-----------------------------------------------------------------------------
-- |
-- Module: Data.Enumerator.Text
-- Copyright: 2010 John Millikin
-- License: MIT
--
-- Maintainer: jmillikin@gmail.com
-- Portability: portable
--
-- Character-oriented alternatives to "Data.Enumerator.List". Note that the
-- enumeratees in this module must unpack their inputs to work properly. If
-- you do not need to handle leftover input on a char-by-char basis, the
-- chunk-oriented versions will be much faster.
--
-- This module is intended to be imported qualified:
--
-- @
-- import qualified Data.Enumerator.Text as ET
-- @
--
-- Since: 0.2
--
-----------------------------------------------------------------------------

module Data.Enumerator.Text (

	-- * IO
	  enumHandle
	, enumFile
	, iterHandle
	
	-- * List analogues
	
	-- ** Folds
	, fold
	, foldM
	
	-- ** Maps
	, Data.Enumerator.Text.map
	, Data.Enumerator.Text.mapM
	, Data.Enumerator.Text.concatMap
	, concatMapM
	
	-- ** Accumulating maps
	, mapAccum
	, mapAccumM
	
	-- ** Infinite streams
	, Data.Enumerator.Text.iterate
	, iterateM
	, Data.Enumerator.Text.repeat
	, repeatM
	
	-- ** Bounded streams
	, Data.Enumerator.Text.replicate
	, replicateM
	, generateM
	, unfold
	, unfoldM
	
	-- ** Filters
	, Data.Enumerator.Text.filter
	, filterM
	
	-- ** Consumers
	, Data.Enumerator.Text.take
	, takeWhile
	, consume
	
	-- ** Unsorted
	, Data.Enumerator.Text.head
	, Data.Enumerator.Text.drop
	, Data.Enumerator.Text.dropWhile
	, require
	, isolate
	, splitWhen
	, lines
	
	-- * Text codecs
	, Codec
	, encode
	, decode
	, utf8
	, utf16_le
	, utf16_be
	, utf32_le
	, utf32_be
	, ascii
	, iso8859_1

	) where

import Prelude hiding (head, drop, takeWhile, lines)
import qualified Prelude
import Data.Enumerator hiding ( head, drop, generateM, filterM, consume
                              , concatMapM, iterateM, repeatM, replicateM
                              , foldM)
import Data.Enumerator.Util (tSpanBy, tlSpanBy, reprWord, reprChar, textToStrict)
import Control.Monad.IO.Class (MonadIO)
import qualified Control.Exception as Exc
import Control.Arrow (first)
import Data.Maybe (catMaybes)
import qualified Data.Text as T
import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as B8
import qualified Data.Text.Encoding as TE
import Data.Word (Word8, Word16)
import Data.Bits ((.&.), (.|.), shiftL)
import qualified System.IO as IO
import System.IO.Error (isEOFError)
import qualified Data.Text.IO as TIO
import Data.Char (ord)
import System.IO.Unsafe (unsafePerformIO)
import qualified Data.Text.Lazy as TL
import qualified Data.Enumerator.List as EL
import qualified Control.Monad as CM
import Control.Monad.Trans.Class (lift)
import Control.Monad (liftM)



-- | Consume the entire input stream with a strict left fold, one character
-- at a time.
--
-- Since: 0.4.8

fold :: Monad m => (b -> Char -> b) -> b
     -> Iteratee T.Text m b
fold step = EL.fold (T.foldl' step)


-- | Consume the entire input stream with a strict monadic left fold, one
-- character at a time.
--
-- Since: 0.4.8

foldM :: Monad m => (b -> Char -> m b) -> b
      -> Iteratee T.Text m b
foldM step = EL.foldM (\b txt -> CM.foldM step b (T.unpack txt))


-- | Enumerates a stream of characters by repeatedly applying a function to
-- some state.
--
-- Similar to 'iterate'.
--
-- Since: 0.4.8

unfold :: Monad m => (s -> Maybe (Char, s)) -> s -> Enumerator T.Text m b
unfold f = checkContinue1 $ \loop s k -> case f s of
	Nothing -> continue k
	Just (c, s') -> k (Chunks [T.singleton c]) >>== loop s'


