Re: Question about the pack OBJ_OFS_DELTA format

From: Jeff King <peff@peff.net>
Date: Jan 10, 2020, 09:57 UTC
Message-ID: <20200110095707.GA459765@coredump.intra.peff.net>
In-Reply-To: <1032627506.120165.1578644787174@email.ionos.de>

On Fri, Jan 10, 2020 at 09:26:27AM +0100, Erik Fastermann wrote:

Show 13 quoted lines

> I get: ee 01 8c 63
> 
> The first two bytes, the type and the size are correctly computed.
> 
> So the next varint should be the offset.
> 
> 8c: 10001100 --- 63: 01100011
> 
> -> 1100011_0001100
> 
> -> 12684 ???
> 
> The result is the same when calculating it manually and with my program.

The pack-format.txt file says:

       offset encoding:
            n bytes with MSB set in all but the last one.
            The offset is then the number constructed by
            concatenating the lower 7 bit of each byte, and
            for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
            to the result.

but I think is missing two bits of information:

  - the bytes are in most-significant to least-significant order, which
    IIRC is the opposite of the size varint

  - each 7-bit byte sneaks in some extra data by implicitly adding "1"
    to all but the last byte

So the low seven bits of "8c" is "12". Add one and multiply by 2^7 gets you 1664. The low seven of "63" is 99. No addition or multiply because it's the last byte.

The result is 1763, which is what you expected.

It does seem like the documentation could be a lot better. I had to dig into the source (packfile.c:get_delta_base is pretty clear, but if you're trying to do a non-GPL clean-room implementation, then obviously don't look at it).

-Peff