Subject: Re: [PATCH] Update, and clear up the pack format documentation a bit

From: "Shawn O. Pearce" <[email-masked]>
Date: Sun, 6 Apr 2008 00:51:32 -0400

Peter Eriksen <[email-masked]> wrote:
> The current documentation does not mention the ofs_delta pack
> object type. This patch is also supposed to make the text a bit
> more readable, since it moves the object entry header
> description earlier.
...
> diff --git a/Documentation/technical/pack-format.txt
> b/Documentation/technical/pack-format.txt
> index aa87756..35ee01d 100644
> --- a/Documentation/technical/pack-format.txt
> +++ b/Documentation/technical/pack-format.txt
>       compressed delta data
> + 
> +     (for object type ofs_delta)
> +     n-byte offset (n*7-bit as above, but with size0 being 7 bit)     
> +     compressed delta data
> +

That is not correct.  The ofs_delta is encoded as an n-byte offset
that is subtracted from the current object's first byte (the byte
holding the type/representation field and first 4 bits of length).

The n-byte encoding for an ofs_delta is different then the one
used for the length.  We add 1 for each byte where the MSB is 1.
We also store the data in big-endian form (the most significant
byte is first and the least significant byte is last).

See get_delta_base in sha1_file.c for the details of this.

In pack v4 I planned on using this particular encoding in more
of the format than just here.

-- 
Shawn.