-- |
-- Module      : Data.Git.Delta
-- License     : BSD-style
-- Maintainer  : Vincent Hanquez <vincent@snarc.org>
-- Stability   : experimental
-- Portability : unix
--
module Data.Git.Delta
        ( Delta(..)
        , DeltaCmd(..)
        , deltaParse
        , deltaRead
        , deltaApply
        ) where

import Data.Attoparsec
import qualified Data.Attoparsec as A
import qualified Data.Attoparsec.Lazy as AL
import Data.ByteString (ByteString)
import qualified Data.ByteString as B
import qualified Data.ByteString.Lazy as L
import Data.Bits
import Data.Word

import Control.Applicative ((<$>), many)

-- | a delta is a source size, a destination size and a list of delta cmd
data Delta = Delta Word64 Word64 [DeltaCmd]
        deriving (Show,Eq)

-- | possible commands in a delta
data DeltaCmd =
          DeltaCopy ByteString   -- command to insert this bytestring
        | DeltaSrc Word64 Word64 -- command to copy from source (offset, size)
        deriving (Show,Eq)

-- | parse a delta.
-- format is 2 variable sizes, followed by delta cmds. for each cmd:
-- * if first byte MSB is set, we copy from source.
-- * otherwise, we copy from delta.
-- * extensions are not handled.
deltaParse = do
        srcSize <- getDeltaHdrSize
        resSize <- getDeltaHdrSize
        dcmds   <- many (anyWord8 >>= parseWithCmd)
        return $ Delta srcSize resSize dcmds
        where
                getDeltaHdrSize = do
                        z <- A.takeWhile (\w -> w `testBit` 7)
                        l <- anyWord8
                        return $ unbytes 0 $ (map (\w -> w `clearBit` 7) (B.unpack z) ++ [l])
                -- use a foldl ..
                unbytes _  []     = 0
                unbytes sh (x:xs) = (fromIntegral x) `shiftL` sh + unbytes (sh+7) xs
                -- parse one command, either an extension, a copy from src, or a copy from delta.
                parseWithCmd cmd
                        | cmd == 0        = error "delta extension not supported"
                        | cmd `testBit` 7 = do
                                o1 <- word8cond (cmd `testBit` 0) 0
                                o2 <- word8cond (cmd `testBit` 1) 8
                                o3 <- word8cond (cmd `testBit` 2) 16 
                                o4 <- word8cond (cmd `testBit` 3) 24
                                s1 <- word8cond (cmd `testBit` 4) 0
                                s2 <- word8cond (cmd `testBit` 5) 8
                                s3 <- word8cond (cmd `testBit` 6) 16 
                                let offset = o1 .|. o2 .|. o3 .|. o4
                                let size   = s1 .|. s2 .|. s3
                                return $ DeltaSrc offset (if size == 0 then 0x10000 else size)
                        | otherwise       = DeltaCopy <$> A.take (fromIntegral cmd)
                word8cond cond sh = do
                        if cond then (flip shiftL sh . fromIntegral) <$> anyWord8 else return 0

-- | read one delta from a lazy bytestring.
deltaRead = AL.maybeResult . AL.parse deltaParse

-- | apply a delta on a lazy bytestring, returning a new bytestring.
deltaApply :: L.ByteString -> Delta -> L.ByteString
deltaApply src (Delta srcSize _ deltaCmds)
        | L.length src /= fromIntegral srcSize = error "source size do not match"
        | otherwise                            = -- FIXME use a bytestring builder here.
                L.fromChunks $ concatMap resolve deltaCmds where
                        resolve (DeltaSrc o s) = L.toChunks $ takeAt (fromIntegral s) (fromIntegral o) src
                        resolve (DeltaCopy b)  = [b]
                        takeAt sz at = L.take sz . L.drop at