threads / discuss / 44530

v2.11 new diff heuristic?

Subject: v2.11 new diff heuristic?

## tl;dr

4 messages between Nov 22, 2016 and Nov 22, 2016.

replies: 3people: 4as markdown or json

Robert Dailey· Nov 22, 2016, 14:42 UTC · lore

The release notes mention a new heuristic for diff:

* Output from "git diff" can be made easier to read by selecting
which lines are common and which lines are added/deleted
intelligently when the lines before and after the changed section
are the same. A command line option is added to help with the
experiment to find a good heuristics.

However, it lacks information on exactly how to use this new feature. I dug into the git diff documentation here:

https://git-scm.com/docs/git-diff

It mentions a "--compaction-heuristic" option. Is this the new heuristic outlined by the release notes? If not, which is it? Is the compaction heuristic compatible with the histogram diff algorithm? Is there a config option to turn this on all the time? For that matter, is this something I can keep on all the time or is it only useful in certain situations?

There's still so much more about this feature I would like to know.

Jacob Keller· Nov 22, 2016, 17:18 UTC · re: Robert Dailey · lore

Re: v2.11 new diff heuristic?

On November 22, 2016 6:42:36 AM PST, Robert Dailey <rcdailey.lists@gmail.com> wrote:

Show 21 quoted lines

>The release notes mention a new heuristic for diff:
>
>* Output from "git diff" can be made easier to read by selecting
>which lines are common and which lines are added/deleted
>intelligently when the lines before and after the changed section
>are the same. A command line option is added to help with the
>experiment to find a good heuristics.
>
>However, it lacks information on exactly how to use this new feature.
>I dug into the git diff documentation here:
>
>https://git-scm.com/docs/git-diff
>
>It mentions a "--compaction-heuristic" option. Is this the new
>heuristic outlined by the release notes? If not, which is it? Is the
>compaction heuristic compatible with the histogram diff algorithm? Is
>there a config option to turn this on all the time? For that matter,
>is this something I can keep on all the time or is it only useful in
>certain situations?
>
>There's still so much more about this feature I would like to know.

Hi,

Yes for now the compaction heuristic option has an undocumented config. (I forget the exact name off the top of my head). Currently it is being evaluated and likely we want to make it default in the near future once we are certain that it helps and doesn't make any difference worse.

So long term you will not need any special knobs to benefit.

Thanks, Jake

Stefan Beller· Nov 22, 2016, 17:51 UTC · re: Robert Dailey · lore

Re: v2.11 new diff heuristic?

On Tue, Nov 22, 2016 at 6:42 AM, Robert Dailey <rcdailey.lists@gmail.com> wrote:

Show 15 quoted lines

> The release notes mention a new heuristic for diff:
>
> * Output from "git diff" can be made easier to read by selecting
> which lines are common and which lines are added/deleted
> intelligently when the lines before and after the changed section
> are the same. A command line option is added to help with the
> experiment to find a good heuristics.
>
> However, it lacks information on exactly how to use this new feature.
> I dug into the git diff documentation here:
>
> https://git-scm.com/docs/git-diff
>
> It mentions a "--compaction-heuristic" option. Is this the new
> heuristic outlined by the release notes?

yes.

> Is the
> compaction heuristic compatible with the histogram diff algorithm?

yes as the compaction heuristic is applied after the actual diff is performed.

> Is
> there a config option to turn this on all the time? For that matter,
> is this something I can keep on all the time or is it only useful in
> certain situations?

I think you can set diff.compactionHeuristic and it will use it by default.

>
> There's still so much more about this feature I would like to know.

The background story (and what this new compaction heuristic is doing) is found at https://github.com/mhagger/diff-slider-tools

Jeff King· Nov 22, 2016, 17:55 UTC · re: Robert Dailey · lore

Re: v2.11 new diff heuristic?

On Tue, Nov 22, 2016 at 08:42:36AM -0600, Robert Dailey wrote:

Show 6 quoted lines

> I dug into the git diff documentation here:
> 
> https://git-scm.com/docs/git-diff
> 
> It mentions a "--compaction-heuristic" option. Is this the new
> heuristic outlined by the release notes? If not, which is it?

No. The docs on git-scm.com are only for released versions, so that is the "old" attempt at a similar feature from v2.9. The "new" one goes by the name "--indent-heuristic" (and "diff.indentHeuristic" in the config). It is not the default in v2.11, but it probably will become so in a future version.

Show 7 quoted lines

> Is the
> compaction heuristic compatible with the histogram diff algorithm? Is
> there a config option to turn this on all the time? For that matter,
> is this something I can keep on all the time or is it only useful in
> certain situations?
> 
> There's still so much more about this feature I would like to know.

Yes, you can keep it all the time. Yes, it's compatible with histogram (and patience); it happens as a separate post-processing step after the actual diff.

You can find more discussion about how it works in the commit message of 433860f3d0beb0c6f205290bd16cda413148f098.

-Peff