Re: Finding a branch point in git

[Felipe Contreras <felipe.contreras@gmail.com>]

On Thu, May 31, 2012 at 10:37 PM, PJ Weisberg
<pj@irregularexpressions.net> wrote:
> On Thu, May 31, 2012 at 8:27 AM, Felipe Contreras
> <felipe.contreras@gmail.com> wrote:
>> On Wed, May 30, 2012 at 11:54 PM, Jeff King <peff@peff.net> wrote:
>>> On Wed, May 30, 2012 at 07:07:39PM +0200, Felipe Contreras wrote:
>>>
>>>> On Mon, May 28, 2012 at 9:06 PM, Jeff King <peff@peff.net> wrote:
>>>> > 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.
>>>
>>> I'm not sure that's possible, though, in the face of criss-cross merges.
>>> How do we distinguish the history above from one in which branch Y was
>>> forked from master at G, and then branch X was forked from branch Y at
>>> H?
>>
>> That is true, but we could use the algorithm used by name-rev: G would
>> have a distance 3 from branch X, and distance 2 from Y, so it would be
>> considered part of branch Y.
>>
>> Sure, it's not possible to know _for sure_, but this is a bit like
>> renames; we don't really know if a file was renamed or not, but we can
>> make a good guess.
>
> Obviously G is part of branch Y, branch X, and branch master.  I'm
> sure I'm missing the whole point of this exercise, because it seems to
> me that it's just needlessly confusing to say anything else.

It's not about which commit is part of which branch, but figuring out
which is the first commit that was created being in branch Y.

I don't see a tremendous value on this, but there is some, for example:

 % git rebase --onto master branch_Y@{tail} branch_Y

Depending on the case, this might not be what you want, but sometimes
it might, the problem is that you can't know automatically if
branch_Y@{tail} is G, or K. So normally you would fire gitk and select
the commit that you want, which is not a big deal, but still, it would
be nice if the tail of a branch was recorded somehow.

To be clear, if I do:

 % git checkout -b branch_Y
 % git commit <- this would be branch_Y's tail

 % git rebase --onto master <- now branch_Y's tail would be right on
top of master

Cheers.

-- 
Felipe Contreras