-- Copyright (C) 2009-2012 John Millikin <john@john-millikin.com>
--
-- This program is free software: you can redistribute it and/or modify
-- it under the terms of the GNU General Public License as published by
-- the Free Software Foundation, either version 3 of the License, or
-- any later version.
--
-- This program is distributed in the hope that it will be useful,
-- but WITHOUT ANY WARRANTY; without even the implied warranty of
-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-- GNU General Public License for more details.
--
-- You should have received a copy of the GNU General Public License
-- along with this program.  If not, see <http://www.gnu.org/licenses/>.

module DBus.Internal.Wire
    ( Endianness(..)
    , MarshalError
    , marshalErrorMessage

    , UnmarshalError
    , unmarshalErrorMessage

    , marshalMessage
    , unmarshalMessage
    , unmarshalMessageM
    ) where

import qualified Control.Applicative
import           Control.Monad (ap, liftM, when, unless)
import qualified Data.ByteString
import           Data.ByteString (ByteString)
import qualified Data.ByteString.Builder as Builder
import qualified Data.ByteString.Char8
import qualified Data.ByteString.Lazy as Lazy
import           Data.Int (Int16, Int32, Int64)
import qualified Data.Map
import           Data.Map (Map)
import           Data.Maybe (fromJust, listToMaybe, fromMaybe)
import           Data.Monoid
import           Data.Text (Text)
import qualified Data.Text.Encoding
import qualified Data.Vector
import           Data.Vector (Vector)
import           Data.Word (Word8, Word16, Word32, Word64)
import           Foreign.C.Types (CInt)
import           System.Posix.Types (Fd(..))
import           Prelude

import qualified Data.Serialize.Get as Get
import           Data.Serialize.IEEE754 (getFloat64be, getFloat64le, putFloat64be, putFloat64le)
import           Data.Serialize.Put (runPut)

import           DBus.Internal.Message
import           DBus.Internal.Types

data Endianness = LittleEndian | BigEndian
    deriving (Show, Eq)

encodeEndianness :: Endianness -> Word8
encodeEndianness LittleEndian = 0x6C
encodeEndianness BigEndian    = 0x42

decodeEndianness :: Word8 -> Maybe Endianness
decodeEndianness 0x6C = Just LittleEndian
decodeEndianness 0x42 = Just BigEndian
decodeEndianness _    = Nothing

alignment :: Type -> Word8
alignment TypeBoolean = 4
alignment TypeWord8 = 1
alignment TypeWord16 = 2
alignment TypeWord32 = 4
alignment TypeWord64 = 8
alignment TypeInt16 = 2
alignment TypeInt32 = 4
alignment TypeInt64 = 8
alignment TypeDouble = 8
alignment TypeUnixFd = 4
alignment TypeString = 4
alignment TypeObjectPath = 4
alignment TypeSignature = 1
alignment (TypeArray _) = 4
alignment (TypeDictionary _ _) = 4
alignment (TypeStructure _) = 8
alignment TypeVariant = 1

{-# INLINE padding #-}
padding :: Word64 -> Word8 -> Word64
padding current count = required where
    count' = fromIntegral count
    missing = mod current count'
    required = if missing > 0
        then count' - missing
        else 0

data WireR s a
    = WireRL String
    | WireRR a !s

newtype Wire s a = Wire
    { unWire :: Endianness -> s -> WireR s a
    }

instance Functor (Wire s) where
    {-# INLINE fmap #-}
    fmap = liftM

instance Control.Applicative.Applicative (Wire s) where
    {-# INLINE pure #-}
    pure = return

    {-# INLINE (<*>) #-}
    (<*>) = ap

instance Monad (Wire s) where
    {-# INLINE return #-}
    return a = Wire (\_ s -> WireRR a s)

    {-# INLINE (>>=) #-}
    m >>= k = Wire $ \e s -> case unWire m e s of
        WireRL err -> WireRL err
        WireRR a s' -> unWire (k a) e s'

