Re: Finding a branch point in git

From: Felipe Contreras <felipe.contreras@gmail.com>
Date: May 30, 2012, 17:07 UTC
Message-ID: <CAMP44s0UBsVicuEcwACsm1zTT_jGau_Q20hJv4J_6uvancYJRQ@mail.gmail.com>
In-Reply-To: <20120528190639.GA2478@sigill.intra.peff.net>

On Mon, May 28, 2012 at 9:06 PM, Jeff King <peff@peff.net> wrote:

Show 18 quoted lines

> On Mon, May 28, 2012 at 02:36:04PM +0200, Felipe Contreras wrote:
>
>> > What about a history with multiple branches?
>> >
>> > --X--A--B--C--D----E  (master)
>> >      \           /
>> >       G--H--I---J   (branch X)
>> >           \    /
>> >            K--L    (branch Y)
>> [...]
>>
>> Yes, but then you would need to specify a second branch. I would avoid
>> that if possible.
>
> I agree that is less nice. But I don't think the operation is
> well-defined with a single branch. If you ask for "when did branch X
> split", then in the above graph it is unclear if you meant "split from
> master", or "split from Y".

If you look from the context that I explained in the first mail; it would be from *any* branch; IOW; find the first commit from branch X (G), and then find the parent. That would be the first commit where branch X started.

> Maybe you could assume "master", or assume "git symbolic-ref HEAD" as
> the second branch?

No.

I think it would be easy to support this case if somehow there was a way to find all the commits that comprise a branch;

 % git log branch ^<any-other-branch>

I could swear I saw such an option, but I've been looking for days and I can't find it.

IOW; all the commits were 'git branch --contains' would show 'branch' and nothing else.

Show 12 quoted lines

>> There's also another case that doesn't work:
>>
>> -- X -- A -- B (master)
>>          \
>>           \
>>            C (branch A)
>>
>> Shouldn't be hard to add checks for those cases I think.
>
> Actually, I think that one extends naturally. They are never merged, so
> your rev-list never finds a merge commit, and you can just take the
> merge base of the branch tips.

Sure, I can _just_ do that, but I need to do it :)

So, here it is... I hope people can find ways to simplify it:

find_merge ()
{
	local selection extra
	test "$2" && extra=" into $2"
	git rev-list --min-parents=2 --grep="Merge branch '$1'$extra"
--topo-order ${3:---all} | tail -1
}

branch_point ()
{
	local first_merge second_merge merge
	first_merge=$(find_merge $1 "" "$1 $2")
	second_merge=$(find_merge $2 $1 $first_merge)
	merge=${second_merge:-$first_merge}

	if [ "$merge" ]; then
		git merge-base $merge^1 $merge^2
	else
		git merge-base $1 $2
	fi
}

And I've added tests: https://raw.github.com/gist/2837595/0734e7f17a4597f81c5129e3cbfee09a183e93cd/branch-point

Seems to work :)

Cheers.

-- 
Felipe Contreras