Re: Understanding version 4 packs

From: Nicolas Pitre <nico@cam.org>
Date: Mar 24, 2007, 23:24 UTC
Message-ID: <alpine.LFD.0.83.0703241913110.18328@xanadu.home>
In-Reply-To: <20070324202356.GA20734@bohr.gbar.dtu.dk>

On Sat, 24 Mar 2007, Peter Eriksen wrote:

Show 7 quoted lines

> There is a new tree type called OBJ_DICT_TREE, which looks something
> like the following:
> 
> +-----------------+------------------------------------------------+----
> |  Table offset   |  SHA-1 of the blob corresponding to the path.  | ...
> +-----------------+------------------------------------------------+----
>       6 bytes                     20 bytes

Actually it is a 2-byte index in the path table, and a 4-byte index in a common SHA1 table. So each tree entry is 6 bytes total.

> These new tree objects will remain uncompressed in the pack file, but
> sorted with, and deltaed against other tree objects. All normal tree
> objects are converted to OBJ_DICT_TREE when packing, and are converted
> back on the fly to callers who need an ordinary OBJ_TREE.

Right.

> The index (.idx) files are extended to have a 4 byte pointer to the
> offset of this file name table in the pack file for easy lookup.

Right. And it will lose the SHA1 entries since they are already available in the pack.

Show 6 quoted lines

> There is something similar with a table of common strings in commit
> objects (e.g. author and timezone), and a new object OBJ_DICT_COMMIT,
> but I have not understood that quite yet.
> 
> Is there something, I have gotten wrong with regards to my
> understanding?

I don't think so. Note that the code is still a work in progress and the resulting pack/index is not yet fully conform to the format we envisaged.

Nicolas