Re: commit-graph: change in "best" merge-base when ambiguous

[Michael Haggerty <mhagger@alum.mit.edu>]

On 05/25/2018 12:08 AM, Jakub Narebski wrote:
> Derrick Stolee <stolee@gmail.com> writes:
>> On 5/22/2018 1:39 AM, Michael Haggerty wrote:
>>> On 05/21/2018 08:10 PM, Derrick Stolee wrote:
>>>> [...]
>>> This may be beyond the scope of what you are working on, but there are
>>> significant advantages to selecting a "best" merge base from among the
>>> candidates. Long ago [1] I proposed that the "best" merge base is the
>>> merge base candidate that minimizes the number of non-merge commits that
>>> are in
>>>
>>>      git rev-list $candidate..$branch
>>>
>>> that are already in master:
>>>
>>>      git rev-list $master
>>>
>>> (assuming merging branch into master), which is equivalent to choosing
>>> the merge base that minimizes
>>>
>>>      git rev-list --count $candidate..$branch
> 
> Is the above correct...
> 
>>> In fact, this criterion is symmetric if you exchange branch ↔ master,
>>> which is a nice property, and indeed generalizes pretty simply to
>>> computing the merge base of more than two commits.
> 
> ...as it doesn't seem to have the described symmetry.

The first email that I referenced [1] demonstrates this in the section
"Symmetry; generalization to more than two branches". The same thing is
demonstrated in a simpler way using set notation in a later email in
that thread [2].

Michael

[1] https://public-inbox.org/git/539A25BF.4060501@alum.mit.edu/
[2] https://public-inbox.org/git/53A06264.9080205@alum.mit.edu/