-- | Enumerates a stream of characters by repeatedly applying a computation
-- to some state.
--
-- Similar to 'iterateM'.
--
-- Since: 0.4.8

unfoldM :: Monad m => (s -> m (Maybe (Char, s))) -> s -> Enumerator T.Text m b
unfoldM f = checkContinue1 $ \loop s k -> do
	fs <- lift (f s)
	case fs of
		Nothing -> continue k
		Just (c, s') -> k (Chunks [T.singleton c]) >>== loop s'


-- | @'map' f@ applies /f/ to each input character and feeds the
-- resulting outputs to the inner iteratee.
--
-- Since: 0.4.8

map :: Monad m => (Char -> Char) -> Enumeratee T.Text T.Text m b
map f = Data.Enumerator.Text.concatMap (\x -> T.singleton (f x))


-- | @'mapM' f@ applies /f/ to each input character and feeds the
-- resulting outputs to the inner iteratee.
--
-- Since: 0.4.8

mapM :: Monad m => (Char -> m Char) -> Enumeratee T.Text T.Text m b
mapM f = Data.Enumerator.Text.concatMapM (\x -> liftM T.singleton (f x))


-- | @'concatMap' f@ applies /f/ to each input character and feeds the
-- resulting outputs to the inner iteratee.
--
-- Since: 0.4.8

concatMap :: Monad m => (Char -> T.Text) -> Enumeratee T.Text T.Text m b
concatMap f = Data.Enumerator.Text.concatMapM (return . f)


-- | @'concatMapM' f@ applies /f/ to each input character and feeds the
-- resulting outputs to the inner iteratee.
--
-- Since: 0.4.8

concatMapM :: Monad m => (Char -> m T.Text) -> Enumeratee T.Text T.Text m b
concatMapM f = checkDone (continue . step) where
	step k EOF = yield (Continue k) EOF
	step k (Chunks xs) = loop k (TL.unpack (TL.fromChunks xs))
	
	loop k [] = continue (step k)
	loop k (x:xs) = do
		fx <- lift (f x)
		k (Chunks [fx]) >>==
			checkDoneEx (Chunks [T.pack xs]) (\k' -> loop k' xs)


-- | Similar to 'map', but with a stateful step function.
--
-- Since: 0.4.9

mapAccum :: Monad m => (s -> Char -> (s, Char)) -> s -> Enumeratee T.Text T.Text m b
mapAccum f s0 = checkDone (continue . step s0) where
	step _ k EOF = yield (Continue k) EOF
	step s k (Chunks xs) = loop s k xs
	
	loop s k [] = continue (step s k)
	loop s k (x:xs) = case T.uncons x of
		Nothing -> loop s k xs
		Just (c, x') -> case f s c of
			(s', ai) -> k (Chunks [T.singleton ai]) >>==
				checkDoneEx (Chunks (x':xs)) (\k' -> loop s' k' (x':xs))


-- | Similar to 'mapM', but with a stateful step function.
--
-- Since: 0.4.9

mapAccumM :: Monad m => (s -> Char -> m (s, Char)) -> s -> Enumeratee T.Text T.Text m b
mapAccumM f s0 = checkDone (continue . step s0) where
	step _ k EOF = yield (Continue k) EOF
	step s k (Chunks xs) = loop s k xs
	
	loop s k [] = continue (step s k)
	loop s k (x:xs) = case T.uncons x of
		Nothing -> loop s k xs
		Just (c, x') -> do
			(s', ai) <- lift (f s c)
			k (Chunks [T.singleton ai]) >>==
				checkDoneEx (Chunks (x':xs)) (\k' -> loop s' k' (x':xs))


