Re: [PATCH 1/2] xdiff: implement a zealous diff3, or "zdiff3"

[Elijah Newren <newren@gmail.com>]

On Tue, Jun 15, 2021 at 2:40 AM Felipe Contreras
<felipe.contreras@gmail.com> wrote:
>
> Elijah Newren via GitGitGadget wrote:
> > From: Elijah Newren <newren@gmail.com>
> >
> > "zdiff3" is identical to ordinary diff3 except that it allows compaction
> > of common lines on the two sides of history at the beginning or end of
> > the conflict hunk.
>
> That was not the main reason behind zdiff3.
>
> The whole point of zdiff3 was to have something closer to the "merge"
> style, even if not technically correct.
>
> Your proposal is better than diff3 in that respect, but worse than Uwe's
> zdiff3.
>
> If you have this:
>
>   l  b  r
>   =  =  =
>   A  A  A
>
>   B     b
>   C     C
>   D     D
>   E     E
>   F     F
>   I     i
>
> merge will output this:
>
>   A
>
>   <<<<<<< l
>   B
>   =======
>   b
>   >>>>>>> r
>   C
>   D
>   E
>   F
>   <<<<<<< l
>   I
>   =======
>   i
>   >>>>>>> r
>
> This is simple, and useful.
>
> diff3 will output this:
>
>   A
>   <<<<<<< l
>
>   B
>   C
>   D
>   E
>   F
>   I
>   ||||||| b
>   =======
>
>   b
>   C
>   D
>   E
>   F
>   i
>   >>>>>>> r
>
> Not very friendly.
>
> Your zdiff3:
>
>   A
>
>   <<<<<<< l
>   B
>   C
>   D
>   E
>   F
>   I
>   ||||||| b
>   =======
>   b
>   C
>   D
>   E
>   F
>   i
>   >>>>>>> r
>
> Just marginally better.

Your example here is one where diff3 has no original text in the
conflicted region.  Empty text is trivially easy to split, making it a
somewhat uninteresting testcase for zdiff3.  The interesting question
is what do you do when that region is non-empty?  When it's non-empty,
it's not going to match the two sides (i.e. it won't have "C D E F"
lines for your example) -- we know that because when the original also
matches the two sides, the xdiff code will start with multiple
separate conflicts instead of one big one.  So, in such a case, do you
still decide to split the conflict regions, and if so, how do you
split the non-matching original text?