    {-# INLINE (>>) #-}
    m >> k = Wire $ \e s -> case unWire m e s of
        WireRL err -> WireRL err
        WireRR _ s' -> unWire k e s'

throwError :: String -> Wire s a
throwError err = Wire (\_ _ -> WireRL err)

{-# INLINE getState #-}
getState :: Wire s s
getState = Wire (\_ s -> WireRR s s)

{-# INLINE putState #-}
putState :: s -> Wire s ()
putState s = Wire (\_ _ -> WireRR () s)

{-# INLINE chooseEndian #-}
chooseEndian :: a -> a -> Wire s a
chooseEndian big little = Wire (\e s -> case e of
    BigEndian -> WireRR big s
    LittleEndian -> WireRR little s)

type Marshal = Wire MarshalState

newtype MarshalError = MarshalError String
    deriving (Show, Eq)

marshalErrorMessage :: MarshalError -> String
marshalErrorMessage (MarshalError s) = s

data MarshalState = MarshalState
    !Builder.Builder
    {-# UNPACK #-} !Word64

marshal :: Value -> Marshal ()
marshal (ValueAtom x) = marshalAtom x
marshal (ValueBytes xs) = marshalStrictBytes xs
marshal (ValueVector t xs) = marshalVector t xs
marshal (ValueMap kt vt xs) = marshalMap kt vt xs
marshal (ValueStructure xs) = marshalStructure xs
marshal (ValueVariant x) = marshalVariant x

marshalAtom :: Atom -> Marshal ()
marshalAtom (AtomWord8 x) = marshalWord8 x
marshalAtom (AtomWord16 x) = marshalWord16 x
marshalAtom (AtomWord32 x) = marshalWord32 x
marshalAtom (AtomWord64 x) = marshalWord64 x
marshalAtom (AtomInt16 x) = marshalInt16 x
marshalAtom (AtomInt32 x) = marshalInt32 x
marshalAtom (AtomInt64 x) = marshalInt64 x
marshalAtom (AtomDouble x) = marshalDouble x
marshalAtom (AtomUnixFd x) = marshalUnixFd x
marshalAtom (AtomBool x) = marshalBool x
marshalAtom (AtomText x) = marshalText x
marshalAtom (AtomObjectPath x) = marshalObjectPath x
marshalAtom (AtomSignature x) = marshalSignature x

appendB :: Word64 -> Builder.Builder -> Marshal ()
appendB size bytes = Wire (\_ (MarshalState builder count) -> let
    builder' = mappend builder bytes
    count' = count + size
    in WireRR () (MarshalState builder' count'))

appendS :: ByteString -> Marshal ()
appendS bytes = appendB
    (fromIntegral (Data.ByteString.length bytes))
    (Builder.byteString bytes)

appendL :: Lazy.ByteString -> Marshal ()
appendL bytes = appendB
    (fromIntegral (Lazy.length bytes))
    (Builder.lazyByteString bytes)

pad :: Word8 -> Marshal ()
pad count = do
    (MarshalState _ existing) <- getState
    let padding' = fromIntegral (padding existing count)
    appendS (Data.ByteString.replicate padding' 0)

marshalBuilder :: Word8
               -> (a -> Builder.Builder)
               -> (a -> Builder.Builder)
               -> a -> Marshal ()
marshalBuilder size be le x = do
    builder <- chooseEndian (be x) (le x)
    pad size
    appendB (fromIntegral size) builder

type Unmarshal = Wire UnmarshalState

newtype UnmarshalError = UnmarshalError String
    deriving (Show, Eq)

unmarshalErrorMessage :: UnmarshalError -> String
unmarshalErrorMessage (UnmarshalError s) = s

data UnmarshalState = UnmarshalState
    {-# UNPACK #-} !ByteString
    {-# UNPACK #-} !Word64

