From: Petr Baudis <pasky@suse.cz>
Date: Thu, 19 Oct 2006 12:33:49 GMT
Subject: Re: VCS comparison table
Message-ID: <20061019123349.GE20017@pasky.or.cz>
In-Reply-To: <vpqirigqzpd.fsf@ecrins.imag.fr>

Dear diary, on Thu, Oct 19, 2006 at 02:04:14PM CEST, I got a letter
where Matthieu Moy <Matthieu.Moy@imag.fr> said that...
> What people call "leftmost ancestor" is the revision which used to be
> the tip at the time you commited. For example, if you do "bzr diff;
> bzr commit" the diff shown before is the same as the one got with
> "bzr diff -r last:1" right after the commit.

The lack of parents ordering in Git is directly connected with
fast-forwarding.

Consider

 repo1   repo2

   a       a
  /       /
 b       c

Now repo2 merges with repo1:

 repo1   repo2

   a       a
  /       / \
 b       c   b
          \ /
           m

repo1 tip ('b') is not ancestor of repo2 tip ('c') so a three-way merge
is done and a new 'm' merge commit is created.

And now repo1 merges with repo2:

 repo1   repo2

   a       a
  / \     / \
 c   b   c   b
  \ /     \ /
   m       m

Because previous repo1 tip ('b') was ancestor of repo2 tip ('m'), a
fast-forward happenned and repo1 tip simply moved to 'm'. But this
"flipped" the development from repo1 POV - you cannot assume anymore
that the first ("leftmost") parent is special.

-- 
				Petr "Pasky" Baudis
Stuff: http://pasky.or.cz/
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