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{-# LANGUAGE CPP #-}
-----------------------------------------------------------------------------
-- |
-- Copyright : (c) 2006-2007 Duncan Coutts
-- License : BSD-style
--
-- Maintainer : duncan@haskell.org
-- Portability : portable (H98 + FFI)
--
-- String encoding conversion
--
-----------------------------------------------------------------------------
module Codec.Text.IConv (
-- | This module provides pure functions for converting the string encoding
-- of strings represented by lazy 'ByteString's. This makes it easy to use
-- either in memory or with disk or network IO.
--
-- For example, a simple Latin1 to UTF-8 conversion program is just:
--
-- > import Codec.Text.IConv as IConv
-- > import Data.ByteString.Lazy as ByteString
-- >
-- > main = ByteString.interact (convert "LATIN1" "UTF-8")
--
-- Or you could lazily read in and convert a UTF-8 file to UTF-32 using:
--
-- > content <- fmap (IConv.convert "UTF-8" "UTF-32") (readFile file)
--
-- This module uses the POSIX @iconv()@ library function. The primary
-- advantage of using iconv is that it is widely available, most systems
-- have a wide range of supported string encodings and the conversion speed
-- it typically good. The iconv library is available on all unix systems
-- (since it is required by the POSIX.1 standard) and GNU libiconv is
-- available as a standalone library for other systems, including Windows.
-- * Simple conversion API
convert,
EncodingName,
-- * Variant that is lax about conversion errors
convertFuzzy,
Fuzzy(..),
-- * Variants that are pedantic about conversion errors
convertStrictly,
convertLazily,
ConversionError(..),
reportConversionError,
Span(..),
) where
import Prelude hiding (length, span)
import Control.Exception (assert)
import qualified Control.Exception as Exception
import Foreign.C.Error as C.Error (Errno, errnoToIOError)
import qualified Data.ByteString.Lazy as L (ByteString, toChunks, fromChunks)
import qualified Data.ByteString.Lazy.Internal as L (defaultChunkSize)
import qualified Data.ByteString as S
import qualified Codec.Text.IConv.Internal as IConv
import Codec.Text.IConv.Internal (IConv)
-- | A string encoding name, eg @\"UTF-8\"@ or @\"LATIN1\"@.
--
-- The range of string encodings available is determined by the capabilities
-- of the underlying iconv implementation.
--
-- When using the GNU C or libiconv libraries, the permitted values are listed
-- by the @iconv --list@ command, and all combinations of the listed values
-- are supported.
--
type EncodingName = String
-- | Output spans from encoding conversion. When nothing goes wrong we
-- expect just a bunch of 'Span's. If there are conversion errors we get other
-- span types.
--
data Span =
-- | An ordinary output span in the target encoding
Span !S.ByteString
-- | An error in the conversion process. If this occurs it will be the
-- last span.
| ConversionError !ConversionError
data ConversionError =
-- | The conversion from the input to output string encoding is not
-- supported by the underlying iconv implementation. This is usually
-- because a named encoding is not recognised or support for it
-- was not enabled on this system.
--
-- The POSIX standard does not guarantee that all possible combinations
-- of recognised string encoding are supported, however most common
-- implementations do support all possible combinations.
--
UnsuportedConversion EncodingName EncodingName
-- | This covers two possible conversion errors:
--
-- * There is a byte sequence in the input that is not valid in the input
-- encoding.
--
-- * There is a valid character in the input that has no corresponding
-- character in the output encoding.
--
-- Unfortunately iconv does not let us distinguish these two cases. In
-- either case, the Int parameter gives the byte offset in the input of
-- the unrecognised bytes or unconvertable character.
--
| InvalidChar Int
-- | This error covers the case where the end of the input has trailing
-- bytes that are the initial bytes of a valid character in the input
-- encoding. In other words, it looks like the input ended in the middle of
-- a multi-byte character. This would often be an indication that the input
-- was somehow truncated. Again, the Int parameter is the byte offset in
-- the input where the incomplete character starts.
--
| IncompleteChar Int
-- | An unexpected iconv error. The iconv spec lists a number of possible
-- expected errors but does not guarantee that there might not be other
-- errors.
--
-- This error can occur either immediately, which might indicate that the
-- iconv installation is messed up somehow, or it could occur later which
-- might indicate resource exhaustion or some other internal iconv error.
--
-- Use 'Foreign.C.Error.errnoToIOError' to get slightly more information
-- on what the error could possibly be.
