Re: [PATCH updated] Add "--dirstat" for some directory statistics

Linus Torvalds [off-list ref] writes:

On Tue, 12 Feb 2008, Junio C Hamano wrote:

quoted

You can drop the "line count" idea and instead count the "amount
of damage" done to the preimage, just like we do in the rename
similarity computation.

The attached patch is just an outline.  There may need special
cases for unmerged paths.

Ouch. I like the concept and the result, but an not a huge fan of the the 
implementation. The problem with this is that it makes the whole damage 
computation something separate from the earlier phases.

So it actually seems to add a lot of cost. Right now there is basically 
zero added cost to just adding a "--dirstat" to one of my common 
operations, which is

	git diff -M --stat --summary

(that's what I do on merges). 

Wouldn't it be nicer to merge it with one of the stages we did earlier by 
simply saving the data. If not the --stat phase, then the -M phase? 

We could add one ulong to diff_filepair struct to record
"damage", and populate it inside diffcore_rename().

Obviously:

 - You would need to diffcore_count_changes() in separate phase
   if you do not use -M;

 - Even if you use -M, we do not run diffcore_count_changes()
   for unrenamed paths during diffcore_rename(), so you would
   need to do that separately.

By the way, if you run "git diff --summary --dirstat -M" (that
is, without --stat), we will not have to run diffstat at all.
Instead, we only run diffcore_count_changes(), which can be
reused for renamed paths if we keep it in diff_filepair struct.