-- | @'iterate' f x@ enumerates an infinite stream of repeated applications
-- of /f/ to /x/.
--
-- Analogous to 'Prelude.iterate'.
--
-- Since: 0.4.8

iterate :: Monad m => (Char -> Char) -> Char -> Enumerator T.Text m b
iterate f = checkContinue1 $ \loop s k -> k (Chunks [T.singleton s]) >>== loop (f s)


-- | Similar to 'iterate', except the iteration function is monadic.
--
-- Since: 0.4.8

iterateM :: Monad m => (Char -> m Char) -> Char -> Enumerator T.Text m b
iterateM f base = worker (return base) where
	worker = checkContinue1 $ \loop m_char k -> do
		char <- lift m_char
		k (Chunks [T.singleton char]) >>== loop (f char)


-- | Enumerates an infinite stream of a single character.
--
-- Analogous to 'Prelude.repeat'.
--
-- Since: 0.4.8

repeat :: Monad m => Char -> Enumerator T.Text m b
repeat char = EL.repeat (T.singleton char)


-- | Enumerates an infinite stream of characters. Each character is computed
-- by the underlying monad.
--
-- Since: 0.4.8

repeatM :: Monad m => m Char -> Enumerator T.Text m b
repeatM next = EL.repeatM (liftM T.singleton next)


-- | @'replicate' n x@ enumerates a stream containing /n/ copies of /x/.
--
-- Since: 0.4.8

replicate :: Monad m => Integer -> Char -> Enumerator T.Text m b
replicate n byte = EL.replicate n (T.singleton byte)


-- | @'replicateM' n m_x@ enumerates a stream of /n/ characters, with each
-- character computed by /m_x/.
--
-- Since: 0.4.8

replicateM :: Monad m => Integer -> m Char -> Enumerator T.Text m b
replicateM n next = EL.replicateM n (liftM T.singleton next)


-- | Like 'repeatM', except the computation may terminate the stream by
-- returning 'Nothing'.
--
-- Since: 0.4.8

generateM :: Monad m => m (Maybe Char) -> Enumerator T.Text m b
generateM next = EL.generateM (liftM (liftM T.singleton) next)


-- | Applies a predicate to the stream. The inner iteratee only receives
-- characters for which the predicate is @True@.
--
-- Since: 0.4.8

filter :: Monad m => (Char -> Bool) -> Enumeratee T.Text T.Text m b
filter p = Data.Enumerator.Text.concatMap (\x -> T.pack [x | p x])


-- | Applies a monadic predicate to the stream. The inner iteratee only
-- receives characters for which the predicate returns @True@.
--
-- Since: 0.4.8

filterM :: Monad m => (Char -> m Bool) -> Enumeratee T.Text T.Text m b
filterM p = Data.Enumerator.Text.concatMapM (\x -> liftM T.pack (CM.filterM p [x]))


-- | @'take' n@ extracts the next /n/ characters from the stream, as a lazy
-- Text.
--
-- Since: 0.4.5

take :: Monad m => Integer -> Iteratee T.Text m TL.Text
take n | n <= 0 = return TL.empty
take n = continue (loop id n) where
	loop acc n' (Chunks xs) = iter where
		lazy = TL.fromChunks xs
		len = toInteger (TL.length lazy)
		
		iter = if len < n'
			then continue (loop (acc . (TL.append lazy)) (n' - len))
			else let
				(xs', extra) = TL.splitAt (fromInteger n') lazy
				in yield (acc xs') (toChunks extra)
	loop acc _ EOF = yield (acc TL.empty) EOF


-- | @'takeWhile' p@ extracts input from the stream until the first character
-- which does not match the predicate.
--
-- Since: 0.4.5

takeWhile :: Monad m => (Char -> Bool) -> Iteratee T.Text m TL.Text
takeWhile p = continue (loop id) where
	loop acc (Chunks []) = continue (loop acc)
	loop acc (Chunks xs) = iter where
		lazy = TL.fromChunks xs
		(xs', extra) = tlSpanBy p lazy
		iter = if TL.null extra
			then continue (loop (acc . (TL.append lazy)))
			else yield (acc xs') (toChunks extra)
	loop acc EOF = yield (acc TL.empty) EOF


