    {-# OPTIONS_GHC  -XNoImplicitPrelude -funbox-strict-fields #-}
    {-# LANGUAGE BangPatterns #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.IO.Encoding.UTF16
    -- Copyright   :  (c) The University of Glasgow, 2009
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable
    --
    -- UTF-16 Codecs for the IO library
    --
    -- Portions Copyright   : (c) Tom Harper 2008-2009,
    --                        (c) Bryan O'Sullivan 2009,
    --                        (c) Duncan Coutts 2009
    --
    -----------------------------------------------------------------------------

    module GHC.IO.Encoding.UTF16 (
      utf16,
      utf16_decode,
      utf16_encode,

      utf16be,
      utf16be_decode,
      utf16be_encode,

      utf16le,
      utf16le_decode,
      utf16le_encode,
      ) where

    import GHC.Base
    import GHC.Real
    import GHC.Num
    -- import GHC.IO
    import GHC.IO.Exception
    import GHC.IO.Buffer
    import GHC.IO.Encoding.Types
    import GHC.Word
    import Data.Bits
    import Data.Maybe
    import GHC.IORef

    #if DEBUG
    import System.Posix.Internals
    import Foreign.C
    import GHC.Show
    import GHC.Ptr

    puts :: String -> IO ()
    puts s = do withCStringLen (s++"\n") $ \(p,len) ->
                    c_write 1 (castPtr p) (fromIntegral len)
                return ()
    #endif

    -- -----------------------------------------------------------------------------
    -- The UTF-16 codec: either UTF16BE or UTF16LE with a BOM

    utf16  :: TextEncoding
    utf16 = TextEncoding { textEncodingName = "UTF-16",
                           mkTextDecoder = utf16_DF,
     	               mkTextEncoder = utf16_EF }

    utf16_DF :: IO (TextDecoder (Maybe DecodeBuffer))
    utf16_DF = do
      seen_bom <- newIORef Nothing
      return (BufferCodec {
                 encode   = utf16_decode seen_bom,
                 close    = return (),
                 getState = readIORef seen_bom,
                 setState = writeIORef seen_bom
              })

    utf16_EF :: IO (TextEncoder Bool)
    utf16_EF = do
      done_bom <- newIORef False
      return (BufferCodec {
                 encode   = utf16_encode done_bom,
                 close    = return (),
                 getState = readIORef done_bom,
                 setState = writeIORef done_bom
              })

    utf16_encode :: IORef Bool -> EncodeBuffer
    utf16_encode done_bom input
      output@Buffer{ bufRaw=oraw, bufL=_, bufR=ow, bufSize=os }
     = do
      b <- readIORef done_bom
      if b then utf16_native_encode input output
           else if os - ow < 2
                   then return (input,output)
                   else do
                        writeIORef done_bom True
                        writeWord8Buf oraw ow     bom1
                        writeWord8Buf oraw (ow+1) bom2
                        utf16_native_encode input output{ bufR = ow+2 }

    utf16_decode :: IORef (Maybe DecodeBuffer) -> DecodeBuffer
    utf16_decode seen_bom
      input@Buffer{  bufRaw=iraw, bufL=ir, bufR=iw,  bufSize=_  }
      output
     = do
       mb <- readIORef seen_bom
       case mb of
         Just decode -> decode input output
         Nothing ->
           if iw - ir < 2 then return (input,output) else do
           c0 <- readWord8Buf iraw ir
           c1 <- readWord8Buf iraw (ir+1)
           case () of
            _ | c0 == bomB && c1 == bomL -> do
                   writeIORef seen_bom (Just utf16be_decode)
                   utf16be_decode input{ bufL= ir+2 } output
              | c0 == bomL && c1 == bomB -> do
                   writeIORef seen_bom (Just utf16le_decode)
                   utf16le_decode input{ bufL= ir+2 } output
              | otherwise -> do
                   writeIORef seen_bom (Just utf16_native_decode)
                   utf16_native_decode input output

    bomB, bomL, bom1, bom2 :: Word8
    bomB = 0xfe
    bomL = 0xff

    -- choose UTF-16BE by default for UTF-16 output
    utf16_native_decode :: DecodeBuffer
    utf16_native_decode = utf16be_decode

    utf16_native_encode :: EncodeBuffer
    utf16_native_encode = utf16be_encode

    bom1 = bomB
    bom2 = bomL

    -- -----------------------------------------------------------------------------
    -- UTF16LE and UTF16BE

