diffcore and directory renames

I often found myself lost when looking at a diff where a couple of large dirs where renamed, and a handful of files were modified to take the rename into account - not a rare situation, I'd say. In such a case, the diffs themselves are mostly hidden among numerous rename entries.

To that, I felt that git ought to know better, and could easily present a directory rename as such. Among the advantages of having such "dir rename detection", aside from readability which we should not neglect, we'd have more accurate "svn dcommit" for such renames and, and better decisions made when merging a branch with adds a file into a dir, with a branch which renames that dir (ok, maybe not that frequent, but still).

Looking closer at the current behaviour, I realized that git-diff-tree *is* already able to report such renames in raw mode :

$ ./git-diff-tree 0f1027 -M 0f1027e1aceb4bc5fa682776ab9f72935e2cd1b3 :040000 040000 6f6159f0245784352414ff38ffb68bae80f30bd6 6f6159f0245784352414ff38ffb68bae80f30bd6 R100 ppc moved

... but patch output breaks this into file moves, and even other commands which I would have expected to display the same do in fact not (eg. "git show --raw"). Is there any reason for this ? I would have expected git-show to do the same as diff-tree, and it would not be so much of a problem to introduce a special form of the "rename" chunk to handle dir renames ?

Since current diff-tree behaviour is based on comparing tree hashs, it only detects 100%-similarity renames, and naturally diff-index and diff-files cannot even do that much yet. However, an optional factorization pass following rename detection could quite easily detect if, when a containing directory disappears because of a set of renames - this is what I have started to look at.

Since what I propose is indeed just a factorization of file renames, it could be adequate to represent them as such instead of directory renames, so as to read something like:

|diff --git a/ppc/* b/moved/
|rename from ppc/*
|rename to moved/

This would have the additional advantage of also working when the target destination was pre-existing, and when merging several dirs into a single one - while at the same time to mistaking the reader too much into thinking that git cares about directories so much ;)

Callers should also be careful about interpreting the output when mixing this behaviour with file filtering, obviously, so it is not meant to be activated by default.

Does any of this sound sensible ?

On the technical side, I figured out that the best way to plug the factorization was inside diffcore_rename to take advantage of the fact it has a list of all renames, that's not going to be published out of this function - that only left me the "cleanup:" label as insertion point, not that I find it conceptually very comfortable :)

But if I have understood correctly, diffcore only gets access to changed filepairs, and does not know about the trees it is comparing.

The most sensible way to go I can think of would be to give to diffcore enough information (src and dst tree-hash|"INDEX"|"WORKING COPY") to be able to know for directory existence. I'd rather check it does not sound completely wrong to anyone before going into that - opinions ?

Best regards,