{- | This module implements UTF-16 encoding and decoding as in RFC 2781
 -}
module Data.Encoding.UTF16
	(UTF16(..)
	) where

import Data.Encoding.Base
import Data.Char(ord,chr)
import Data.Bits
import Data.Int
import Data.Word
import Data.ByteString
import qualified Data.ByteString.Lazy as LBS
import Data.ByteString.Base(w2c,c2w)
import Prelude hiding (length)
import Control.Exception
import Data.Dynamic (toDyn)

data UTF16 = UTF16

utf16enc :: (EncodeState,String) -> Maybe (Word8,(EncodeState,String))
utf16enc (Done,[])   = Nothing
utf16enc (Done,x:xs)
	| n<=0x0000FFFF = Just
		(fromIntegral $ n `shiftR` 8
		,(Put1 (fromIntegral $ n),xs))
	| n<=0x0010FFFF = Just
		(fromIntegral $ 0xD8 .|. (n' `shiftR` 18)
		,(Put3 (fromIntegral $ (n' `shiftR` 10))
			(fromIntegral $
			0xDC .|. ((n' `shiftR` 8) .&. 0x03))
			(fromIntegral n'),xs))
	| otherwise = throwDyn $ HasNoRepresentation x
	where
	n  = ord x
	n' = n - 0x10000
utf16enc (Put3 w1 w2 w3,xs) = Just (w1,(Put2 w2 w3,xs))
utf16enc (Put2 w1 w2,xs) = Just (w1,(Put1 w2,xs))
utf16enc (Put1 w1,xs) = Just (w1,(Done,xs))

{-# SPECIALIZE utf16dec :: Bool -> Word8 -> Word8 -> Word8 -> Word8 -> (Char,Int) #-}
{-# SPECIALIZE utf16dec :: Bool -> Word8 -> Word8 -> Word8 -> Word8 -> (Char,Int64) #-}
utf16dec :: Num a => Bool -> Word8 -> Word8 -> Word8 -> Word8 -> (Char,a)
utf16dec be s1 s2 s3 s4
	| w1< 0xD8 || w1> 0xDF
		= (chr $ ((fromIntegral w1) `shiftL` 8) .|. (fromIntegral w2),2)
	| w1> 0xDB = throwDyn $ IllegalCharacter w1
	| w3< 0xDC || w3>0xDF = throwDyn $ IllegalCharacter w3
	| otherwise = (chr $ (((fromIntegral w1 .&. 0x03) `shiftL` 18)
		.|. ((fromIntegral w2) `shiftL` 10)
		.|. ((fromIntegral w3 .&. 0x03) `shiftL` 8)
		.|. (fromIntegral w4)) + 0x10000,4)
	where
	(w1,w2,w3,w4) = if be then (s1,s2,s3,s4) else (s2,s1,s4,s3)

instance Encoding UTF16 where
	encode _ str = unfoldr utf16enc (Put2 0xFE 0xFF,str)
	encodeLazy _ str = LBS.unfoldr utf16enc (Put2 0xFE 0xFF,str)
	encodable _ c = ord c <= 0x0010FFFF
	decode _ str = case findByteOrder str of
		Nothing -> decode' True 0
		Just big -> decode' big 2
		where
		decode' be i = c:decode' be (i+took)
			where
			(c,took) = mapException (\ex -> case ex of
				ErrorCall _ -> DynException (toDyn UnexpectedEnd)
				_ -> ex) (utf16dec be s1 s2 s3 s4)
			s1 = index str i
			s2 = index str (i+1)
			s3 = index str (i+2)
			s4 = index str (i+3)
	decodeLazy _ str = case findByteOrderLazy str of
		Nothing -> decode' True 0
		Just big -> decode' big 2
		where
		decode' be i = c:decode' be (i+took)
			where
			(c,took) = mapException (\ex -> case ex of
				ErrorCall _ -> DynException (toDyn UnexpectedEnd)
				_ -> ex) (utf16dec be s1 s2 s3 s4)
			s1 = LBS.index str i
			s2 = LBS.index str (i+1)
			s3 = LBS.index str (i+2)
			s4 = LBS.index str (i+3)
	decodable _ str = case findByteOrder str of
		Nothing  -> check' True (length str) 0
		Just big -> check' big  (length str) 2
		where
		check' be m i
			| m == i   = True
			| m == i+1 = False
			| w1< 0xD8 || w1> 0xDF = check' be m (i+2)
			| w1> 0xDB = False
			| m <= i+3 = False
			| w3< 0xDC || w3>0xDF = False
			| otherwise = check' be m (i+4)
			where
			(w1,w3) = if be then (s1,s3) else (s2,s4)
			s1 = index str i
			s2 = index str (i+1)
			s3 = index str (i+2)
			s4 = index str (i+3)

findByteOrder :: ByteString -> Maybe Bool
findByteOrder str
	| length str < 2 = Nothing
	| w1 == 0xFE && w2 == 0xFF = Just True
	| w1 == 0xFF && w2 == 0xFE = Just False
	| otherwise = Nothing
	where
	w1 = index str 0
	w2 = index str 1


findByteOrderLazy :: LBS.ByteString -> Maybe Bool
findByteOrderLazy str = case LBS.unpack (LBS.take 2 str) of
	[w1,w2]
		| w1 == 0xFE && w2 == 0xFF -> Just True
		| w1 == 0xFF && w2 == 0xFE -> Just False
		| otherwise -> Nothing
	_ -> Nothing