Re: [PATCH v2 3/3] diff: Introduce --diff-algorithm command line option

On 14.01.2013 22:06, Junio C Hamano wrote:

Michal Privoznik [off-list ref] writes:

quoted

+--diff-algorithm={patience|minimal|histogram|myers}::
+	Choose a diff algorithm. The variants are as follows:
++
+--
+`myers`;;
+	The basic greedy diff algorithm.
+`minimal`;;
+	Spend extra time to make sure the smallest possible diff is
+	produced.
+`patience`;;
+	Use "patience diff" algorithm when generating patches.
+`histogram`;;
+	This algorithm extends the patience algorithm to "support
+	low-occurrence common elements".
+--
++
+For instance, if you configured diff.algorithm variable to a
+non-default value and want to use the default one, then you
+have to use `--diff-algorithm=myers` option.
+
+You should prefer this option over the `--minimal`, `--patience` and
+`--histogram` which are kept just for backwards compatibility.

Much better; I'd drop the last paragraph, though.

I also think we really should consider "default" synonym for
whichever happens to be the built-in default (currently myers).

quoted

diff --git a/diff.c b/diff.c
index e9a7e4d..3e021d5 100644
--- a/diff.c
+++ b/diff.c

@@ -144,7 +144,7 @@ static int git_config_rename(const char *var, const char *value)
 	return git_config_bool(var,value) ? DIFF_DETECT_RENAME : 0;
 }
 
-static long parse_algorithm_value(const char *value)
+long parse_algorithm_value(const char *value)
 {
 	if (!value || !strcasecmp(value, "myers"))
 		return 0;

@@ -3633,6 +3633,16 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
 	else if (!strcmp(arg, "--histogram"))
 		options->xdl_opts = DIFF_WITH_ALG(options, HISTOGRAM_DIFF);
+	else if (!prefixcmp(arg, "--diff-algorithm=")) {
+		long value = parse_algorithm_value(arg+17);
+		if (value < 0)
+			return error("option diff-algorithm accepts \"myers\", "
+				     "\"minimal\", \"patience\" and \"histogram\"");
+		/* clear out previous settings */
+		DIFF_XDL_CLR(options, NEED_MINIMAL);
+		options->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
+		options->xdl_opts |= value;

This makes me wonder if other places that use DIFF_WITH_ALG() also
need to worry about clearing NEED_MINIMAL?

Not really. In my approach, --minimal looks at yet another algorithm.
However, current code sees it as orthogonal to the myers algorithm. The
flag doesn't have any effect on --patience or --histogram at all.

So I think we need to clear the flag only when using --diff-algorithm.

Michal