Re: [PATCH 0/3] Teach Git about the patience diff algorithm

From

Johannes Schindelin <johannes.schindelin@gmx.de>

Date

Jan 7, 2009, 20:38 UTC

Message-ID

<alpine.DEB.1.00.0901072121260.7496@intel-tinevez-2-302>

In-Reply-To

<1231359317.6011.12.camel@maia.lan>

Hi,

On Thu, 8 Jan 2009, Sam Vilain wrote:

> On Tue, 2009-01-06 at 20:40 +0100, Johannes Schindelin wrote:
> > Although I would like to see it in once it is fleshed out -- even if it 
> > does not meet our usefulness standard -- because people said Git is 
> > inferior for not providing a patience diff.  If we have --patience, we can 
> > say "but we have it, it's just not useful, check for yourself".
> 
> Whatever happens, the current deterministic diff algorithm needs to stay
> for generating patch-id's... those really can't be allowed to change.

- patience diff is slower,
- patience diff is often not worth it (produces the same, maybe even 
  worse output), and
- patience diff needs the existing code as a fallback anyway.

Where it could possibly help to change existing behavior is with merging.

So maybe somebody has some time to play with, and can apply this patch:

-- snip --

diff --git a/ll-merge.c b/ll-merge.c
index fa2ca52..f731026 100644
--- a/ll-merge.c
+++ b/ll-merge.c
@@ -79,6 +79,8 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 	memset(&xpp, 0, sizeof(xpp));
 	if (git_xmerge_style >= 0)
 		style = git_xmerge_style;
+	if (getenv("GIT_PATIENCE_MERGE"))
+		xpp.flags |= XDF_PATIENCE_DIFF;
 	return xdl_merge(orig,
 			 src1, name1,
 			 src2, name2,
-- snap --

After compiling and installing, something like this should be fun to 
watch:

	$ git rev-list --all --parents | \
	  grep " .* " | \
	  while read commit parent1 parent2 otherparents
	  do
		test -z "$otherparents" || continue
		git checkout $parent1 &&
		git merge $parent2 &&
		git diff > without-patience.txt &&
		git reset --hard $parent1 &&
		GIT_PATIENCE_MERGE=1 git merge $parent2 &&
		git diff > with-patience.txt &&
		if ! cmp without-patience.txt with-patience.txt
		then
			echo '==============================='
			echo "differences found in merge $commit"
			echo "without patience: $(wc -l < without-patience.txt)"
			echo "with patience: $(wc -l < with-patience.txt)"
			echo '-------------------------------'
			echo 'without patience:'
			cat without-patience.txt
			echo '-------------------------------'
			echo 'with patience:'
			cat with-patience.txt
		fi ||
		exit
	  done | tee patience-merge.out

Ciao,
Dscho