-- | @'consume' = 'takeWhile' (const True)@
--
-- Since: 0.4.5

consume :: Monad m => Iteratee T.Text m TL.Text
consume = continue (loop id) where
	loop acc (Chunks []) = continue (loop acc)
	loop acc (Chunks xs) = iter where
		lazy = TL.fromChunks xs
		iter = continue (loop (acc . (TL.append lazy)))
	loop acc EOF = yield (acc TL.empty) EOF


-- | Get the next character from the stream, or 'Nothing' if the stream has
-- ended.
--
-- Since: 0.4.5

head :: Monad m => Iteratee T.Text m (Maybe Char)
head = continue loop where
	loop (Chunks xs) = case TL.uncons (TL.fromChunks xs) of
		Just (char, extra) -> yield (Just char) (toChunks extra)
		Nothing -> head
	loop EOF = yield Nothing EOF


-- | @'drop' n@ ignores /n/ characters of input from the stream.
--
-- Since: 0.4.5

drop :: Monad m => Integer -> Iteratee T.Text m ()
drop n | n <= 0 = return ()
drop n = continue (loop n) where
	loop n' (Chunks xs) = iter where
		lazy = TL.fromChunks xs
		len = toInteger (TL.length lazy)
		iter = if len < n'
			then drop (n' - len)
			else yield () (toChunks (TL.drop (fromInteger n') lazy))
	loop _ EOF = yield () EOF


-- | @'dropWhile' p@ ignores input from the stream until the first character
-- which does not match the predicate.
--
-- Since: 0.4.5

dropWhile :: Monad m => (Char -> Bool) -> Iteratee T.Text m ()
dropWhile p = continue loop where
	loop (Chunks xs) = iter where
		lazy = TL.dropWhile p (TL.fromChunks xs)
		iter = if TL.null lazy
			then continue loop
			else yield () (toChunks lazy)
	loop EOF = yield () EOF


-- | @'require' n@ buffers input until at least /n/ characters are available,
-- or throws an error if the stream ends early.
--
-- Since: 0.4.5

require :: Monad m => Integer -> Iteratee T.Text m ()
require n | n <= 0 = return ()
require n = continue (loop id n) where
	loop acc n' (Chunks xs) = iter where
		lazy = TL.fromChunks xs
		len = toInteger (TL.length lazy)
		iter = if len < n'
			then continue (loop (acc . (TL.append lazy)) (n' - len))
			else yield () (toChunks (acc lazy))
	loop _ _ EOF = throwError (Exc.ErrorCall "require: Unexpected EOF")


-- | @'isolate' n@ reads at most /n/ characters from the stream, and passes
-- them to its iteratee. If the iteratee finishes early, characters continue
-- to be consumed from the outer stream until /n/ have been consumed.
--
-- Since: 0.4.5

isolate :: Monad m => Integer -> Enumeratee T.Text T.Text m b
isolate n step | n <= 0 = return step
isolate n (Continue k) = continue loop where
	loop (Chunks []) = continue loop
	loop (Chunks xs) = iter where
		lazy = TL.fromChunks xs
		len = toInteger (TL.length lazy)
		
		iter = if len <= n
			then k (Chunks xs) >>== isolate (n - len)
			else let
				(s1, s2) = TL.splitAt (fromInteger n) lazy
				in k (toChunks s1) >>== (\step -> yield step (toChunks s2))
	loop EOF = k EOF >>== (\step -> yield step EOF)
isolate n step = drop n >> return step


-- | Split on characters satisfying a given predicate.
--
-- Since: 0.4.8

splitWhen :: Monad m => (Char -> Bool) -> Enumeratee T.Text T.Text m b
splitWhen p = loop where
	loop = checkDone step
	step k = isEOF >>= \eof -> if eof
		then yield (Continue k) EOF
		else do
			lazy <- takeWhile (not . p)
			let text = textToStrict lazy
			eof <- isEOF
			drop 1
			if TL.null lazy && eof
				then yield (Continue k) EOF
				else k (Chunks [text]) >>== loop


