{-# LANGUAGE OverloadedStrings #-}

-- |
-- Module      : Data.ByteString.Base16
-- Copyright   : (c) 2011 MailRank, Inc.
--
-- License     : BSD
-- Maintainer  : bos@mailrank.com
-- Stability   : experimental
-- Portability : GHC
--
-- Fast and efficient encoding and decoding of base16-encoded strings.

module Data.ByteString.Base16
    (
      encode
    , decode
    ) where

import Data.Bits ((.&.), shiftL, shiftR)
import Data.ByteString.Char8 (empty)
import Data.ByteString.Internal (ByteString(..), createAndTrim', unsafeCreate)
import Data.ByteString.Unsafe (unsafeIndex)
import Data.Word (Word8)
import Foreign.ForeignPtr (ForeignPtr, withForeignPtr)
import Foreign.Ptr (Ptr, minusPtr, plusPtr)
import Foreign.Storable (peek, poke)
import System.IO.Unsafe (unsafePerformIO)

digits :: ByteString
digits = "0123456789abcdef"
{-# NOINLINE digits #-}

-- | Encode a string into base16 form.  The result will always be a
-- multiple of 2 bytes in length.
--
-- Example:
--
-- > encode "foo"  == "666f6f"
encode :: ByteString -> ByteString
encode (PS sfp soff slen)
    | slen > maxBound `div` 2 =
        error "Data.ByteString.Base16.encode: input too long"
    | otherwise = unsafeCreate (slen*2) $ \dptr ->
                    withForeignPtr sfp $ \sptr ->
                      enc (sptr `plusPtr` soff) dptr
 where
  enc sptr = go sptr where
    e = sptr `plusPtr` slen
    go s d | s == e = return ()
           | otherwise = do
      x <- peek8 s
      poke d . unsafeIndex digits $ x `shiftR` 4
      poke (d `plusPtr` 1) . unsafeIndex digits $ x .&. 0xf
      go (s `plusPtr` 1) (d `plusPtr` 2)

-- | Decode a string from base16 form. The first element of the
-- returned tuple contains the decoded data. The second element starts
-- at the first invalid base16 sequence in the original string.
--
-- Examples:
--
-- > decode "666f6f"  == ("foo", "")
-- > decode "66quux"  == ("f", "quux")
-- > decode "666quux" == ("f", "6quux")
decode :: ByteString -> (ByteString, ByteString)
decode (PS sfp soff slen) =
  unsafePerformIO . createAndTrim' (slen `div` 2) $ \dptr ->
      withForeignPtr sfp $ \sptr ->
        dec (sptr `plusPtr` soff) dptr
 where
  dec sptr = go sptr where
    e = sptr `plusPtr` if odd slen then slen - 1 else slen
    go s d | s == e = let len = e `minusPtr` sptr
                      in return (0, len `div` 2, ps sfp (soff+len) (slen-len))
           | otherwise = do
      let hex w
              | w >= 48 && w <= 57  = w - 48
              | w >= 97 && w <= 102 = w - 97 + 10
              | w >= 65 && w <= 70  = w - 65 + 10
              | otherwise           = 0xff
      hi <- hex `fmap` peek8 s
      lo <- hex `fmap` peek8 (s `plusPtr` 1)
      if lo == 0xff || hi == 0xff
        then let len = s `minusPtr` sptr
             in return (0, len `div` 2, ps sfp (soff+len) (slen-len))
        else do
          poke d . fromIntegral $ lo + (hi `shiftL` 4)
          go (s `plusPtr` 2) (d `plusPtr` 1)

peek8 :: Ptr Word8 -> IO Int
peek8 p = fromIntegral `fmap` peek p

ps :: ForeignPtr Word8 -> Int -> Int -> ByteString
ps fp off len
    | len <= 0 = empty
    | otherwise = PS fp off len