    utf16be :: TextEncoding
    utf16be = TextEncoding { textEncodingName = "UTF-16BE",
                             mkTextDecoder = utf16be_DF,
     	                 mkTextEncoder = utf16be_EF }

    utf16be_DF :: IO (TextDecoder ())
    utf16be_DF =
      return (BufferCodec {
                 encode   = utf16be_decode,
                 close    = return (),
                 getState = return (),
                 setState = const $ return ()
              })

    utf16be_EF :: IO (TextEncoder ())
    utf16be_EF =
      return (BufferCodec {
                 encode   = utf16be_encode,
                 close    = return (),
                 getState = return (),
                 setState = const $ return ()
              })

    utf16le :: TextEncoding
    utf16le = TextEncoding { textEncodingName = "UTF16-LE",
                             mkTextDecoder = utf16le_DF,
     	                 mkTextEncoder = utf16le_EF }

    utf16le_DF :: IO (TextDecoder ())
    utf16le_DF =
      return (BufferCodec {
                 encode   = utf16le_decode,
                 close    = return (),
                 getState = return (),
                 setState = const $ return ()
              })

    utf16le_EF :: IO (TextEncoder ())
    utf16le_EF =
      return (BufferCodec {
                 encode   = utf16le_encode,
                 close    = return (),
                 getState = return (),
                 setState = const $ return ()
              })

    utf16be_decode :: DecodeBuffer
    utf16be_decode
      input@Buffer{  bufRaw=iraw, bufL=ir0, bufR=iw,  bufSize=_  }
      output@Buffer{ bufRaw=oraw, bufL=_,   bufR=ow0, bufSize=os }
     = let
           loop !ir !ow
             | ow >= os || ir >= iw  =  done ir ow
             | ir + 1 == iw          =  done ir ow
             | otherwise = do
                  c0 <- readWord8Buf iraw ir
                  c1 <- readWord8Buf iraw (ir+1)
                  let x1 = fromIntegral c0 `shiftL` 8 + fromIntegral c1
                  if validate1 x1
                     then do ow' <- writeCharBuf oraw ow (unsafeChr (fromIntegral x1))
                             loop (ir+2) ow'
                     else if iw - ir < 4 then done ir ow else do
                          c2 <- readWord8Buf iraw (ir+2)
                          c3 <- readWord8Buf iraw (ir+3)
                          let x2 = fromIntegral c2 `shiftL` 8 + fromIntegral c3
                          if not (validate2 x1 x2) then invalid else do
                          ow' <- writeCharBuf oraw ow (chr2 x1 x2)
                          loop (ir+4) ow'
             where
               invalid = if ir > ir0 then done ir ow else ioe_decodingError

           -- lambda-lifted, to avoid thunks being built in the inner-loop:
           done !ir !ow = return (if ir == iw then input{ bufL=0, bufR=0 }
                                              else input{ bufL=ir },
                             output{ bufR=ow })
        in
        loop ir0 ow0

    utf16le_decode :: DecodeBuffer
    utf16le_decode
      input@Buffer{  bufRaw=iraw, bufL=ir0, bufR=iw,  bufSize=_  }
      output@Buffer{ bufRaw=oraw, bufL=_,   bufR=ow0, bufSize=os }
     = let
           loop !ir !ow
             | ow >= os || ir >= iw  =  done ir ow
             | ir + 1 == iw          =  done ir ow
             | otherwise = do
                  c0 <- readWord8Buf iraw ir
                  c1 <- readWord8Buf iraw (ir+1)
                  let x1 = fromIntegral c1 `shiftL` 8 + fromIntegral c0
                  if validate1 x1
                     then do ow' <- writeCharBuf oraw ow (unsafeChr (fromIntegral x1))
                             loop (ir+2) ow'
                     else if iw - ir < 4 then done ir ow else do
                          c2 <- readWord8Buf iraw (ir+2)
                          c3 <- readWord8Buf iraw (ir+3)
                          let x2 = fromIntegral c3 `shiftL` 8 + fromIntegral c2
                          if not (validate2 x1 x2) then invalid else do
                          ow' <- writeCharBuf oraw ow (chr2 x1 x2)
                          loop (ir+4) ow'
             where
               invalid = if ir > ir0 then done ir ow else ioe_decodingError