unmarshal :: Type -> Unmarshal Value
unmarshal TypeWord8 = liftM toValue unmarshalWord8
unmarshal TypeWord16 = liftM toValue unmarshalWord16
unmarshal TypeWord32 = liftM toValue unmarshalWord32
unmarshal TypeWord64 = liftM toValue unmarshalWord64
unmarshal TypeInt16 = liftM toValue unmarshalInt16
unmarshal TypeInt32 = liftM toValue unmarshalInt32
unmarshal TypeInt64 = liftM toValue unmarshalInt64
unmarshal TypeDouble = liftM toValue unmarshalDouble
unmarshal TypeUnixFd = liftM toValue unmarshalUnixFd
unmarshal TypeBoolean = liftM toValue unmarshalBool
unmarshal TypeString = liftM toValue unmarshalText
unmarshal TypeObjectPath = liftM toValue unmarshalObjectPath
unmarshal TypeSignature = liftM toValue unmarshalSignature
unmarshal (TypeArray TypeWord8) = liftM toValue unmarshalByteArray
unmarshal (TypeArray t) = liftM (ValueVector t) (unmarshalArray t)
unmarshal (TypeDictionary kt vt) = unmarshalDictionary kt vt
unmarshal (TypeStructure ts) = unmarshalStructure ts
unmarshal TypeVariant = unmarshalVariant

{-# INLINE consume #-}
consume :: Word64 -> Unmarshal ByteString
consume count = do
    (UnmarshalState bytes offset) <- getState
    let count' = fromIntegral count
    let (x, bytes') = Data.ByteString.splitAt count' bytes
    let lenConsumed = Data.ByteString.length x
    if lenConsumed == count'
        then do
            putState (UnmarshalState bytes' (offset + count))
            return x
        else throwError ("Unexpected EOF at offset " ++ show (offset + fromIntegral lenConsumed))

skipPadding :: Word8 -> Unmarshal ()
skipPadding count = do
    (UnmarshalState _ offset) <- getState
    bytes <- consume (padding offset count)
    unless (Data.ByteString.all (== 0) bytes)
        (throwError ("Value padding " ++ show bytes ++ " contains invalid bytes."))

skipTerminator :: Unmarshal ()
skipTerminator = do
    byte <- unmarshalWord8
    when (byte /= 0) (throwError "Textual value is not NUL-terminated.")

fromMaybeU :: Show a => String -> (a -> Maybe b) -> a -> Unmarshal b
fromMaybeU label f x = case f x of
    Just x' -> return x'
    Nothing -> throwError ("Invalid " ++ label ++ ": " ++ show x)

unmarshalGet :: Word8 -> Get.Get a -> Get.Get a -> Unmarshal a
unmarshalGet count be le = do
    skipPadding count
    bytes <- consume (fromIntegral count)
    get <- chooseEndian be le
    let Right ret = Get.runGet get bytes
    return ret

marshalWord8 :: Word8 -> Marshal ()
marshalWord8 x = appendB 1 (Builder.word8 x)

unmarshalWord8 :: Unmarshal Word8
unmarshalWord8 = liftM Data.ByteString.head (consume 1)

marshalWord16 :: Word16 -> Marshal ()
marshalWord16 = marshalBuilder 2
    Builder.word16BE
    Builder.word16LE

marshalWord32 :: Word32 -> Marshal ()
marshalWord32 = marshalBuilder 4
    Builder.word32BE
    Builder.word32LE

marshalWord64 :: Word64 -> Marshal ()
marshalWord64 = marshalBuilder 8
    Builder.word64BE
    Builder.word64LE

marshalInt16 :: Int16 -> Marshal ()
marshalInt16 = marshalWord16 . fromIntegral

marshalInt32 :: Int32 -> Marshal ()
marshalInt32 = marshalWord32 . fromIntegral

marshalInt64 :: Int64 -> Marshal ()
marshalInt64 = marshalWord64 . fromIntegral

unmarshalWord16 :: Unmarshal Word16
unmarshalWord16 = unmarshalGet 2
    Get.getWord16be
    Get.getWord16le

unmarshalWord32 :: Unmarshal Word32
unmarshalWord32 = unmarshalGet 4
    Get.getWord32be
    Get.getWord32le

