Re: [PATCH v2] Add description of OFS_DELTA to the pack format description

From: Shawn O. Pearce <spearce@spearce.org>
Date: Apr 6, 2008, 20:28 UTC
Message-ID: <20080406202821.GN10274@spearce.org>
In-Reply-To: <20080406201907.GB7075@bohr.gbar.dtu.dk>

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

Show 17 quoted lines

> 
> diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
> index aa87756..953c7ee 100644
> --- a/Documentation/technical/pack-format.txt
> +++ b/Documentation/technical/pack-format.txt
> @@ -103,10 +103,24 @@ Pack file entry: <+
>       packed object data:
>          If it is not DELTA, then deflated bytes (the size above
>  		is the size before compression).
> -	If it is DELTA, then
> +	If it is REF_DELTA, then
>  	  20-byte base object name SHA1 (the size above is the
>  		size of the delta data that follows).
>            delta data, deflated.
> +	If it is OFS_DELTA, then
> +	  n-byte offset (see below) counted from the type-byte
> +                of the header of the ofs-delta entry.

Except that only a positive offset value is here, as it is always a treated as a negative offset (you can only refer to a base that was before you, as that is the only way to know the base's offset in the pack).

Show 10 quoted lines

> +                (the size above is the size of the delta data
> +                that follows).
> +          delta data, deflated.
> +
> +     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.

Yes, that sounds correct.

-- 
Shawn.