Re: [PATCH 1/2] git-commit: Disallow unchanged tree in non-merge mode

[Junio C Hamano <gitster@pobox.com>]

Linus Torvalds <torvalds@linux-foundation.org> writes:

> Yeah, we should probably:
>  - default to something larger but still reasonably sane (ie not 100, but 
>    perhaps 1000)
>  - special-case the "identical rename", and have a higher limit for that 
>    (we already handle the identicals as a separate pass before we even 
>    start doing the similarity analysis - and it's the similarity analysis 
>    that can be the really expensive part)
>
> There's really no point in trying to do rename analysis for tons and tons 
> of files - even if we find perfect renames, the diff is going to be 
> unreadable by a human, so realistically nobody is ever going to care! A 
> machine won't care whether it was done as a create/delete or a rename, and 
> a human won't be bothered to read about thousands of renames, so we're 
> just wasting time trying to make it prettier.
>
> So quite arguably, the only case we really care about for renames is when 
> the numbers are small enough to be human-readable.

I agree with that.  At the same time we might want to revisit
the earlier "build a full matrix and pick the best ones"
approach commit 5c97558c9a813a0a775c438a79cfc438def00c22 (Detect
renames in diff family) introduced.

A tangent.

I've been thinking about updating the diffcore-rename for some
time to give bonus points to a filepair whose neighbors are
detected to be renames.  E.g. if you have this pair of preimage
and postimage:

	(preimage)		(postimage)

	arch/i386/foo.c		arch/x86/foo-32.c
	arch/i386/bar.c		arch/x86/bar-32.c
	arch/i386/baz.c		arch/x86/baz-32.c

and if foo.c and bar.c are found to be very similar to foo-32.c
and bar-32.c while baz.c and baz-32.c are not that much, we may
want to take hints from the movement of neighbouring files and
boost the similarity score between baz.c and baz-32.c pair.

It would be a quite an interesting coding challenge for anybody
who wants to get his hands dirty.  Would this be worth it in
practice?  I dunno.