unmarshalWord64 :: Unmarshal Word64
unmarshalWord64 = unmarshalGet 8
    Get.getWord64be
    Get.getWord64le

unmarshalInt16 :: Unmarshal Int16
unmarshalInt16 = liftM fromIntegral unmarshalWord16

unmarshalInt32 :: Unmarshal Int32
unmarshalInt32 = liftM fromIntegral unmarshalWord32

unmarshalInt64 :: Unmarshal Int64
unmarshalInt64 = liftM fromIntegral unmarshalWord64

marshalDouble :: Double -> Marshal ()
marshalDouble x = do
    put <- chooseEndian putFloat64be putFloat64le
    pad 8
    appendS (runPut (put x))

unmarshalDouble :: Unmarshal Double
unmarshalDouble = unmarshalGet 8
    getFloat64be
    getFloat64le

marshalUnixFd :: Fd -> Marshal ()
marshalUnixFd (Fd x)
    | x < 0 = throwError ("Invalid file descriptor: " ++ show x)
    | toInteger x > toInteger (maxBound :: Word32) = throwError ("D-Bus forbids file descriptors exceeding UINT32_MAX: " ++ show x)
    | otherwise = marshalWord32 (fromIntegral x)

unmarshalUnixFd :: Unmarshal Fd
unmarshalUnixFd = do
    x <- unmarshalWord32
    when (toInteger x > toInteger (maxBound :: CInt))
        (throwError ("Invalid file descriptor: " ++ show x))
    return (Fd (fromIntegral x))

marshalBool :: Bool -> Marshal ()
marshalBool False = marshalWord32 0
marshalBool True  = marshalWord32 1

unmarshalBool :: Unmarshal Bool
unmarshalBool = do
    word <- unmarshalWord32
    case word of
        0 -> return False
        1 -> return True
        _ -> throwError ("Invalid boolean: " ++ show word)

marshalText :: Text -> Marshal ()
marshalText text = do
    let bytes = Data.Text.Encoding.encodeUtf8 text
    when (Data.ByteString.any (== 0) bytes)
        (throwError ("String " ++ show text ++ " contained forbidden character: '\\x00'"))
    marshalWord32 (fromIntegral (Data.ByteString.length bytes))
    appendS bytes
    marshalWord8 0

unmarshalText :: Unmarshal Text
unmarshalText = do
    byteCount <- unmarshalWord32
    bytes <- consume (fromIntegral byteCount)
    skipTerminator
    fromMaybeU "text" maybeDecodeUtf8 bytes

maybeDecodeUtf8 :: ByteString -> Maybe Text
maybeDecodeUtf8 bs = case Data.Text.Encoding.decodeUtf8' bs of
    Right text -> Just text
    _ -> Nothing

marshalObjectPath :: ObjectPath -> Marshal ()
marshalObjectPath p = do
    let bytes = Data.ByteString.Char8.pack (formatObjectPath p)
    marshalWord32 (fromIntegral (Data.ByteString.length bytes))
    appendS bytes
    marshalWord8 0

unmarshalObjectPath :: Unmarshal ObjectPath
unmarshalObjectPath = do
    byteCount <- unmarshalWord32
    bytes <- consume (fromIntegral byteCount)
    skipTerminator
    fromMaybeU "object path" parseObjectPath (Data.ByteString.Char8.unpack bytes)

signatureBytes :: Signature -> ByteString
signatureBytes (Signature ts) = Data.ByteString.Char8.pack (concatMap typeCode ts)

marshalSignature :: Signature -> Marshal ()
marshalSignature x = do
    let bytes = signatureBytes x
    marshalWord8 (fromIntegral (Data.ByteString.length bytes))
    appendS bytes
    marshalWord8 0

unmarshalSignature :: Unmarshal Signature
unmarshalSignature = do
    byteCount <- unmarshalWord8
    bytes <- consume (fromIntegral byteCount)
    skipTerminator
    fromMaybeU "signature" parseSignatureBytes bytes

