content: {-# LANGUAGE Trustworthy #-}
  {-# LANGUAGE NoImplicitPrelude, ExistentialQuantification #-}
  {-# OPTIONS_GHC -funbox-strict-fields #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  GHC.IO.Encoding.Types
  -- Copyright   :  (c) The University of Glasgow, 2008-2009
  -- License     :  see libraries/base/LICENSE
  -- 
  -- Maintainer  :  libraries@haskell.org
  -- Stability   :  internal
  -- Portability :  non-portable
  --
  -- Types for text encoding/decoding
  --
  -----------------------------------------------------------------------------
  
  module GHC.IO.Encoding.Types (
      BufferCodec(..),
      TextEncoding(..),
      TextEncoder, TextDecoder,
      EncodeBuffer, DecodeBuffer,
      CodingProgress(..)
    ) where
  
  import GHC.Base
  import GHC.Word
  import GHC.Show
  -- import GHC.IO
  import GHC.IO.Buffer
  
  -- -----------------------------------------------------------------------------
  -- Text encoders/decoders
  
  data BufferCodec from to state = BufferCodec {
    encode :: Buffer from -> Buffer to -> IO (CodingProgress, Buffer from, Buffer to),
     -- ^ The @encode@ function translates elements of the buffer @from@
     -- to the buffer @to@.  It should translate as many elements as possible
     -- given the sizes of the buffers, including translating zero elements
     -- if there is either not enough room in @to@, or @from@ does not
     -- contain a complete multibyte sequence.
     --
     -- The fact that as many elements as possible are translated is used by the IO
     -- library in order to report translation errors at the point they
     -- actually occur, rather than when the buffer is translated.
     --
     -- To allow us to use iconv as a BufferCode efficiently, character buffers are
     -- defined to contain lone surrogates instead of those private use characters that
     -- are used for roundtripping. Thus, Chars poked and peeked from a character buffer
     -- must undergo surrogatifyRoundtripCharacter and desurrogatifyRoundtripCharacter
     -- respectively.
     --
     -- For more information on this, see Note [Roundtripping] in GHC.IO.Encoding.Failure.
    
    recover :: Buffer from -> Buffer to -> IO (Buffer from, Buffer to),
     -- ^ The @recover@ function is used to continue decoding
     -- in the presence of invalid or unrepresentable sequences. This includes
     -- both those detected by @encode@ returning @InvalidSequence@ and those
     -- that occur because the input byte sequence appears to be truncated.
     --
     -- Progress will usually be made by skipping the first element of the @from@
     -- buffer. This function should only be called if you are certain that you
     -- wish to do this skipping and if the @to@ buffer has at least one element
     -- of free space. Because this function deals with decoding failure, it assumes
     -- that the from buffer has at least one element.
     --
     -- @recover@ may raise an exception rather than skipping anything.
     --
     -- Currently, some implementations of @recover@ may mutate the input buffer.
     -- In particular, this feature is used to implement transliteration.
    
    close  :: IO (),
     -- ^ Resources associated with the encoding may now be released.
     -- The @encode@ function may not be called again after calling
     -- @close@.
  
    getState :: IO state,
     -- ^ Return the current state of the codec.
     --
     -- Many codecs are not stateful, and in these case the state can be
     -- represented as '()'.  Other codecs maintain a state.  For
     -- example, UTF-16 recognises a BOM (byte-order-mark) character at
     -- the beginning of the input, and remembers thereafter whether to
     -- use big-endian or little-endian mode.  In this case, the state
     -- of the codec would include two pieces of information: whether we
     -- are at the beginning of the stream (the BOM only occurs at the
     -- beginning), and if not, whether to use the big or little-endian
     -- encoding.
  
    setState :: state -> IO ()
     -- restore the state of the codec using the state from a previous
     -- call to 'getState'.
   }
  
  type DecodeBuffer = Buffer Word8 -> Buffer Char
                    -> IO (CodingProgress, Buffer Word8, Buffer Char)
  
  type EncodeBuffer = Buffer Char -> Buffer Word8
                    -> IO (CodingProgress, Buffer Char, Buffer Word8)
  
  type TextDecoder state = BufferCodec Word8 CharBufElem state
  type TextEncoder state = BufferCodec CharBufElem Word8 state
  
  -- | A 'TextEncoding' is a specification of a conversion scheme
  -- between sequences of bytes and sequences of Unicode characters.
  --
  -- For example, UTF-8 is an encoding of Unicode characters into a sequence
  -- of bytes.  The 'TextEncoding' for UTF-8 is 'utf8'.
  data TextEncoding
    = forall dstate estate . TextEncoding  {
          textEncodingName :: String,
                     -- ^ a string that can be passed to 'mkTextEncoding' to
                     -- create an equivalent 'TextEncoding'.
          mkTextDecoder :: IO (TextDecoder dstate),
                     -- ^ Creates a means of decoding bytes into characters: the result must not
                     -- be shared between several byte sequences or simultaneously across threads
          mkTextEncoder :: IO (TextEncoder estate)
                     -- ^ Creates a means of encode characters into bytes: the result must not
                     -- be shared between several character sequences or simultaneously across threads
    }
  
  instance Show TextEncoding where
    -- | Returns the value of 'textEncodingName'
    show te = textEncodingName te
  
  data CodingProgress = InputUnderflow  -- ^ Stopped because the input contains insufficient available elements,
                                        -- or all of the input sequence has been sucessfully translated.
                      | OutputUnderflow -- ^ Stopped because the output contains insufficient free elements
                      | InvalidSequence -- ^ Stopped because there are sufficient free elements in the output
                                        -- to output at least one encoded ASCII character, but the input contains
                                        -- an invalid or unrepresentable sequence
                      deriving (Eq, Show)
  
  