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

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

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

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