pack-delta.hs

readCopyInstruction :: (Integral a) => Word8 -> Get (a, a)
readCopyInstruction opcode = do
        -- off -> offset in the source buffer where the copy will start
        -- this will read the correct subsequent bytes and shift them based on
        -- the set bit
        offset <- foldM readIfBitSet 0 $ zip [0x01, 0x02, 0x04, 0x08] [0,8..]
        -- bytes to copy
        len'   <- foldM readIfBitSet 0 $ zip [0x10, 0x20, 0x40] [0,8..]
        let len = if coerce len' == 0 then 0x10000 else len'
        -- FIXME add guard condition from `patch-delta.c`: if (unsigned_add_overflows(cp_off, cp_size) || ...
        return $ (coerce offset, coerce len)
    where calculateVal off shift           = if shift /= 0 then (\x -> off .|. (x `shiftL` shift)::Int) . fromIntegral else fromIntegral
          readIfBitSet off (test, shift)   = if opcode .&. test /= 0 then liftM (calculateVal off shift) getWord8 else return off
          coerce