--
| UnexpectedError C.Error.Errno
reportConversionError :: ConversionError -> IOError
reportConversionError conversionError = case conversionError of
UnsuportedConversion fromEncoding toEncoding
-> err $ "cannot convert from string encoding "
++ show fromEncoding ++ " to string encoding "
++ show toEncoding
InvalidChar inputPos -> err $ "invalid input sequence at byte offset "
++ show inputPos
IncompleteChar inputPos -> err $ "incomplete input sequence at byte offset "
++ show inputPos
UnexpectedError errno -> C.Error.errnoToIOError
"Codec.Text.IConv: unexpected error" errno
Nothing Nothing
where err msg = userError $ "Codec.Text.IConv: " ++ msg
{-# NOINLINE convert #-}
-- | Convert text from one named string encoding to another.
--
-- * The conversion is done lazily.
--
-- * An exception is thrown if conversion between the two encodings is not
-- supported.
--
-- * An exception is thrown if there are any encoding conversion errors.
--
convert :: EncodingName -- ^ Name of input string encoding
-> EncodingName -- ^ Name of output string encoding
-> L.ByteString -- ^ Input text
-> L.ByteString -- ^ Output text
convert fromEncoding toEncoding =
-- lazily convert the list of spans into an ordinary lazy ByteString:
L.fromChunks
. foldr span []
. convertLazily fromEncoding toEncoding
where
span (Span c) cs = c : cs
span (ConversionError e) _ =
#if MIN_VERSION_base(4,0,0)
Exception.throw (reportConversionError e)
#else
Exception.throw (Exception.IOException (reportConversionError e))
#endif
data Fuzzy = Transliterate | Discard
-- | Convert text ignoring encoding conversion problems.
--
-- If invalid byte sequences are found in the input they are ignored and
-- conversion continues if possible. This is not always possible especially
-- with stateful encodings. No placeholder character is inserted into the
-- output so there will be no indication that invalid byte sequences were
-- encountered.
--
-- If there are characters in the input that have no direct corresponding
-- character in the output encoding then they are dealt in one of two ways,
-- depending on the 'Fuzzy' argument. We can try and 'Transliterate' them into
-- the nearest corresponding character(s) or use a replacement character
-- (typically @\'?\'@ or the Unicode replacement character). Alternatively they
-- can simply be 'Discard'ed.
--
-- In either case, no exceptions will occur. In the case of unrecoverable
-- errors, the output will simply be truncated. This includes the case of
-- unrecognised or unsupported encoding names; the output will be empty.
--
-- * This function only works with the GNU iconv implementation which provides
-- this feature beyond what is required by the iconv specification.
--
convertFuzzy :: Fuzzy -- ^ Whether to try and transliterate or
-- discard characters with no direct conversion
-> EncodingName -- ^ Name of input string encoding
-> EncodingName -- ^ Name of output string encoding
-> L.ByteString -- ^ Input text
-> L.ByteString -- ^ Output text
convertFuzzy fuzzy fromEncoding toEncoding =
-- lazily convert the list of spans into an ordinary lazy ByteString:
L.fromChunks
. foldr span []
. convertInternal IgnoreInvalidChar fromEncoding (toEncoding ++ mode)
where
mode = case fuzzy of
Transliterate -> "//IGNORE,TRANSLIT"
Discard -> "//IGNORE"
span (Span c) cs = c : cs
span (ConversionError _) cs = cs
{-# NOINLINE convertStrictly #-}
-- | This variant does the conversion all in one go, so it is able to report
-- any conversion errors up front. It exposes all the possible error conditions
-- and never throws exceptions
--
-- The disadvantage is that no output can be produced before the whole input
-- is consumed. This might be problematic for very large inputs.
--
convertStrictly :: EncodingName -- ^ Name of input string encoding
-> EncodingName -- ^ Name of output string encoding
-> L.ByteString -- ^ Input text
-> Either L.ByteString
ConversionError -- ^ Output text or conversion error
convertStrictly fromEncoding toEncoding =
-- strictly convert the list of spans into an ordinary lazy ByteString
-- or an error
strictify []
. convertLazily fromEncoding toEncoding
where
strictify :: [S.ByteString] -> [Span] -> Either L.ByteString ConversionError
strictify cs [] = Left (L.fromChunks (reverse cs))
strictify cs (Span c : ss) = strictify (c:cs) ss
strictify _ (ConversionError e:_) = Right e
{-# NOINLINE convertLazily #-}
-- | This version provides a more complete but less convenient conversion
-- interface. It exposes all the possible error conditions and never throws
-- exceptions.
