Re: [PATCH] revision.c: implement --reverse=before for walks

> > > > >    if (revs->reverse_output_stage) {
> > > > > +        if (revs->reverse == 2 && revs->max_count == 0)
> > > > > +            return NULL;
> > > > > +
> >
> > PS: something I spotted on a second read. [Ignoring reverse=after
> > mode] This hunk looks to me like a nice little optimization (return
> > nothing if we know max_count says we yield no commits). Of course, I
> > could see that being viable early in the function, right? When asking
> > get_revision for commits, if max_count is 0, just return NULL.
> >
> > For reverse=after mode, this condition is only true if the max_count
> > was 0 in the previous conditional, also, since we use max_count=-1
> > before iterating get_revision_internal. That means the original
> > max_count isn't touched. At any rate, it _seems_ to me that the whole
> > function could benefit from this optimization… but I wonder if it is
> > _necessary_ for correctness of reverse=after in some way that I'm not
> > seeing? Since the current version doesn't need the early bailout, why
> > does reverse=after?
>
> Just to clarify, "reverse = 2" is "--reverse=before" and not
> "--reverse=after".

> With "reverse = 2", the snippet of code you're referencing is not an
> optimization but a requirement for correctness. With "reverse = 1" we
> just keep the max_count as is and it's used by get_revision_internal()
> to stop if that limit is reached. What we find in 'reversed' are already
> just the commits we need to return.
>
> With "reverse = 2", we first set max_count to -1 and then retrieve the
> whole history, then we set max_count to its original value. Then we
> return the commits on each call of get_revision(). Now, unlike with
> "reverse = 1", we have the whole history in 'reversed', because of that
> we need to know when to stop. That's the reason we decrement max_count
> only for "reverse = 2" and why "max_count == 0" is checked only for
> "reverse = 2".

> > > > >        c = pop_commit(&revs->commits);
> > > > > +        if (revs->reverse == 2)
> > > > > +            revs->max_count--;
> > > >
> > > > Hm. Why do we decrement here? Again, not an area I’m familiar with, but a bit surprising.
> > >
> > > get_revision() (in revision.c) handles the reverse option and updates
> > > the "struct git_graph". get_revision() then calls
> > > get_revision_internal(), which handles commit boundaries and max_count,
> > > here is where it gets decreased. Since max_count gets decreased
> > > everytime get_revision_internal() is called, if we were to leave
> > > max_count as is before the walk (in get_revision() at line 4558), the
> > > walk would stop before reaching the root commit. This is why the current
> > > --reverse option is applied only after commit limiting options. So
> > > instead we set max_count at -1 walking the whole history and storing it
> > > in 'reversed'. Now we're in "reverse_output_stage = 1", and in this
> > > state we never call get_revision_internal() again, instead we pop
> > > commits from 'reversed'. Because of this we have to handle max_count
> > > outside get_revision_internal(), so we decrement it in the snippet of
> > > code you referenced.
> > >
> > > A bit verbose but hopefully it'll get my point across.
> >
> > I don't 100% follow, but I'm out of my depth :)
> >
> > I think I see that get_revision() effectively has 2 modes pertaining
> > to reverse: reverse and reverse output stage (the former falls
> > directly into the latter, though).
> >
> > After some setup, the reverse mode calls get_revision_internal() as
> > you said. That decrements max_count as a way of counting how many
> > commits we've seen through the loop, so if we asked for 5 we'd only
> > process 5 commits.
> >
> > Then we fall into the output stage mode, which pops a commit [1].
> >
> > With this patch, in reverse=after we disable max_count in the first
> > (reverse) mode, as you said. Ok: we get the whole (filtered) history
> > then, at which point we can now shrink. That makes sense.
> >
> > Then in the reverse output stage mode, we pretend to have one less
> > max_count. That's what I can't figure out. Is it because of the
> > pop_commit()? I guess I'm not totally seeing how that interacted with
> > the max_count in the original code: does the current code yield one
> > extra commit in get_revision_internal() ?
>
> I'm not sure I understand what you're referencing with "Then in the
> reverse output stage mode, we pretend to have one less max_count".
>
> If you're referring to line 4573, then...
>
> > You wrote that "we never call get_revision_internal() again," but I
> > don't see why that's true with this patch and not true before it.
> >
> > I do agree that _somebody_ has to handle max_count after
> > get_revision() returns with reverse=after. I'm just not sure what
> >
> >     if (revs->reverse == 2)
> >         revs->max_count--;
> >
> > is doing.
>
> ...we're not pretending we have fewer commits. Every subsequent call to
> get_revision() after the first call will never enter the branch at line
> 4548 and will only enter the branch at 4568. Everytime we pop a commit
> from 'reversed' we decrease max_count so we can limit only to the amount
> of commits the user wants.
>
> So, to recap, with "reverse = 2", on the first call to get_revision() we
> walk the whole history and store it in 'reversed' in reversed order and
> return the first commit.
> On subsequent calls to get_revision() we do not walk the history again,
> we simply return the commits that have been stored in 'reversed'.
> Everytime we pop a commit we have to decrease max_count, and we check
> againts max_count to know if we shouldn't return anymore commits (by
> returning NULL).

> > Of course if I'm the only one confused and others make sense of it,
> > that's ok, too.
>
> No, I completely understand. I did have to retouch the function a few
> times after writing the tests :P
>
> > [1]: I traced this to 498bcd3159 (rev-list: fix --reverse interaction
> > with --parents, 2008-08-29), but I can't fathom what the pop is doing
> > there.
>
> It's pretty much doing the same thing it does now, it's returning stored
> commits. In both versions, the initial setup when "revs->reverse" is
> true, becomes "dead code" after the first call.