-- | @'lines' = 'splitWhen' (== '\n')@
--
-- Since: 0.4.8

lines :: Monad m => Enumeratee T.Text T.Text m b
lines = splitWhen (== '\n')



-- | Read lines of text from the handle, and stream them to an 'Iteratee'.
-- If an exception occurs during file IO, enumeration will stop and 'Error'
-- will be returned. Exceptions from the iteratee are not caught.
--
-- The handle should be opened with an appropriate text encoding, and
-- in 'IO.ReadMode' or 'IO.ReadWriteMode'.
--
-- Since: 0.2

enumHandle :: MonadIO m => IO.Handle
           -> Enumerator T.Text m b
enumHandle h = checkContinue0 $ \loop k -> do
	let getText = Exc.catch
		(Just `fmap` TIO.hGetLine h)
		(\err -> if isEOFError err
			then return Nothing
			else Exc.throwIO err)
	
	maybeText <- tryIO getText
	case maybeText of
		Nothing -> continue k
		Just text -> k (Chunks [text]) >>== loop
	


-- | Opens a file path in text mode, and passes the handle to 'enumHandle'.
-- The file will be closed when the 'Iteratee' finishes.
--
-- Since: 0.2

enumFile :: FilePath -> Enumerator T.Text IO b
enumFile path step = do
	h <- tryIO (IO.openFile path IO.ReadMode)
	Iteratee $ Exc.finally
		(runIteratee (enumHandle h step))
		(IO.hClose h)


-- | Read text from a stream and write it to a handle. If an exception
-- occurs during file IO, enumeration will stop and 'Error' will be
-- returned.
--
-- The handle should be opened with an appropriate text encoding, and
-- in 'IO.WriteMode' or 'IO.ReadWriteMode'.
--
-- Since: 0.2

iterHandle :: MonadIO m => IO.Handle
           -> Iteratee T.Text m ()
iterHandle h = continue step where
	step EOF = yield () EOF
	step (Chunks []) = continue step
	step (Chunks chunks) = do
		tryIO (mapM_ (TIO.hPutStr h) chunks)
		continue step


data Codec = Codec
	{ codecName :: T.Text
	, codecEncode
		:: T.Text
		-> (B.ByteString, Maybe (Exc.SomeException, T.Text))
	, codecDecode
		:: B.ByteString
		-> (T.Text, Either
			(Exc.SomeException, B.ByteString)
			B.ByteString)
	}

instance Show Codec where
	showsPrec d c = showParen (d > 10) $
		showString "Codec " . shows (codecName c)


-- | Convert text into bytes, using the provided codec. If the codec is
-- not capable of representing an input character, an error will be thrown.
--
-- Since: 0.2

encode :: Monad m => Codec
       -> Enumeratee T.Text B.ByteString m b
encode codec = checkDone (continue . step) where
	step k EOF = yield (Continue k) EOF
	step k (Chunks xs) = loop k xs
	
	loop k [] = continue (step k)
	loop k (x:xs) = let
		(bytes, extra) = codecEncode codec x
		extraChunks = Chunks $ case extra of
			Nothing -> xs
			Just (_, text) -> text:xs
		
		checkError k' = case extra of
			Nothing -> loop k' xs
			Just (exc, _) -> throwError exc
		
		in if B.null bytes
			then checkError k
			else k (Chunks [bytes]) >>==
				checkDoneEx extraChunks checkError


