Re: Understanding version 4 packs

[Nicolas Pitre <nico@cam.org>]

On Sat, 24 Mar 2007, Peter Eriksen wrote:

> 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.

> 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