On Thu, Aug 30, 2018 at 12:20 PM Jeff King <peff@xxxxxxxx> wrote: > > 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@xxxxxxxx> 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). The `Myers` (our default) diff algorithm is really the Myers algorithm + a heuristic that cuts off the long tail when it is very costly to compute the minimal diff. The `minimal` diff is the true Myers algorithm and I'd vouch for its correctness and being the minimal number of lines in the diff output. The Myers is implemented before https://github.com/git/git/blob/master/xdiff/xdiffi.c#L135 and the heuristics is after that line. > > I have a patch cooking (which was sent out as > > https://public-inbox.org/git/20180810221857.87399-1-sbeller@xxxxxxxxxx/) > > > > 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). An interesting (to me) approach for digging into that would include finding these examples at scale, which I presented in https://public-inbox.org/git/20180810001010.58870-1-sbeller@xxxxxxxxxx/ but I guess reading the code would work just as fine. Thanks, Stefan
