Subject: Re: Rename detection within in files WAS: [PATCH 2/6] t7408: merge short tests, factor out testing method

From: René Scharfe <[email-masked]>
Date: Sun, 7 Aug 2016 11:24:42 +0200

Am 06.08.2016 um 01:26 schrieb Stefan Beller:
> When moving code around, we usually get large chunks of text. If the contributor
> is not 100% trustworthy, we need to review all the code without much intelectual
> joy. Essentially the reviewer is just making sure the parts of the text are the
> same.
>
> I'd like to propose a new addition to the diff format that makes this use case
> easier. The idea is to mark up lines that were just moved around in the file
> instead of adding and removing them.
>
> Currently we have 3 characters that
> are allowed to start a line within a hunk:
> ' ' to indicate context
> '+' to add a line
> '-' to remove a line
>
> I'd propose to add the following characters:
> '*' which is the same as '+', but it indicates that the line was moved
>      from somewhere else without change.
> 'X' The same as '-', with the addition that this line was moved to a different
>      place without change.
>
> The patch below uses these new '*' and 'X'. Each hunk that makes use of these
> additions, is followed other sections, [moved-from, moved-to] that indicate
> where the corresponding line is.

Interesting idea. It should be easy to convert the result into a regular 
unified diff for consumption with patch(1) or git am/apply by replacing 
the new flags with + and - and removing the moved-* hunks.

Your example ignores whitespace changes at the start of the line and 
within it, the added "-C $working_dir", s/expected/expect/; is this all 
intended?  Only a single blank line was moved verbatim.

The moved-from and moved-to hunks make this diff quite verbose.

If multiple lines from different sources are moved to the same hunk then 
you'd get multiple moved-from hunks following that single destination, 
right?  (Same with lines moved from a single hunk to multiple 
destinations and moved-to.)

But does it even warrent a new format? It's a display problem; the 
necessary information is already in the diffs we have today.  A 
graphical diff viewer could connect moved blocks with lines, like 
http://www.araxis.com/merge/ does in its side-by-side view.  A 
Thunderbird extension (or a bookmarklet or browser extendiion for 
webmail users) could do that for an email-based workflow.

Still, what about adding information about moved lines as an extended 
header (like that index line)?  Line numbers are included in hunk 
headers and can serve as orientation.  A reader would have to do some 
mental arithmetic (ugh), but incompatible format changes would be 
avoided.  For your example it should look something like this:

	move from t/t7408-submodule-reference.sh:52,1
	move to t/t7408-submodule-reference.sh:22,1

>
> There are multiple things to tackle when going for such an addition:
> * How to present this to the user (it's covered in this email)
> * how to find the renamed lines algorithmically.
>    (there are already approaches to that, e.g. https://github.com/stefanbeller/duplo
>    which is http://duplo.sourceforge.net/ with no substantial additions)
>
> Any comments welcome,
>
> Thanks,
> Stefan
>
> ---
>   t/t7408-submodule-reference.sh | 50 +++++++++++++++---------------------------
>   1 file changed, 15 insertions(+), 29 deletions(-), 6 moved lines
>
> diff --git a/t/t7408-submodule-reference.sh b/t/t7408-submodule-reference.sh
> index afcc629..1416cbd 100755
> --- a/t/t7408-submodule-reference.sh
> +++ b/t/t7408-submodule-reference.sh
> @@ -10,6 +10,16 @@ base_dir=$(pwd)
>
>   U=$base_dir/UPLOAD_LOG
>
> +test_alternate_usage()
> +{
> +	alternates_file=$1
> +	working_dir=$2
> +	test_line_count = 1 $alternates_file &&
> *	echo "0 objects, 0 kilobytes" >expect &&
> *	git -C $working_dir count-objects >current &&
> *	diff expect current
> +}
> +

Post-image line 22.

>   test_expect_success 'preparing first repository' '
>   	test_create_repo A &&
>   	(
> @@ move-source 42,6 @@ test_expect_success 'that reference gets used with add' '
>   test_expect_success 'that reference gets used with add' '
>   	(
>   		cd super/sub &&
> X		echo "0 objects, 0 kilobytes" > expected &&
> X		git count-objects > current &&
> X		diff expected current
>   	)
>   '
> @@ -42,44 +52,20 @@ test_expect_success 'preparing superproject' '
>   	)
>   '
>
> -test_expect_success 'submodule add --reference' '
> +test_expect_success 'submodule add --reference uses alternates' '
>   	(
>   		cd super &&
>   		git submodule add --reference ../B "file://$base_dir/A" sub &&
>   		git commit -m B-super-added
> -	)
> -'
> -

Pre-image line 52.

> -test_expect_success 'after add: existence of info/alternates' '
> -	test_line_count = 1 super/.git/modules/sub/objects/info/alternates
> -'
> -
> -test_expect_success 'that reference gets used with add' '
> -	(
> -		cd super/sub &&
> X		echo "0 objects, 0 kilobytes" > expected &&
> X		git count-objects > current &&
> X		diff expected current
> -	)
> -'
> -
> -test_expect_success 'cloning superproject' '
> -	git clone super super-clone
> -'
> -
> @@ move-to 10,6 @@ test_alternate_usage
> +	alternates_file=$1
> +	working_dir=$2
> +	test_line_count = 1 $alternates_file &&
> *	echo "0 objects, 0 kilobytes" >expect &&
> *	git -C $working_dir count-objects >current &&
> *	diff expect current
> +}
> +
> --
> 2.9.2.572.g9d9644e.dirty
>