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

On Tue, Jun 15, 2021 at 2:40 AM Felipe Contreras
[off-list ref] wrote:

Elijah Newren via GitGitGadget wrote:

quoted

From: Elijah Newren <redacted>

"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?