threads / discuss / 44799

Counter-intuitive result from diff -C --stat

Subject: Counter-intuitive result from diff -C --stat

## tl;dr

2 messages between Dec 31, 2016 and Jan 1, 2017.

replies: 1people: 2as markdown or json

Mike Hommey· Dec 28, 2016, 08:09 UTC · lore

Hi,

So I was checking out differences between two branches, accounting for file moves with -C, and was surprised by the number of insertions and deletions that it indicated, because it was telling me I had removed more than I added, which I really don't think is true.

I took a closer look, and what happens is that I had a lot of stuff in a __init__.py file that I moved to another file, while keeping a now new, empty, __init__.py file.

Which means while diff counts the deletions from __init__.py, it doesn't count the additions from the move because it is a move, leading to a counter-intuitive result.

Here's how to reproduce in case code makes more sense than prose:

/tmp$ git init g
Initialized empty Git repository in /tmp/g/.git/
/tmp$ cd g
/tmp/g$ echo foo > foo
/tmp/g$ git add foo
/tmp/g$ git commit -m foo
[master (root-commit) 14749a7] foo
 1 file changed, 1 insertion(+)
 create mode 100644 foo
/tmp/g$ git mv foo bar
/tmp/g$ touch foo
/tmp/g$ git add foo
/tmp/g$ git commit -m bar
[master 9fbf50e] bar
 2 files changed, 1 insertion(+), 1 deletion(-)
 create mode 100644 bar
/tmp/g$ git diff HEAD~ -C --stat
 foo => bar | 0
 foo        | 1 -
 2 files changed, 1 deletion(-)

I'm actually not sure what the right thing would be. I guess this is a case where -B should help, but it doesn't.

Any thoughts?

Mike

Junio C Hamano· Jan 1, 2017, 01:15 UTC · re: Mike Hommey · lore

Re: Counter-intuitive result from diff -C --stat

Mike Hommey <mh@glandium.org> writes:

Show 14 quoted lines

> Hi,
>
> So I was checking out differences between two branches, accounting for
> file moves with -C, and was surprised by the number of insertions and
> deletions that it indicated, because it was telling me I had removed
> more than I added, which I really don't think is true.
>
> I took a closer look, and what happens is that I had a lot of stuff in
> a __init__.py file that I moved to another file, while keeping a now
> new, empty, __init__.py file.
>
> Which means while diff counts the deletions from __init__.py, it doesn't
> count the additions from the move because it is a move, leading to a
> counter-intuitive result.

Intuition is in the eyes of observer.

A pairing of the original and the result you saw might be not very useful (which I have no opinion on), but in the context of the chosen pairing of the original and the result, in order to produce the final result, you started from a copy of the original and removed quite a lot while adding just a bit, so what you saw was an outcome that was deliberately designed.