Re: Best method of detecting if list of commit refs is a parent

From: Junio C Hamano <gitster@pobox.com>
Date: Aug 18, 2008, 04:41 UTC
Message-ID: <7vfxp3ndec.fsf@gitster.siamese.dyndns.org>
In-Reply-To: <e47324780808171924j237688faj9e13740f89e75fdf@mail.gmail.com>

"Thomas Harning Jr." <harningt@gmail.com> writes:

Show 7 quoted lines

> I'm working on my 'Stick' git bug-tracking tool and am working on the
> functionality to get a list of relevant bug-items at a specific point
> in history.
>
> Before I get into figuring out the 'best' way to do this, I thought
> I'd at least get the simple single-item case of detecting if a
> specific commit can be walked to from another commit...

It is unclear from your description what you are trying to do here, but if you want to know if a commit A is reachable from commit B, the standard way to do so is:

        git merge-base A B

If the output from the command is the object name of commit A, that means A is reachable from B. Otherwise A is not reachable from B.

This is because "merge-base" computes the set of ancestors common to A and B that are not reachable from other ancestors common to A and B.

Examples.

(1) This one is obvious.

        A---o---o---o---B

(2) Merge-base between A and B is M.  The one before M is also an ancestor
    common to A and B, but because it is reachable from M which is another
    ancestor common between A and B, it won't be part of the merge-base
    output.

              o---A
             /
        o---M---o---B

If you have bunch of commits A B C D E F and if you would want to know which one of them is reachable from X, you could of course run merge-base once for each of A..F. Another way to do this would be to run:

	git rev-list A B C D E F ^X

and look at the output. The ones among A..F that appear in the output are not reachable from X. The ones that are reachable from X do not appear in the output.

This is because "rev-list" outputs everything reachable from the given commits without ^ prefix, excluding the ones that are reacahble from the ones prefixed with ^.