           -- lambda-lifted, to avoid thunks being built in the inner-loop:
           done !ir !ow = return (if ir == iw then input{ bufL=0, bufR=0 }
                                              else input{ bufL=ir },
                             output{ bufR=ow })
        in
        loop ir0 ow0

    ioe_decodingError :: IO a
    ioe_decodingError = ioException
         (IOError Nothing InvalidArgument "utf16_decode"
              "invalid UTF-16 byte sequence" Nothing Nothing)

    utf16be_encode :: EncodeBuffer
    utf16be_encode
      input@Buffer{  bufRaw=iraw, bufL=ir0, bufR=iw,  bufSize=_  }
      output@Buffer{ bufRaw=oraw, bufL=_,   bufR=ow0, bufSize=os }
     = let
          done !ir !ow = return (if ir == iw then input{ bufL=0, bufR=0 }
                                             else input{ bufL=ir },
                                 output{ bufR=ow })
          loop !ir !ow
            | ir >= iw     =  done ir ow
            | os - ow < 2  =  done ir ow
            | otherwise = do
               (c,ir') <- readCharBuf iraw ir
               case ord c of
                 x | x < 0x10000 -> do
                        writeWord8Buf oraw ow     (fromIntegral (x `shiftR` 8))
                        writeWord8Buf oraw (ow+1) (fromIntegral x)
                        loop ir' (ow+2)
                   | otherwise -> do
                        if os - ow < 4 then done ir ow else do
                        let
                             n1 = x - 0x10000
                             c1 = fromIntegral (n1 `shiftR` 18 + 0xD8)
                             c2 = fromIntegral (n1 `shiftR` 10)
                             n2 = n1 .&. 0x3FF
                             c3 = fromIntegral (n2 `shiftR` 8 + 0xDC)
                             c4 = fromIntegral n2
                        --
                        writeWord8Buf oraw ow     c1
                        writeWord8Buf oraw (ow+1) c2
                        writeWord8Buf oraw (ow+2) c3
                        writeWord8Buf oraw (ow+3) c4
                        loop ir' (ow+4)
        in
        loop ir0 ow0

    utf16le_encode :: EncodeBuffer
    utf16le_encode
      input@Buffer{  bufRaw=iraw, bufL=ir0, bufR=iw,  bufSize=_  }
      output@Buffer{ bufRaw=oraw, bufL=_,   bufR=ow0, bufSize=os }
     = let
          done !ir !ow = return (if ir == iw then input{ bufL=0, bufR=0 }
                                             else input{ bufL=ir },
                                 output{ bufR=ow })
          loop !ir !ow
            | ir >= iw     =  done ir ow
            | os - ow < 2  =  done ir ow
            | otherwise = do
               (c,ir') <- readCharBuf iraw ir
               case ord c of
                 x | x < 0x10000 -> do
                        writeWord8Buf oraw ow     (fromIntegral x)
                        writeWord8Buf oraw (ow+1) (fromIntegral (x `shiftR` 8))
                        loop ir' (ow+2)
                   | otherwise ->
                        if os - ow < 4 then done ir ow else do
                        let
                             n1 = x - 0x10000
                             c1 = fromIntegral (n1 `shiftR` 18 + 0xD8)
                             c2 = fromIntegral (n1 `shiftR` 10)
                             n2 = n1 .&. 0x3FF
                             c3 = fromIntegral (n2 `shiftR` 8 + 0xDC)
                             c4 = fromIntegral n2
                        --
                        writeWord8Buf oraw ow     c2
                        writeWord8Buf oraw (ow+1) c1
                        writeWord8Buf oraw (ow+2) c4
                        writeWord8Buf oraw (ow+3) c3
                        loop ir' (ow+4)
        in
        loop ir0 ow0

    chr2 :: Word16 -> Word16 -> Char
    chr2 (W16# a#) (W16# b#) = C# (chr# (upper# +# lower# +# 0x10000#))
        where
          !x# = word2Int# a#
          !y# = word2Int# b#
          !upper# = uncheckedIShiftL# (x# -# 0xD800#) 10#
          !lower# = y# -# 0xDC00#
    {-# INLINE chr2 #-}

    validate1    :: Word16 -> Bool
    validate1 x1 = (x1 >= 0 && x1 < 0xD800) || x1 > 0xDFFF
    {-# INLINE validate1 #-}

    validate2       ::  Word16 -> Word16 -> Bool
    validate2 x1 x2 = x1 >= 0xD800 && x1 <= 0xDBFF &&
                      x2 >= 0xDC00 && x2 <= 0xDFFF
    {-# INLINE validate2 #-}