    --
    -- |
    -- Module      :  Codec.Binary.UTF8.String
    -- Copyright   :  (c) Eric Mertens 2007
    -- License     :  BSD3-style (see LICENSE)
    --
    -- Maintainer:    emertens@galois.com
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- Support for encoding UTF8 Strings to and from @[Word8]@
    --

    module Codec.Binary.UTF8.String (
          encode
        , decode
        , encodeString
        , decodeString
      ) where

    import Data.Word        (Word8)
    import Data.Bits        ((.|.),(.&.),shiftL,shiftR)
    import Data.Char        (chr,ord)

    default(Int)

    -- | Encode a string using 'encode' and store the result in a 'String'.
    encodeString :: String -> String
    encodeString xs = map (toEnum . fromEnum) (encode xs)

    -- | Decode a string using 'decode' using a 'String' as input.
    -- | This is not safe but it is necessary if UTF-8 encoded text
    -- | has been loaded into a 'String' prior to being decoded.
    decodeString :: String -> String
    decodeString xs = decode (map (toEnum . fromEnum) xs)

    replacement_character :: Char
    replacement_character = '\xfffd'

    -- | Encode a Haskell String to a list of Word8 values, in UTF8 format.
    encode :: String -> [Word8]
    encode = concatMap (map fromIntegral . go . ord)
     where
      go oc
       | oc <= 0x7f       = [oc]

       | oc <= 0x7ff      = [ 0xc0 + (oc `shiftR` 6)
                            , 0x80 + oc .&. 0x3f
                            ]

       | oc <= 0xffff     = [ 0xe0 + (oc `shiftR` 12)
                            , 0x80 + ((oc `shiftR` 6) .&. 0x3f)
                            , 0x80 + oc .&. 0x3f
                            ]
       | otherwise        = [ 0xf0 + (oc `shiftR` 18)
                            , 0x80 + ((oc `shiftR` 12) .&. 0x3f)
                            , 0x80 + ((oc `shiftR` 6) .&. 0x3f)
                            , 0x80 + oc .&. 0x3f
                            ]

    --
    -- | Decode a UTF8 string packed into a list of Word8 values, directly to String
    --
    decode :: [Word8] -> String
    decode [    ] = ""
    decode (c:cs)
      | c < 0x80  = chr (fromEnum c) : decode cs
      | c < 0xc0  = replacement_character : decode cs
      | c < 0xe0  = multi1
      | c < 0xf0  = multi_byte 2 0xf  0x800
      | c < 0xf8  = multi_byte 3 0x7  0x10000
      | c < 0xfc  = multi_byte 4 0x3  0x200000
      | c < 0xfe  = multi_byte 5 0x1  0x4000000
      | otherwise = replacement_character : decode cs
      where
        multi1 = case cs of
          c1 : ds | c1 .&. 0xc0 == 0x80 ->
            let d = ((fromEnum c .&. 0x1f) `shiftL` 6) .|.  fromEnum (c1 .&. 0x3f)
            in if d >= 0x000080 then toEnum d : decode ds
                                else replacement_character : decode ds
          _ -> replacement_character : decode cs

        multi_byte :: Int -> Word8 -> Int -> [Char]
        multi_byte i mask overlong = aux i cs (fromEnum (c .&. mask))
          where
            aux 0 rs acc
              | overlong <= acc && acc <= 0x10ffff &&
                (acc < 0xd800 || 0xdfff < acc)     &&
                (acc < 0xfffe || 0xffff < acc)      = chr acc : decode rs
              | otherwise = replacement_character : decode rs

            aux n (r:rs) acc
              | r .&. 0xc0 == 0x80 = aux (n-1) rs
                                   $ shiftL acc 6 .|. fromEnum (r .&. 0x3f)

            aux _ rs     _ = replacement_character : decode rs