Re: [PATCH] Make sure diff-helper can tell rename/copy in the new diff-raw format.

From: Linus Torvalds <torvalds@osdl.org>
Date: May 23, 2005, 14:49 UTC
Message-ID: <Pine.LNX.4.58.0505230736180.2307@ppc970.osdl.org>
In-Reply-To: <7vfywe769d.fsf@assigned-by-dhcp.cox.net>

On Mon, 23 May 2005, Junio C Hamano wrote:

>
> This adds tests to make sure that diff-helper can tell renames
> from copies using the same "everything but the last one are
> copies and the last one is either rename or stay" logic.

Btw, I still disagree with this notion that the order of the use of the names makes a difference.

I think that when we generate a diff, we should always have the _option_ to make sure that we generate it in a format where you can apply it "incrementally". But I disagree with the notion that it's something fundamental, and in fact, I even think that it's not necessarily a good default.

For example, let's say that you have modified "fileA" _and_ you have created a "fileB" that is a copy of the original "fileA" with some _other_ slight modifications. We'll call the SHA1's involved "sha_A", "sha_A'" and "sha_B"

I think it's perfectly valid to say

	:100644 100644 <sha_A> <sha_A'> M	fileA	fileA
	:100644 100644 <sha_A> <sha_B> C89	fileA	fileB

which says "fileA" was modified from orig-A to new-A, and "fileB" is a copy based on orig-A.

(I've used a new syntax just to confuse the issue, with the extra field for "what happened", aka "M" for "modify", "C89" for "copy a 89% similar file").

Now, when the above is turned into a "diff", that diff is no longer something you can apply "incrementally" - you have to apply it as if you're applying all differences to the "original tree". But the thing is, that's actually what I _want_, because I was planning on writing a tool that applies patches that applies them all-or-nothing.

Also, it turns out that this kind of "non-incremental" diff is the kind that I personally want to see as a _human_, because quite frankly, my brain-capacity is that of a demented ocelot, and I can't _remember_ what happened in other parts of the diff. I much prefer the stateless "oh, this file X is in that relation Y to the previous version of file Z".

I do that partly because I actually routinely edit patches. If you have the incremental format, that's practically impossible, while the stateless version is fine.

See?

So I think all the clever "don't re-use files we have modified" etc is actually wrong. If you want to make a traditional diff that can be applied with normal "patch", you just don't use the -M or -C flags.

		Linus