Re: Possible bug: identical lines added/removed in git diff

[Jeff King <peff@peff.net>]

On Thu, Aug 30, 2018 at 12:16:22PM -0700, Stefan Beller wrote:

> On Wed, Aug 29, 2018 at 7:54 PM Jeff King <peff@peff.net> wrote:
> >
> > On Wed, Aug 29, 2018 at 10:10:25PM -0400, Gabriel Holodak wrote:
> >
> > > > Could you cut down to a real minimal reproduction, i.e. just these 20
> > > > lines or so?
> > >
> > > I'm working on getting down to a minimal reproduction, a few lines at
> > > a time. One thing that seems strange: as I've removed lines, there are
> > > a bunch of lines that don't matter. Then I'll find some lines that, if
> > > removed, completely fix the issue. But the ordering for these
> > > apparently important lines doesn't matter. They just have to be
> > > somewhere in the file to cause the duplicated diffs.
> > >
> > > I'll upload again when I've figured out all the unimportant lines to remove.
> >
> > Yeah, I reproduced based on your initial post, but noticed that when I
> > cut it down the problem went away.
> 
> Oh, I had to look further down than I did initially. Now I can reproduce it
> from the initial data as well.
> 
> Note that it goes away with --minimal.

That's interesting. I did wonder if this was in fact a bug, or simply
that Myers does not promise to find the absolute minimal diff. I'm
_still_ not sure, especially because the minimization is so obvious in
this case (literally the first "-" and the first "+" line of a
contiguous hunk are identical).

> I have a patch cooking (which was sent out as
> https://public-inbox.org/git/20180810221857.87399-1-sbeller@google.com/)
> 
> and one of the weaknesses in that patch is the lack of explanation on
> when the heuristic is applied as I have not fully understood it yet.

I'm not sure I understand it either. But at least knowing that --minimal
changes the output gives a lead for investigation (I don't really have
time to dig into it in the next few days, though).

-Peff