-- | Convert bytes into text, using the provided codec. If the codec is
-- not capable of decoding an input byte sequence, an error will be thrown.
--
-- Since: 0.2

decode :: Monad m => Codec
       -> Enumeratee B.ByteString T.Text m b
decode codec = checkDone (continue . step B.empty) where
	step _   k EOF = yield (Continue k) EOF
	step acc k (Chunks xs) = loop acc k xs
	
	loop acc k [] = continue (step acc k)
	loop acc k (x:xs) = let
		(text, extra) = codecDecode codec (B.append acc x)
		extraChunks = Chunks (either snd id extra : xs)
		
		checkError k' = case extra of
			Left (exc, _) -> throwError exc
			Right bytes -> loop bytes k' xs
		
		in if T.null text
			then checkError k
			else k (Chunks [text]) >>==
				checkDoneEx extraChunks checkError

byteSplits :: B.ByteString
           -> [(B.ByteString, B.ByteString)]
byteSplits bytes = loop (B.length bytes) where
	loop 0 = [(B.empty, bytes)]
	loop n = B.splitAt n bytes : loop (n - 1)

splitSlowly :: (B.ByteString -> T.Text)
            -> B.ByteString
            -> (T.Text, Either
            	(Exc.SomeException, B.ByteString)
            	B.ByteString)
splitSlowly dec bytes = valid where
	valid = firstValid (Prelude.map decFirst splits)
	splits = byteSplits bytes
	firstValid = Prelude.head . catMaybes
	tryDec = tryEvaluate . dec
	
	decFirst (a, b) = case tryDec a of
		Left _ -> Nothing
		Right text -> Just (text, case tryDec b of
			Left exc -> Left (exc, b)
			
			-- this case shouldn't occur, since splitSlowly
			-- is only called when parsing failed somewhere
			Right _ -> Right B.empty)

utf8 :: Codec
utf8 = Codec name enc dec where
	name = T.pack "UTF-8"
	enc text = (TE.encodeUtf8 text, Nothing)
	dec bytes = case splitQuickly bytes of
		Just (text, extra) -> (text, Right extra)
		Nothing -> splitSlowly TE.decodeUtf8 bytes

	splitQuickly bytes = loop 0 >>= maybeDecode where

		required x0
			| x0 .&. 0x80 == 0x00 = 1
			| x0 .&. 0xE0 == 0xC0 = 2
			| x0 .&. 0xF0 == 0xE0 = 3
			| x0 .&. 0xF8 == 0xF0 = 4
			
			-- Invalid input; let Text figure it out
			| otherwise           = 0

		maxN = B.length bytes
		
		loop n | n == maxN = Just (TE.decodeUtf8 bytes, B.empty)
		loop n = let
			req = required (B.index bytes n)
			tooLong = first TE.decodeUtf8 (B.splitAt n bytes)
			decodeMore = loop $! n + req
			in if req == 0
				then Nothing
				else if n + req > maxN
					then Just tooLong
					else decodeMore

utf16_le :: Codec
utf16_le = Codec name enc dec where
	name = T.pack "UTF-16-LE"
	enc text = (TE.encodeUtf16LE text, Nothing)
	dec bytes = case splitQuickly bytes of
		Just (text, extra) -> (text, Right extra)
		Nothing -> splitSlowly TE.decodeUtf16LE bytes

	splitQuickly bytes = maybeDecode (loop 0) where
		maxN = B.length bytes
		
		loop n |  n      == maxN = decodeAll
		       | (n + 1) == maxN = decodeTo n
		loop n = let
			req = utf16Required
				(B.index bytes 0)
				(B.index bytes 1)
			decodeMore = loop $! n + req
			in if n + req > maxN
				then decodeTo n
				else decodeMore
		
		decodeTo n = first TE.decodeUtf16LE (B.splitAt n bytes)
		decodeAll = (TE.decodeUtf16LE bytes, B.empty)

utf16_be :: Codec
utf16_be = Codec name enc dec where
	name = T.pack "UTF-16-BE"
	enc text = (TE.encodeUtf16BE text, Nothing)
	dec bytes = case splitQuickly bytes of
		Just (text, extra) -> (text, Right extra)
		Nothing -> splitSlowly TE.decodeUtf16BE bytes

