Re: history damage in linux.git

> On Thu, Apr 21, 2016 at 10:08 AM, Jeff King <peff@peff.net> wrote:
>>
>> Right, because it makes the names longer. We give the second-parent
>> traversal a heuristic cost. If we drop that cost to "1", like:
>
> So I dropped it to 500 (removed the two last digits), and it gave a
> reasonable answer. With 1000, it gave the same "based on 4.6" answer
> as the current 65536 value does.
>
>> which is technically true, but kind of painful to read. It may be that a
>> reasonable weight is somewhere between "1" and "65535", though.
>
> Based on my tests, the "right" number is somewhere in the 500-1000
> range for this particular case. But it's still a completely made up
> number.
>
>> However, I think the more fundamental confusion with git-describe is
>> that people expect the shortest distance to be the "first" tag that
>> contained the commit, and that is clearly not true in a branchy history.
>
> Yeah.
>
> And I don't think people care *too* much, because I'm sure this has
> happened before, it's just that before when it happened it wasn't
> quite _so_ far off the expected path..
>
>> I actually think most people would be happy with an algorithm more like:
>>
>>   1. Find the "oldest" tag (either by timestamp, or by version-sorting
>>      the tags) that contains the commit in question.
>
> Yes, we might want to base the "distance" at least partly on the age
> of the base commits.
>
>>   2. Find the "simplest" path from that tag to the commit, where we
>>      are striving mostly for shortness of explanation, not of path (so
>>      "~500" is way better than "~20^2~30^2~14", even though the latter
>>      is technically a shorter path).
>
> Well, so the three different paths I've seen are:
>
>  - standard git (65536), and 1000:
>    aed06b9 tags/v4.6-rc1~9^2~792
>
>  - non-first-parent cost: 500:
>    aed06b9 tags/v3.13-rc7~9^2~14^2~42
>
>  - non-first parent cost: 1:
>    aed06b9 tags/v3.13~5^2~4^2~2^2~1^2~42
>
> so there clearly are multiple valid answers.
>
> I would actually claim that the middle one is the best one - but I
> claim that based on your algorithm case #1. The last one may be the
> shortest actual path, but it's a shorter path to a newer tag that is a
> superset of the older tag, so the middle one is actually not just
> better based on age, but is a better choice based on "minimal actual
> history".
>
>                Linus