On Thu, Aug 30, 2018 at 12:20 PM Jeff King [off-list ref] wrote:
On Thu, Aug 30, 2018 at 12:16:22PM -0700, Stefan Beller wrote:
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On Wed, Aug 29, 2018 at 7:54 PM Jeff King [off-list ref] wrote:
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On Wed, Aug 29, 2018 at 10:10:25PM -0400, Gabriel Holodak wrote:
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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.
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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).
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@google.com/
but I guess reading the code would work just as fine.
Thanks,
Stefan