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

[Linus Torvalds <torvalds@osdl.org>]

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