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

From: "Shawn O. Pearce" <[email-masked]>
Date: Sun, 6 Apr 2008 15:07:24 -0400

Peter Eriksen <[email-masked]> wrote:
> diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
> index aa87756..f6b1405 100644
> --- a/Documentation/technical/pack-format.txt
> +++ b/Documentation/technical/pack-format.txt
> @@ -103,10 +103,20 @@ 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) (the size above is the
> +		size of the delta data that follows).
> +          delta data, deflated.

You are missing the description that the position within _this_
packfile is determined by:

	offset_of_type_byte - n_byte_offset

The above can still be taken to mean the offset is from the start
of the packfile or something like that.

> +     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
> +          adding 2^7 + 2^14 + ... + 2^(7*(n-1)) to the result.

Hmm.  I've just spent 5 minutes reading this and I'm still not
certain what the result is.  2^0 only gets added to the result if
there is at least 2 bytes of data in the offset encoding; in your
description above it sounds like we should add 2^0 (1) even when
there is only 1 offset byte (n=1), which is not correct.

-- 
Shawn.