Re: feature suggestion: optimize common parts for checkout --conflict=diff3

From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: "Antoine Pelisse" <apelisse@gmail.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	git <git@vger.kernel.org>,
	kernel@pengutronix.de
Subject: Re: feature suggestion: optimize common parts for checkout --conflict=diff3
Date: Thu, 07 Mar 2013 10:21:52 -0800	[thread overview]
Message-ID: <7vk3pjm5of.fsf@alter.siamese.dyndns.org> (raw)
In-Reply-To: <7v1ubrnmtu.fsf@alter.siamese.dyndns.org> (Junio C. Hamano's message of "Thu, 07 Mar 2013 09:26:05 -0800")

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

> You could make it "1234<AB|5=AX><C|=C><DE|6=YE>789", and that is
> technically correct (what there were in the shared original for the
> conflicted part is 5 and then 6), but the representation pretends
> that it knows more than there actually is information, which may be
> somewhat misleading.  All these three are equally plausible split of
> the original "56":
>
> 	1234<AB|=AX><C|=C><DE|56=YE>789
> 	1234<AB|5=AX><C|=C><DE|6=YE>789
> 	1234<AB|56=AX><C|=C><DE|=YE>789
>
> and picking one over others would be a mere heuristic.  All three
> are technically correct representations and it is just the matter of
> which one is the easiest to understand.  So, this is the kind of
> "misleading but not incorrect".

I forgot to say that youu could even do something silly like:

	1234<AB|=AX><C|56=C><DE|=YE>789

;-)

> In all these cases, the middle part would look like this:
>
>       <<<<<<< ours
>       C
>       ||||||| base
>       =======
>       C
>       >>>>>>> theirs
>
> in order to honor the explicit "I want to view all three versions to
> examine the situation" aka "--conflict=diff3" option.  We cannot
> reduce it to just "C".  That will make it "not just misleading but
> is actively wrong".

I also forgot to say that the issue is the same to reduce

 	1234<AB|=AX><C|=C><DE|56=YE>789

to

 	1234<A|=A><B|=X><C|=C><D|56=Y><E|=E>789

which is unconditionally correct and then for all x reduce <x|=x> to
x, yielding

 	1234A<B|=X>C<D|56=Y>E789

which your zealous-diff3 would do.  So squashing that <C|=C> in the
middle would be consistent if you take the zealous-diff3 route.

But again, that is discarding the information of the original, which
the user explicitly asked from "diff3 -m", i.e. show all three to
examine the situation. If the user wants to operate _without_ the
original, the user would have asked for "RCS merge" style output, so
I am still not sure if that is a sensible mode of operation for diff3
to begin with.