Subject: Re: [PATCH] merge-recursive: fix parsing of "diff-algorithm" option

From: John Keeping <[email-masked]>
Date: Thu, 26 Sep 2013 22:13:31 +0100

On Thu, Sep 26, 2013 at 01:47:20PM -0700, Jonathan Nieder wrote:
> John Keeping wrote:
> 
> > The "diff-algorithm" option to the recursive merge strategy takes the
> > name of the algorithm as an option, but it uses strcmp on the option
> > string to check if it starts with "diff-algorithm=", meaning that this
> > options cannot actually be used.
> >
> > Fix this by switching to prefixcmp.  At the same time, clarify the
> > following line by using strlen instead of a hard-coded length, which
> > also makes it consistent with nearby code.
> >
> > Reported-by: Luke Noel-Storr <[email-masked]>
> > Signed-off-by: John Keeping <[email-masked]>
> 
> Thanks, both.
> 
> [...]
> > --- a/merge-recursive.c
> > +++ b/merge-recursive.c
> > @@ -2069,8 +2069,8 @@ int parse_merge_opt(struct merge_options *o, const char *s)
> >  		o->xdl_opts = DIFF_WITH_ALG(o, PATIENCE_DIFF);
> >  	else if (!strcmp(s, "histogram"))
> >  		o->xdl_opts = DIFF_WITH_ALG(o, HISTOGRAM_DIFF);
> > -	else if (!strcmp(s, "diff-algorithm=")) {
> > -		long value = parse_algorithm_value(s+15);
> > +	else if (!prefixcmp(s, "diff-algorithm=")) {
> > +		long value = parse_algorithm_value(s + strlen("diff-algorithm="));
> >  		if (value < 0)
> >  			return -1;
> 
> While we're here:
> 
> Part of the problem is that there are no tests for this option (or for
> 'git diff --diff-algorithm', or for the '[diff] algorithm'
> configuration), so we didn't notice it didn't work until someone
> actually tried it.
> 
> Do you have any examples of a diff or merge that works better with
> some particular diff algorithm, that could be used in tests?

The following is the same diff, but the first time using histogram and
then with myers.

I'm not sure whether we should encode specific diff output into general
tests though - I assume there's a reason why there aren't already tests
for this, but perhaps it's just that no one's written them yet.

-- >8 --
diff --git a/test.c b/test.c
index f3eae9e..55f0c37 100644
--- a/test.c
+++ b/test.c
@@ -1,9 +1,9 @@
-void bye(void)
-{
-	printf("goodbye\n");
-}
-
 void hello(void)
 {
 	printf("hello\n");
 }
+
+void bye(void)
+{
+	printf("goodbye\n");
+}

diff --git a/test.c b/test.c
index f3eae9e..55f0c37 100644
--- a/test.c
+++ b/test.c
@@ -1,9 +1,9 @@
-void bye(void)
+void hello(void)
 {
-	printf("goodbye\n");
+	printf("hello\n");
 }
 
-void hello(void)
+void bye(void)
 {
-	printf("hello\n");
+	printf("goodbye\n");
 }