module Data.ByteString.Char8.Util (
      decodeHex
    , fromInt
    , hexString
    , previous
    , subBS
    , toInt
    , toBytes
    , utf16BEToutf8
  ) where

import Data.ByteString (ByteString, snoc)
import qualified Data.ByteString as BS (empty, foldl, pack, singleton)
import qualified Data.ByteString.Char8 as Char8 (drop, index, take, unpack)
import Data.Text.Encoding (encodeUtf8, decodeUtf16BE)
import Prelude hiding (length)

previous :: Char -> Int -> ByteString -> Int
previous char position byteString
  | Char8.index byteString position == char = position
  | otherwise = previous char (position - 1) byteString

subBS :: Int -> Int -> ByteString -> ByteString
subBS offset length = Char8.take length . Char8.drop offset

hexString :: (Num a, Read a) => String -> a
hexString s = read $ "0x" ++ s

fromInt :: Int -> ByteString
fromInt n
  | n < 0x100 = BS.singleton $ toEnum n
  | otherwise = fromInt (n `div` 0x100) `snoc` (toEnum (n `mod` 0x100))

toBytes :: Int -> Int -> ByteString
toBytes 0 _ = BS.empty
toBytes size n = toBytes (size - 1) (n `div` 0x100) `snoc` (toEnum (n `mod` 0x100))

toInt :: ByteString -> Int
toInt = BS.foldl (\n w -> 0x100*n + fromEnum w) 0

decodeHex :: ByteString -> ByteString
decodeHex = BS.pack . fmap hexString . pairDigits . Char8.unpack
  where
    pairDigits "" = []
    pairDigits [c] = [[c]]
    pairDigits (firstChar:secondChar:end) = (firstChar:[secondChar]):pairDigits end

utf16BEToutf8 :: ByteString -> ByteString
utf16BEToutf8 = encodeUtf8 . decodeUtf16BE