	splitQuickly bytes = maybeDecode (loop 0) where
		maxN = B.length bytes
		
		loop n |  n      == maxN = decodeAll
		       | (n + 1) == maxN = decodeTo n
		loop n = let
			req = utf16Required
				(B.index bytes 1)
				(B.index bytes 0)
			decodeMore = loop $! n + req
			in if n + req > maxN
				then decodeTo n
				else decodeMore
		
		decodeTo n = first TE.decodeUtf16BE (B.splitAt n bytes)
		decodeAll = (TE.decodeUtf16BE bytes, B.empty)

utf16Required :: Word8 -> Word8 -> Int
utf16Required x0 x1 = required where
	required = if x >= 0xD800 && x <= 0xDBFF
		then 4
		else 2
	x :: Word16
	x = (fromIntegral x1 `shiftL` 8) .|. fromIntegral x0

utf32_le :: Codec
utf32_le = Codec name enc dec where
	name = T.pack "UTF-32-LE"
	enc text = (TE.encodeUtf32LE text, Nothing)
	dec bs = case utf32SplitBytes TE.decodeUtf32LE bs of
		Just (text, extra) -> (text, Right extra)
		Nothing -> splitSlowly TE.decodeUtf32LE bs

utf32_be :: Codec
utf32_be = Codec name enc dec where
	name = T.pack "UTF-32-BE"
	enc text = (TE.encodeUtf32BE text, Nothing)
	dec bs = case utf32SplitBytes TE.decodeUtf32BE bs of
		Just (text, extra) -> (text, Right extra)
		Nothing -> splitSlowly TE.decodeUtf32BE bs

utf32SplitBytes :: (B.ByteString -> T.Text)
                -> B.ByteString
                -> Maybe (T.Text, B.ByteString)
utf32SplitBytes dec bytes = split where
	split = maybeDecode (dec toDecode, extra)
	len = B.length bytes
	lenExtra = mod len 4
	
	lenToDecode = len - lenExtra
	(toDecode, extra) = if lenExtra == 0
		then (bytes, B.empty)
		else B.splitAt lenToDecode bytes

ascii :: Codec
ascii = Codec name enc dec where
	name = T.pack "ASCII"
	enc text = (bytes, extra) where
		(safe, unsafe) = tSpanBy (\c -> ord c <= 0x7F) text
		bytes = B8.pack (T.unpack safe)
		extra = if T.null unsafe
			then Nothing
			else Just (illegalEnc name (T.head unsafe), unsafe)
	
	dec bytes = (text, extra) where
		(safe, unsafe) = B.span (<= 0x7F) bytes
		text = T.pack (B8.unpack safe)
		extra = if B.null unsafe
			then Right B.empty
			else Left (illegalDec name (B.head unsafe), unsafe)

iso8859_1 :: Codec
iso8859_1 = Codec name enc dec where
	name = T.pack "ISO-8859-1"
	enc text = (bytes, extra) where
		(safe, unsafe) = tSpanBy (\c -> ord c <= 0xFF) text
		bytes = B8.pack (T.unpack safe)
		extra = if T.null unsafe
			then Nothing
			else Just (illegalEnc name (T.head unsafe), unsafe)
	
	dec bytes = (T.pack (B8.unpack bytes), Right B.empty)

illegalEnc :: T.Text -> Char -> Exc.SomeException
illegalEnc name c = Exc.toException . Exc.ErrorCall $
	concat [ "Codec "
	       , show name
	       , " can't encode character "
	       , reprChar c
	       ]

illegalDec :: T.Text -> Word8 -> Exc.SomeException
illegalDec name w = Exc.toException . Exc.ErrorCall $
	concat [ "Codec "
	       , show name
	       , " can't decode byte "
	       , reprWord w
	       ]

tryEvaluate :: a -> Either Exc.SomeException a
tryEvaluate = unsafePerformIO . Exc.try . Exc.evaluate

maybeDecode:: (a, b) -> Maybe (a, b)
maybeDecode (a, b) = case tryEvaluate a of
	Left _ -> Nothing
	Right _ -> Just (a, b)


toChunks :: TL.Text -> Stream T.Text
toChunks = Chunks . TL.toChunks