Calculating major.minor.patch from commit hash

From: "Martin Vejnár" <vejnar.martin@gmail.com>
To: git@vger.kernel.org
Subject: Calculating major.minor.patch from commit hash
Date: Thu, 13 Feb 2020 15:40:55 +0100	[thread overview]
Message-ID: <CAJFfRmt65DV5bZa5yTvg9sDb8SS8dFFyX7Zm23T7mAHL58v1Lw@mail.gmail.com> (raw)

I'd like my releases to have major.minor.patch version number. While
increments of major.minor are something one has to do manually, I
don't want to do that for patch.

So I was thinking of having a VERSION file in the repo containing
major.minor and then calculate patch at the given commit C
automatically as the length of the longest path starting at C in the
subgraph containing only commits in which VERSION is the same as C's.

I can do that pretty easily right now by

* identifying commits in which VERSION changes with `git log --
VERSION` and then
* walking the graph in topo order with `git log --format=format:"%H %P"`.

The latter step can terminate early, so it doesn't have to walk the
entire repo, but the latter walks everything and can be quite slow.

Is there a more efficient way to do this?

If not, would there be an interest in a new builtin (git-depth?) that
would calculate the value?
-- 
Martin