Re: git and time

Junio C Hamano [off-list ref] writes:

Linus Torvalds [off-list ref] writes:

quoted

On Sat, 30 Sep 2006, Junio C Hamano wrote:

quoted

Especially, "find the closest ancestor" behaviour means when you
get "tag-gXXXX" as an answer, the tag proper does _not_ contain
the given commit (e.g. commit v1.4.2-g4839bd8 is not part of
v1.4.2).

Correct.

But that just means that we should take the _next_ one in the time-ordered 
list, no?

I do not think so.

Extending the example (sorry for doing the same topic on two
separate threads) I just gave Jeff on "fix based on v0.99",
after finding that the fix is based on v0.99, finding another
commit that immediately followed the v0.99 commit on my master
branch does not help finding out that I very recently merged the
fix in at all.  I think we cannot get away without honestly
doing the first descendant, which is unfortunately a lot more
expensive.

Maybe not *that* expensive.  Here is an outline, thinking aloud.

When describing a commit and a ref, we first run the ancestry
traversal algorithm merge-base uses internally.  If the tip of
the ref is not a descendant of the commit, abort (I'll justify
this in a moment).

Otherwise, we would already have parsed all the necessary
commits we need to determine which commits on the given ref's
ancestry is the first one that is a descendant of the target
commit at this point.  We collect these commits in a set, and
then mark the ones that are immediate children of the target
commit, then the ones that are children of them, etc. until we
find all the descendant of the target commit.

After that, we can bisect the reflog for the ref to find the
first commit that we have marked as a descendant of the target
commit in the above process.

If the tip of the ref is not a descendant of the commit to begin
with, that does not automatically mean that the target commit
has never been part of the ref -- the ref _could_ have contained
it and then later got rewound.  But then the question "when did
the commit has become part of this branch" itself stops being
interesting.  It would not do us much good if we know it was
part of the branch for only two days last week but it is not
contained in the branch anymore.