Re: [rfc] git: combo-blobs

[Linus Torvalds]

On Mon, 11 Apr 2005, Linus Torvalds wrote:
> >      bk changes -R
> > 
> >      bk changes -L
> 
> You'd dowload all the sha1 objects (they don't actually do anything to
> _your_ state - they only show the possible other states), and then it's a 
> "simple thing" to generate a full tree of your local HEAD commit and 
> compare it to a full tree of the remove HEAD commit.

Ok, there's a "rev-tree" program there now to generate these things. 

If you control both ends, or have some other means of a "smart"  
communications protocol, you don't actually have to download the blobs
themselves. Just download the "rev-tree" from the other side, and you can
generate the differences by comparing your rev-tree against theirs.

(And since they are sorted, the compare is very cheap).

The downside? A revtree can get quite large. My "rev-tree" program allows
you to cache previous state so that you don't have to follow the whole
thing down, though, so it's possible to just send incrementals (since a
"commit" _uniquely_ generates the whole rev-tree, you really can do
reasonably smart things and create "superset revtrees" etc).

So the change difference between two commits is literally

	rev-tree [commit-id1]  > commit1-revtree
	rev-tree [commit-id2]  > commit2-revtree
	join -t : commit1-revtree commit2-revtree > common-revisions

(this is also how to find the most common parent - you'd look at just the
head revisions - the ones that aren't referred to by other revisions - in
"common-revision", and figure out the best one. I think.)

		Linus
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