arrayMaximumLength :: Int64
arrayMaximumLength = 67108864

marshalVector :: Type -> Vector Value -> Marshal ()
marshalVector t x = do
    (arrayPadding, arrayBytes) <- getArrayBytes t x
    let arrayLen = Lazy.length arrayBytes
    when (arrayLen > arrayMaximumLength) (throwError ("Marshaled array size (" ++ show arrayLen ++ " bytes) exceeds maximum limit of (" ++ show arrayMaximumLength ++ " bytes)."))
    marshalWord32 (fromIntegral arrayLen)
    appendS (Data.ByteString.replicate arrayPadding 0)
    appendL arrayBytes

marshalStrictBytes :: ByteString -> Marshal ()
marshalStrictBytes bytes = do
    let arrayLen = Lazy.length (Lazy.fromStrict bytes)
    when (fromIntegral arrayLen > arrayMaximumLength) (throwError ("Marshaled array size (" ++ show arrayLen ++ " bytes) exceeds maximum limit of (" ++ show arrayMaximumLength ++ " bytes)."))
    marshalWord32 (fromIntegral arrayLen)
    appendS bytes

getArrayBytes :: Type -> Vector Value -> Marshal (Int, Lazy.ByteString)
getArrayBytes itemType vs = do
    s <- getState
    (MarshalState _ afterLength) <- marshalWord32 0 >> getState
    (MarshalState _ afterPadding) <- pad (alignment itemType) >> getState

    putState (MarshalState mempty afterPadding)
    (MarshalState itemBuilder _) <- Data.Vector.mapM_ marshal vs >> getState

    let itemBytes = Builder.toLazyByteString itemBuilder
        paddingSize = fromIntegral (afterPadding - afterLength)

    putState s
    return (paddingSize, itemBytes)

unmarshalByteArray :: Unmarshal ByteString
unmarshalByteArray = do
    byteCount <- unmarshalWord32
    consume (fromIntegral byteCount)

unmarshalArray :: Type -> Unmarshal (Vector Value)
unmarshalArray itemType = do
    let getOffset = do
            (UnmarshalState _ o) <- getState
            return o
    byteCount <- unmarshalWord32
    skipPadding (alignment itemType)
    start <- getOffset
    let end = start + fromIntegral byteCount
    vs <- untilM (liftM (>= end) getOffset) (unmarshal itemType)
    end' <- getOffset
    when (end' > end) (throwError ("Array data size exeeds array size of " ++ show end))
    return (Data.Vector.fromList vs)

dictionaryToArray :: Map Atom Value -> Vector Value
dictionaryToArray = Data.Vector.fromList . map step . Data.Map.toList where
    step (k, v) = ValueStructure [ValueAtom k, v]

arrayToDictionary :: Vector Value -> Map Atom Value
arrayToDictionary = Data.Map.fromList . map step . Data.Vector.toList where
    step (ValueStructure [ValueAtom k, v]) = (k, v)
    step _ = error "arrayToDictionary: internal error"

marshalMap :: Type -> Type -> Map Atom Value -> Marshal ()
marshalMap kt vt x = let
    structType = TypeStructure [kt, vt]
    array = dictionaryToArray x
    in marshalVector structType array

unmarshalDictionary :: Type -> Type -> Unmarshal Value
unmarshalDictionary kt vt = do
    let pairType = TypeStructure [kt, vt]
    array <- unmarshalArray pairType
    return (ValueMap kt vt (arrayToDictionary array))

marshalStructure :: [Value] -> Marshal ()
marshalStructure vs = do
    pad 8
    mapM_ marshal vs

unmarshalStructure :: [Type] -> Unmarshal Value
unmarshalStructure ts = do
    skipPadding 8
    liftM ValueStructure (mapM unmarshal ts)

marshalVariant :: Variant -> Marshal ()
marshalVariant var@(Variant val) = do
    sig <- case signature [valueType val] of
        Just x' -> return x'
        Nothing -> throwError ("Signature " ++ show (typeCode (valueType val)) ++ " for variant " ++ show var ++ " is malformed or too large.")
    marshalSignature sig
    marshal val

