improved diff tool

Dear git people, I've created a diff algorithm that focuses on creating readable diffs, see https://github.com/pierstitus/klondiff

The git integration as an external diff command works quite well, though it would be nice to integrate it deeper in git, and also in git-gui and gitk. Any way to use ext-diff from the gui tools?

Is there interest to incorporate this algorithm in the main git codebase? And if so, any hints on how to proceed?

Piers

Hi Piers,

On Thu, 30 Aug 2018, Piers Titus van der Torren wrote:

Looks intriguing.

Git GUI and gitk are both Tcl/Tk programs, and will need quite a bit of work to accommodate for your diff mode.

To put things into perspective: the `--color-words` mode is not integrated into Git GUI nor gitk, and it has been around for a while...

The best advice I have is to look at the `--color-words` mode. It comes with its own "consume" function that accumulates lines from the diff, then outputs them in a different way than the regular colored diff. Your mode would want to do it very similarly.

This is the accumulating part:

	https://github.com/git/git/blob/v2.19.0-rc1/diff.c#L1886

and this is the display part:

	https://github.com/git/git/blob/v2.19.0-rc1/diff.c#L2013

Basically, I would suggest to do a `git grep color.words` to find the places where the `--color-words` mode is special-cased, and add new special-casing for your mode. Which, BTW, I would suggest to find a catchier name for ;-)

I have not looked closely at your implementation, but I could imagine that you might want to have at least part of your algorithm step in at a much lower level: If you can use the patience diff algorithm itself for pre-processing and initial diff generation, that's great, just force XDF_PATIENCE_DIFF; Otherwise you will have to implement an alternative, similar to https://github.com/git/git/blob/v2.19.0-rc1/xdiff/xpatience.c.

Ciao, Johannes

On Thu, Aug 30, 2018 at 4:33 AM Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:

Yes it does.

The diff of c07c0923 looks quite interesting as it shows what was intended to happen (indented a lot of code as it was wrapped).

Playing around with that commit and some recently added switches
  --color-moved --color-moved-ws=allow-indentation-change which is
supposed to solve a similar problem did not yield as good results.

It would be awesome to have gitk/git gui working with more diff options.

How is it different from the "Color words" mode that sits in the drop down menu that defaults to "Line diff" ? https://imgur.com/a/qvo4oOC

(Oh, I realize I had to draw the window wide to see it as it hides easily :/)

It depends on the the layering, look at xdiff/xdiffi.c xdl_do_diff() which has

  if (XDF_DIFF_ALG(xpp->flags) == XDF_PATIENCE_DIFF)
    return xdl_do_patience_diff(mf1, mf2, xpp, xe);

  if (XDF_DIFF_ALG(xpp->flags) == XDF_HISTOGRAM_DIFF)
    return xdl_do_histogram_diff(mf1, mf2, xpp, xe);

as that produces the xdiff/ is for producing the diffs.

All the coloring is at a later step, see diff.c (it's a big file), builtin_diff(), which uses fn_out_consume() as a callback from the xdiff/ stuff and assembles a git diff around the raw diff.

diff.c is responsible for everything after the raw diff, including coloring, or getting the format of patches right (such as file names before a diff, matching up (renamed) files for diffing and such)

AFAICT this is more than just a coloring scheme as it both produces a different low level diff, which would need code in the xdiff parts as well as colors, that is in diff.c.

Thanks, Stefan

On Thu, Aug 30, 2018 at 7:22 PM, Stefan Beller <sbeller@google.com> wrote:

About the low level diff, Michael Haggerty's tools for experimenting with diff "slider" heuristics might also help:

https://github.com/mhagger/diff-slider-tools

On Thu, Aug 30 2018, Piers Titus van der Torren wrote:

This looks very nice, it would be great to have it in git. I think it's more useful to focus on getting it into the git C codebase, the user base of gitk/git-gui is tiny by comparison.

Aside from what others have mentioned, maybe this commit helps to get an idea of how to do stuff like this: 3cde4e02ee ("diff: retire "compaction" heuristics", 2016-12-23)

I.e. it's one of our previous diff algorithms being ripped out. Grepping for the various --diff-algorithm=* options in "man git-diff" in the source/history should also give good ideas of where to start hooking in.

I see from your name / some brief spying on you via Google that you might be in the Amsterdam area. I'm not very familiar with the diff part of the codebase, but if you'd like I'd be happy to meet in person sometime to help you get started on various other stuff in git.git if you'd like.