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

From: Shawn O. Pearce <spearce@spearce.org>
Date: Apr 6, 2008, 04:51 UTC
Message-ID: <20080406045132.GA10274@spearce.org>
In-Reply-To: <20080405180759.GA29710@bohr.gbar.dtu.dk>

Peter Eriksen <s022018@student.dtu.dk> 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.

...

Show 11 quoted lines

> 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.