unmarshalVariant :: Unmarshal Value
unmarshalVariant = do
    let getType sig = case signatureTypes sig of
            [t] -> Just t
            _   -> Nothing

    t <- fromMaybeU "variant signature" getType =<< unmarshalSignature
    (toValue . Variant) `liftM` unmarshal t

protocolVersion :: Word8
protocolVersion = 1

messageMaximumLength :: Integer
messageMaximumLength = 134217728

encodeField :: HeaderField -> Value
encodeField (HeaderPath x)        = encodeField' 1 x
encodeField (HeaderInterface x)   = encodeField' 2 x
encodeField (HeaderMember x)      = encodeField' 3 x
encodeField (HeaderErrorName x)   = encodeField' 4 x
encodeField (HeaderReplySerial x) = encodeField' 5 x
encodeField (HeaderDestination x) = encodeField' 6 x
encodeField (HeaderSender x)      = encodeField' 7 x
encodeField (HeaderSignature x)   = encodeField' 8 x
encodeField (HeaderUnixFds x)     = encodeField' 9 x

encodeField' :: IsVariant a => Word8 -> a -> Value
encodeField' code x = toValue (code, toVariant x)

decodeField :: (Word8, Variant)
            -> ErrorM UnmarshalError [HeaderField]
decodeField struct = case struct of
    (1, x) -> decodeField' x HeaderPath "path"
    (2, x) -> decodeField' x HeaderInterface "interface"
    (3, x) -> decodeField' x HeaderMember "member"
    (4, x) -> decodeField' x HeaderErrorName "error name"
    (5, x) -> decodeField' x HeaderReplySerial "reply serial"
    (6, x) -> decodeField' x HeaderDestination "destination"
    (7, x) -> decodeField' x HeaderSender "sender"
    (8, x) -> decodeField' x HeaderSignature "signature"
    (9, x) -> decodeField' x HeaderUnixFds "unix fds"
    _      -> return []

decodeField' :: IsVariant a => Variant -> (a -> b) -> String
             -> ErrorM UnmarshalError [b]
decodeField' x f label = case fromVariant x of
    Just x' -> return [f x']
    Nothing -> throwErrorM (UnmarshalError ("Header field " ++ show label ++ " contains invalid value " ++ show x))

marshalMessage :: Message a => Endianness -> Serial -> a
               -> Either MarshalError ByteString
marshalMessage e serial msg = runMarshal where
    body = messageBody msg
    marshaler = do
        sig <- checkBodySig body
        empty <- getState
        mapM_ (marshal . (\(Variant x) -> x)) body
        (MarshalState bodyBytesB _) <- getState
        putState empty
        marshal (toValue (encodeEndianness e))
        let bodyBytes = Builder.toLazyByteString bodyBytesB
        marshalHeader msg serial sig (fromIntegral (Lazy.length bodyBytes))
        pad 8
        appendL bodyBytes
        checkMaximumSize
    emptyState = MarshalState mempty 0
    runMarshal = case unWire marshaler e emptyState of
        WireRL err -> Left (MarshalError err)
        WireRR _ (MarshalState builder _) -> Right (Lazy.toStrict (Builder.toLazyByteString builder))

checkBodySig :: [Variant] -> Marshal Signature
checkBodySig vs = case signature (map variantType vs) of
    Just x -> return x
    Nothing -> throwError ("Message body " ++ show vs ++ " has too many items")

marshalHeader :: Message a => a -> Serial -> Signature -> Word32
              -> Marshal ()
marshalHeader msg serial bodySig bodyLength = do
    let fields = HeaderSignature bodySig : messageHeaderFields msg
    marshalWord8 (messageTypeCode msg)
    marshalWord8 (messageFlags msg)
    marshalWord8 protocolVersion
    marshalWord32 bodyLength
    marshalWord32 (serialValue serial)
    let fieldType = TypeStructure [TypeWord8, TypeVariant]
    marshalVector fieldType (Data.Vector.fromList (map encodeField fields))

checkMaximumSize :: Marshal ()
checkMaximumSize = do
    (MarshalState _ messageLength) <- getState
    when (toInteger messageLength > messageMaximumLength)
        (throwError ("Marshaled message size (" ++ show messageLength ++ " bytes) exeeds maximum limit of (" ++ show messageMaximumLength ++ " bytes)."))