--
-- The conversion is still lazy. It returns a list of spans, where a span may
-- be an ordinary span of output text or a conversion error. This somewhat
-- complex interface allows both for lazy conversion and for precise reporting
-- of conversion problems. The other functions 'convert' and 'convertStrictly'
-- are actually simple wrappers on this function.
--
convertLazily :: EncodingName -- ^ Name of input string encoding
-> EncodingName -- ^ Name of output string encoding
-> L.ByteString -- ^ Input text
-> [Span] -- ^ Output text spans
convertLazily = convertInternal StopOnInvalidChar
data InvalidCharBehaviour = StopOnInvalidChar | IgnoreInvalidChar
convertInternal :: InvalidCharBehaviour
-> EncodingName -> EncodingName
-> L.ByteString -> [Span]
convertInternal ignore fromEncoding toEncoding input =
IConv.run fromEncoding toEncoding $ \status -> case status of
IConv.InitOk -> do IConv.newOutputBuffer outChunkSize
fillInputBuffer ignore (L.toChunks input)
IConv.UnsupportedConversion -> failConversion (UnsuportedConversion
fromEncoding
toEncoding)
IConv.UnexpectedInitError errno -> failConversion (UnexpectedError errno)
fillInputBuffer :: InvalidCharBehaviour -> [S.ByteString] -> IConv [Span]
fillInputBuffer ignore (inChunk : inChunks) = do
IConv.pushInputBuffer inChunk
drainBuffers ignore inChunks
fillInputBuffer _ignore [] = do
outputBufferBytesAvailable <- IConv.outputBufferBytesAvailable
IConv.finalise
if outputBufferBytesAvailable > 0
then do outChunk <- IConv.popOutputBuffer
return [Span outChunk]
else return []
drainBuffers :: InvalidCharBehaviour -> [S.ByteString] -> IConv [Span]
drainBuffers ignore inChunks = do
inputBufferEmpty_ <- IConv.inputBufferEmpty
outputBufferFull <- IConv.outputBufferFull
assert (not outputBufferFull && not inputBufferEmpty_) $ return ()
-- this invariant guarantees we can always make forward progress
status <- IConv.iconv
case status of
IConv.InputEmpty -> do
inputBufferEmpty <- IConv.inputBufferEmpty
assert inputBufferEmpty $ fillInputBuffer ignore inChunks
IConv.OutputFull -> do
outChunk <- IConv.popOutputBuffer
outChunks <- IConv.unsafeInterleave $ do
IConv.newOutputBuffer outChunkSize
drainBuffers ignore inChunks
return (Span outChunk : outChunks)
IConv.InvalidChar -> invalidChar ignore inChunks
IConv.IncompleteChar -> fixupBoundary ignore inChunks
IConv.UnexpectedError errno -> failConversion (UnexpectedError errno)
-- | The posix iconv api looks like it's designed specifically for streaming
-- and it is, except for one really really annoying corner case...
--
-- Suppose you're converting a stream, say by reading a file in 4k chunks. This
-- would seem to be the canonical use case for iconv, reading and converting an
-- input file. However suppose the 4k read chunk happens to split a multi-byte
-- character. Then iconv will stop just before that char and tell us that its
-- an incomplete char. So far so good. Now what we'd like to do is have iconv
-- remember those last few bytes in its conversion state so we can carry on
-- with the next 4k block. Sadly it does not. It requires us to fix things up
-- so that it can carry on with the next block starting with a complete multi-
-- byte character. Do do that we have to somehow copy those few trailing bytes
-- to the beginning of the next block. That's perhaps not too bad in an
-- imperitive context using a mutable input buffer - we'd just copy the few
-- trailing bytes to the beginning of the buffer and do a short read (ie 4k-n
-- the number of trailing bytes). That's not terribly nice since it means the
-- OS has to do IO on non-page aligned buffers which tends to be slower. It's
-- worse for us though since we're not using a mutable input buffer, we're
-- using a lazy bytestring which is a sequence of immutable buffers.
--
-- So we have to do more cunning things. We could just prepend the trailing
-- bytes to the next block, but that would mean alocating and copying the whole
-- next block just to prepend a couple bytes. This probably happens quite
-- frequently so would be pretty slow. So we have to be even more cunning.
--
-- The solution is to create a very small buffer to cover the few bytes making
-- up the character spanning the block boundary. So we copy the trailing bytes
-- plus a few from the beginning of the next block. Then we run iconv again on
-- that small buffer. How many bytes from the next block to copy is a slightly
-- tricky issue. If we copy too few there's no guarantee that we have enough to
-- give a complete character. We opt for a maximum size of 16, 'tmpChunkSize'
-- on the theory that no encoding in existance uses that many bytes to encode a
-- single character, so it ought to be enough. Yeah, it's a tad dodgey.
