Re: Our merge bases sometimes suck

[Junio C Hamano <gitster@pobox.com>]

Michael Haggerty <mhagger@alum.mit.edu> writes:

> The "best" merge base
> =====================
>
> But not all merge bases are created equal.  It is possible to define a
> "best" merge base that has some nice properties.
>
> Let's focus on the command
>
>     git diff $master...$branch
>
> which is equivalent to
>
>     git diff $(git merge-base $master $branch)..$branch
> ...
> I propose that the best merge base is the merge base "candidate" that
> minimizes the number of non-merge commits that are in
>
>     git rev-list --no-merges $candidate..$branch
>
> but are already in master:
>
>     git rev-list --no-merges $master

I welcome this line of thought very much.

There is one niggle I find somewhat curious but am either too lazy
or too stupid to think it through myself ;-)

The "merge-base" is a symmetric operation, because the three-way
merge, which is the primary customer of its result, fundamentally
is.  From your description, it sounds like the "best" merge base
however may not be symmetric at all.  The merge-base between A and B
that makes "git diff A...B" the easiest to read by minimizing the
distance between it and B may be different from the merge-base
between A and B that makes the other diff "git diff B...A" the
easiest to read.

Or it may not be assymmetric---that is why I said I didn't think it
through.  I am not saying that it is bad if the "best" merge-base is
an asymmetric concept; I am curious if it is asymmetric, and if so
if that is fundamental.