unmarshalMessageM :: Monad m => (Int -> m ByteString)
                  -> m (Either UnmarshalError ReceivedMessage)
unmarshalMessageM getBytes' = runErrorT $ do
    let getBytes count = do
            bytes <- ErrorT (liftM Right (getBytes' count))
            if Data.ByteString.length bytes < count
                then throwErrorT (UnmarshalError "Unexpected end of input while parsing message header.")
                else return bytes

    let Just fixedSig = parseSignature "yyyyuuu"
    fixedBytes <- getBytes 16

    let messageVersion = Data.ByteString.index fixedBytes 3
    when (messageVersion /= protocolVersion) (throwErrorT (UnmarshalError ("Unsupported protocol version: " ++ show messageVersion)))

    let eByte = Data.ByteString.index fixedBytes 0
    endianness <- case decodeEndianness eByte of
        Just x' -> return x'
        Nothing -> throwErrorT (UnmarshalError ("Invalid endianness: " ++ show eByte))

    let unmarshalSig = mapM unmarshal . signatureTypes
    let unmarshal' x bytes = case unWire (unmarshalSig x) endianness (UnmarshalState bytes 0) of
            WireRR x' _ -> return x'
            WireRL err  -> throwErrorT (UnmarshalError err)
    fixed <- unmarshal' fixedSig fixedBytes
    let messageType = fromJust (fromValue (fixed !! 1))
    let flags = fromJust (fromValue (fixed !! 2))
    let bodyLength = fromJust (fromValue (fixed !! 4)) :: Word32
    let serial = fromJust (fromVariant (Variant (fixed !! 5)))

    let fieldByteCount = fromJust (fromValue (fixed !! 6)) :: Word32
    let bodyPadding = padding (fromIntegral fieldByteCount + 16) 8

    -- Forbid messages larger than 'messageMaximumLength'
    let messageLength = 16 + toInteger fieldByteCount + toInteger bodyPadding + toInteger bodyLength
    when (messageLength > messageMaximumLength) $
        throwErrorT (UnmarshalError ("Message size " ++ show messageLength ++ " exceeds limit of " ++ show messageMaximumLength))

    let Just headerSig  = parseSignature "yyyyuua(yv)"
    fieldBytes <- getBytes (fromIntegral fieldByteCount)
    let headerBytes = Data.ByteString.append fixedBytes fieldBytes
    header <- unmarshal' headerSig headerBytes

    let fieldArray = Data.Vector.toList (fromJust (fromValue (header !! 6)))
    fields <- case runErrorM $ concat `liftM` mapM decodeField fieldArray of
        Left err -> throwErrorT err
        Right x -> return x
    _ <- getBytes (fromIntegral bodyPadding)
    let bodySig = findBodySignature fields
    bodyBytes <- getBytes (fromIntegral bodyLength)
    body <- unmarshal' bodySig bodyBytes
    y <- case runErrorM (buildReceivedMessage messageType fields) of
        Right x -> return x
        Left err -> throwErrorT (UnmarshalError ("Header field " ++ show err ++ " is required, but missing"))
    return (y serial flags (map Variant body))

findBodySignature :: [HeaderField] -> Signature
findBodySignature fields = fromMaybe (signature_ []) (listToMaybe [x | HeaderSignature x <- fields])

buildReceivedMessage :: Word8 -> [HeaderField] -> ErrorM String (Serial -> Word8 -> [Variant] -> ReceivedMessage)
buildReceivedMessage 1 fields = do
    path <- require "path" [x | HeaderPath x <- fields]
    member <- require "member name" [x | HeaderMember x <- fields]
    return $ \serial flags body -> let
        iface = listToMaybe [x | HeaderInterface x <- fields]
        dest = listToMaybe [x | HeaderDestination x <- fields]
        sender = listToMaybe [x | HeaderSender x <- fields]
        msg = MethodCall path iface member sender dest True True body
        in ReceivedMethodCall serial (setMethodCallFlags msg flags)