--
-- Having papered over the block boundary, we still have to cross the boundary
-- of this small buffer. It looks like we've still got the same problem,
-- however this time we should have crossed over into bytes that are wholly
-- part of the large following block so we can abandon our small temp buffer
-- an continue with the following block, with a slight offset for the few bytes
-- taken up by the chars that fit into the small buffer.
--
-- So yeah, pretty complex. Check out the proof below of the tricky case.
--
fixupBoundary :: InvalidCharBehaviour -> [S.ByteString] -> IConv [Span]
fixupBoundary _ignore [] = do
inputPos <- IConv.inputPosition
failConversion (IncompleteChar inputPos)
fixupBoundary ignore inChunks@(inChunk : inChunks') = do
inSize <- IConv.inputBufferSize
assert (inSize < tmpChunkSize) $ return ()
let extraBytes = tmpChunkSize - inSize
if S.length inChunk <= extraBytes
then do
IConv.replaceInputBuffer (`S.append` inChunk)
drainBuffers ignore inChunks'
else do
IConv.replaceInputBuffer (`S.append` S.take extraBytes inChunk)
before <- IConv.inputBufferSize
assert (before == tmpChunkSize) $ return ()
status <- IConv.iconv
after <- IConv.inputBufferSize
let consumed = before - after
case status of
IConv.InputEmpty ->
assert (consumed == tmpChunkSize) $
fillInputBuffer ignore (S.drop extraBytes inChunk : inChunks')
IConv.OutputFull -> do
outChunk <- IConv.popOutputBuffer
outChunks <- IConv.unsafeInterleave $ do
IConv.newOutputBuffer outChunkSize
drainBuffers ignore inChunks
return (Span outChunk : outChunks)
IConv.InvalidChar -> invalidChar ignore inChunks
IConv.IncompleteChar ->
assert (inSize < consumed && consumed < tmpChunkSize) $
-- inSize < consumed < tmpChunkSize
-- => { subtract inSize from each side }
-- 0 < consumed - inSize < tmpChunkSize - inSize
-- => { by definition that extraBytes = tmpChunkSize - inSize }
-- 0 < consumed - inSize < extraBytes
-- => { since we're in the False case of the if, we know:
-- not (S.length inChunk <= extraBytes)
-- = S.length inChunk > extraBytes
-- = extraBytes < S.length inChunk }
-- 0 < consumed - inSize < extraBytes < S.length inChunk
--
-- And we're done! We know it's safe to drop (consumed - inSize) from
-- inChunk since it's more than 0 and less than the inChunk size, so
-- we're not being left with an empty chunk (which is not allowed).
drainBuffers ignore (S.drop (consumed - inSize) inChunk : inChunks')
IConv.UnexpectedError errno -> failConversion (UnexpectedError errno)
invalidChar :: InvalidCharBehaviour -> [S.ByteString] -> IConv [Span]
invalidChar StopOnInvalidChar _ = do
inputPos <- IConv.inputPosition
failConversion (InvalidChar inputPos)
invalidChar IgnoreInvalidChar inChunks = do
inputPos <- IConv.inputPosition
let invalidCharError = ConversionError (InvalidChar inputPos)
outputBufferBytesAvailable <- IConv.outputBufferBytesAvailable
if outputBufferBytesAvailable > 0
then do outChunk <- IConv.popOutputBuffer
outChunks <- IConv.unsafeInterleave $ do
IConv.newOutputBuffer outChunkSize
inputBufferEmpty <- IConv.inputBufferEmpty
if inputBufferEmpty
then fillInputBuffer IgnoreInvalidChar inChunks
else drainBuffers IgnoreInvalidChar inChunks
return (Span outChunk : invalidCharError : outChunks)
else do outChunks <- IConv.unsafeInterleave $ do
IConv.newOutputBuffer outChunkSize
inputBufferEmpty <- IConv.inputBufferEmpty
if inputBufferEmpty
then fillInputBuffer IgnoreInvalidChar inChunks
else drainBuffers IgnoreInvalidChar inChunks
return (invalidCharError : outChunks)
failConversion :: ConversionError -> IConv [Span]
failConversion err = do
outputBufferBytesAvailable <- IConv.outputBufferBytesAvailable
IConv.finalise
if outputBufferBytesAvailable > 0
then do outChunk <- IConv.popOutputBuffer
return [Span outChunk, ConversionError err]
else return [ ConversionError err]
outChunkSize :: Int
outChunkSize = L.defaultChunkSize
tmpChunkSize :: Int
tmpChunkSize = 16
|