module Distribution.Utils.String
    ( -- * Encode to/from UTF8
      decodeStringUtf8
    , encodeStringUtf8
    ) where

import Data.Word
import Data.Bits
import Data.Char (chr,ord)

-- | Decode 'String' from UTF8-encoded octets.
--
-- Invalid data in the UTF8 stream (this includes code-points @U+D800@
-- through @U+DFFF@) will be decoded as the replacement character (@U+FFFD@).
--
-- See also 'encodeStringUtf8'
decodeStringUtf8 :: [Word8] -> String
decodeStringUtf8 = go
  where
    go :: [Word8] -> String
    go []       = []
    go (c : cs)
      | c <= 0x7F = chr (fromIntegral c) : go cs
      | c <= 0xBF = replacementChar : go cs
      | c <= 0xDF = twoBytes c cs
      | c <= 0xEF = moreBytes 3 0x800     cs (fromIntegral $ c .&. 0xF)
      | c <= 0xF7 = moreBytes 4 0x10000   cs (fromIntegral $ c .&. 0x7)
      | c <= 0xFB = moreBytes 5 0x200000  cs (fromIntegral $ c .&. 0x3)
      | c <= 0xFD = moreBytes 6 0x4000000 cs (fromIntegral $ c .&. 0x1)
      | otherwise   = replacementChar : go cs

    twoBytes :: Word8 -> [Word8] -> String
    twoBytes c0 (c1:cs')
      | c1 .&. 0xC0 == 0x80
      = let d = (fromIntegral (c0 .&. 0x1F) `shiftL` 6)
             .|. fromIntegral (c1 .&. 0x3F)
         in if d >= 0x80
               then  chr d                : go cs'
               else  replacementChar      : go cs'
    twoBytes _ cs' = replacementChar      : go cs'

    moreBytes :: Int -> Int -> [Word8] -> Int -> [Char]
    moreBytes 1 overlong cs' acc
      | overlong <= acc, acc <= 0x10FFFF, acc < 0xD800 || 0xDFFF < acc
      = chr acc : go cs'

      | otherwise
      = replacementChar : go cs'

    moreBytes byteCount overlong (cn:cs') acc
      | cn .&. 0xC0 == 0x80
      = moreBytes (byteCount-1) overlong cs'
          ((acc `shiftL` 6) .|. fromIntegral cn .&. 0x3F)

    moreBytes _ _ cs' _
      = replacementChar : go cs'

    replacementChar = '\xfffd'


-- | Encode 'String' to a list of UTF8-encoded octets
--
-- Code-points in the @U+D800@-@U+DFFF@ range will be encoded
-- as the replacement character (i.e. @U+FFFD@).
--
-- See also 'decodeUtf8'
encodeStringUtf8 :: String -> [Word8]
encodeStringUtf8 []        = []
encodeStringUtf8 (c:cs)
  | c <= '\x07F' = w8
                 : encodeStringUtf8 cs
  | c <= '\x7FF' = (0xC0 .|.  w8ShiftR  6          )
                 : (0x80 .|. (w8          .&. 0x3F))
                 : encodeStringUtf8 cs
  | c <= '\xD7FF'= (0xE0 .|.  w8ShiftR 12          )
                 : (0x80 .|. (w8ShiftR  6 .&. 0x3F))
                 : (0x80 .|. (w8          .&. 0x3F))
                 : encodeStringUtf8 cs
  | c <= '\xDFFF'= 0xEF : 0xBF : 0xBD -- U+FFFD
                 : encodeStringUtf8 cs
  | c <= '\xFFFF'= (0xE0 .|.  w8ShiftR 12          )
                 : (0x80 .|. (w8ShiftR  6 .&. 0x3F))
                 : (0x80 .|. (w8          .&. 0x3F))
                 : encodeStringUtf8 cs
  | otherwise    = (0xf0 .|.  w8ShiftR 18          )
                 : (0x80 .|. (w8ShiftR 12 .&. 0x3F))
                 : (0x80 .|. (w8ShiftR  6 .&. 0x3F))
                 : (0x80 .|. (w8          .&. 0x3F))
                 : encodeStringUtf8 cs
  where
    w8 = fromIntegral (ord c) :: Word8
    w8ShiftR :: Int -> Word8
    w8ShiftR = fromIntegral . shiftR (ord c)