buildReceivedMessage 2 fields = do
    replySerial <- require "reply serial" [x | HeaderReplySerial x <- fields]
    return $ \serial _ body -> let
        dest = listToMaybe [x | HeaderDestination x <- fields]
        sender = listToMaybe [x | HeaderSender x <- fields]
        msg = MethodReturn replySerial sender dest body
        in ReceivedMethodReturn serial msg

buildReceivedMessage 3 fields = do
    name <- require "error name" [x | HeaderErrorName x <- fields]
    replySerial <- require "reply serial" [x | HeaderReplySerial x <- fields]
    return $ \serial _ body -> let
        dest = listToMaybe [x | HeaderDestination x <- fields]
        sender = listToMaybe [x | HeaderSender x <- fields]
        msg = MethodError name replySerial sender dest body
        in ReceivedMethodError serial msg

buildReceivedMessage 4 fields = do
    path <- require "path" [x | HeaderPath x <- fields]
    member <- require "member name" [x | HeaderMember x <- fields]
    iface <- require "interface" [x | HeaderInterface x <- fields]
    return $ \serial _ body -> let
        dest = listToMaybe [x | HeaderDestination x <- fields]
        sender = listToMaybe [x | HeaderSender x <- fields]
        msg = Signal path iface member sender dest body
        in ReceivedSignal serial msg

buildReceivedMessage messageType fields = return $ \serial _ body -> let
    sender = listToMaybe [x | HeaderSender x <- fields]
    msg = UnknownMessage messageType sender body
    in ReceivedUnknown serial msg

require :: String -> [a] -> ErrorM String a
require _     (x:_) = return x
require label _     = throwErrorM label

unmarshalMessage :: ByteString -> Either UnmarshalError ReceivedMessage
unmarshalMessage bytes = checkError (Get.runGet get bytes) where
    get = unmarshalMessageM getBytes

    -- wrap getByteString, so it will behave like transportGet and return
    -- a truncated result on EOF instead of throwing an exception.
    getBytes count = do
        remaining <- Get.remaining
        Get.getByteString (min remaining count)

    checkError (Left err) = Left (UnmarshalError err)
    checkError (Right x) = x

untilM :: Monad m => m Bool -> m a -> m [a]
untilM test comp = do
    done <- test
    if done
        then return []
        else do
            x <- comp
            xs <- untilM test comp
            return (x:xs)

-------------------------------------------------------------------------------
-- local ErrorT and MonadError, which don't have the silly Error => dependency
-- found in the "transformers" package.
-------------------------------------------------------------------------------

newtype ErrorM e a = ErrorM { runErrorM :: Either e a }

instance Functor (ErrorM e) where
    fmap f m = ErrorM $ case runErrorM m of
        Left err -> Left err
        Right x -> Right (f x)

instance Control.Applicative.Applicative (ErrorM e) where
    pure = return
    (<*>) = ap

instance Monad (ErrorM e) where
    return = ErrorM . Right
    (>>=) m k = case runErrorM m of
        Left err -> ErrorM (Left err)
        Right x -> k x

throwErrorM :: e -> ErrorM e a
throwErrorM = ErrorM . Left

newtype ErrorT e m a = ErrorT { runErrorT :: m (Either e a) }

instance Monad m => Functor (ErrorT e m) where
    fmap = liftM

instance Monad m => Control.Applicative.Applicative (ErrorT e m) where
    pure = return
    (<*>) = ap

instance Monad m => Monad (ErrorT e m) where
    return = ErrorT . return . Right
    (>>=) m k = ErrorT $ do
        x <- runErrorT m
        case x of
            Left l -> return (Left l)
            Right r -> runErrorT (k r)

throwErrorT :: Monad m => e -> ErrorT e m a
throwErrorT = ErrorT . return . Left
