
where
n = L.foldl (\a b -> a `shiftL` 8 .|. fromIntegral b) 0 bs
isneg = not (L.null bs) && L.head bs .&. 0x80 /= 0
|
