threads / discuss / 59330

"git diff" surprising default output

Subject: "git diff" surprising default output

## tl;dr

3 messages between Mar 1, 2023 and Mar 1, 2023.

replies: 2people: 2as markdown or json

Sergey Organov· Mar 1, 2023, 18:25 UTC · lore

Hello,

By default, 'git diff' produces an output that is often rather surprising and difficult to grok, here is a simple example:

$ git diff --no-index -- f0.txt f1.txt

diff --git a/f0.txt b/f1.txt
index 3af00a929..35478f7bd 100644
--- a/f0.txt
+++ b/f1.txt
@@ -3,9 +3,6 @@
 #endif
 #if CANOPEN > 0
     IMD_CANOPEN,
-#endif
-#if MDL_LASER > 0
-    IMD_LASER,
 #endif
     IMD_AUTO,
     IMD_NONE,

I figured the offender is --indent-heuristic that is described in the
manual as:

      Enable the heuristic that shifts diff hunk boundaries to make
      patches easier to read. This is the default.

but apparently rather makes patches harder to read, at least at some
conditions. Turning it off helps to get the expected result:

$ git diff --no-index --no-indent-heuristic -- f0.txt f1.txt
diff --git a/f0.txt b/f1.txt
index 3af00a929..35478f7bd 100644
--- a/f0.txt
+++ b/f1.txt
@@ -4,9 +4,6 @@
 #if CANOPEN > 0
     IMD_CANOPEN,
 #endif
-#if MDL_LASER > 0
-    IMD_LASER,
-#endif
     IMD_AUTO,
     IMD_NONE,
     IMD_COUNT


Changing --diff-algorithm doesn't change the outcome.

So the questions are:

- Is there an example of an advantage the option has?

- Is it possible to flip the default?

Thanks,
-- Sergey Organov

Junio C Hamano· Mar 1, 2023, 19:40 UTC · re: Sergey Organov · lore

Re: "git diff" surprising default output

Sergey Organov <sorganov@gmail.com> writes:

Show 8 quoted lines

> I figured the offender is --indent-heuristic that is described in the
> manual as:
>
>       Enable the heuristic that shifts diff hunk boundaries to make
>       patches easier to read. This is the default.
>
> but apparently rather makes patches harder to read, at least at some
> conditions. Turning it off helps to get the expected result:

As a heuristic, it is not surprising that it has some corner cases where it does not shine. I think the version that was eventually merged starts here

  https://lore.kernel.org/git/cover.1473068229.git.mhagger@alum.mit.edu/

and we should be able to find a few links to the original observation that motivated the change, e.g.

  https://public-inbox.org/git/cover.1470259583.git.mhagger@alum.mit.edu/

Thanks.

Sergey Organov· Mar 1, 2023, 20:28 UTC · re: Junio C Hamano · lore

Re: "git diff" surprising default output

Junio C Hamano <gitster@pobox.com> writes:

Show 21 quoted lines

> Sergey Organov <sorganov@gmail.com> writes:
>
>> I figured the offender is --indent-heuristic that is described in the
>> manual as:
>>
>>       Enable the heuristic that shifts diff hunk boundaries to make
>>       patches easier to read. This is the default.
>>
>> but apparently rather makes patches harder to read, at least at some
>> conditions. Turning it off helps to get the expected result:
>
> As a heuristic, it is not surprising that it has some corner cases
> where it does not shine.  I think the version that was eventually
> merged starts here
>
>   https://lore.kernel.org/git/cover.1473068229.git.mhagger@alum.mit.edu/
>
> and we should be able to find a few links to the original
> observation that motivated the change, e.g.
>
>   https://public-inbox.org/git/cover.1470259583.git.mhagger@alum.mit.edu/

Thanks a lot for the references!

From what I briefly read there it apparently follows that it'd shine at regular C code, yet won't shine at preprocessor directives that are usually out of regular alignment.

It'd be nice to improve this, but I'm not sure it's a simple thing to fix, or even at all possible, though maybe it'd help to somehow favoring chunks that start and end at the same alignment?

Or maybe it's just plain bug in the implementation?

Thanks, -- Sergey Organov