blame: make diff algorithm configurable

The diff algorithm used in 'git-blame(1)' is set to 'myers',
without the possibility to change it aside from the `--minimal` option.

There has been long-standing interest in changing the default diff
algorithm to "histogram", and Git 3.0 was floated as a possible occasion
for taking some steps towards that:

https://lore.kernel.org/git/xmqqed873vgn.fsf@gitster.g/

As a preparation for this move, it is worth making sure that the diff
algorithm is configurable where useful.

Make it configurable in the `git-blame(1)` command by introducing the
`--diff-algorithm` option and make honor the `diff.algorithm` config
variable. Keep Myers diff as the default.

Signed-off-by: Antonin Delpeuch <antonin@delpeuch.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 51 insertions, 1 deletions

diff --git a/builtin/blame.c b/builtin/blame.c
index 2703820258..27b513d27f 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -779,6 +779,19 @@ static int git_blame_config(const char *var, const char *value,
 		}
 	}
 
+	if (!strcmp(var, "diff.algorithm")) {
+		long diff_algorithm;
+		if (!value)
+			return config_error_nonbool(var);
+		diff_algorithm = parse_algorithm_value(value);
+		if (diff_algorithm < 0)
+			return error(_("unknown value for config '%s': %s"),
+				     var, value);
+		xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
+		xdl_opts |= diff_algorithm;
+		return 0;
+	}
+
 	if (git_diff_heuristic_config(var, value, cb) < 0)
 		return -1;
 	if (userdiff_config(var, value) < 0)
@@ -824,6 +837,38 @@ static int blame_move_callback(const struct option *option, const char *arg, int
 	return 0;
 }
 
+static int blame_diff_algorithm_minimal(const struct option *option,
+					const char *arg, int unset)
+{
+	int *opt = option->value;
+
+	BUG_ON_OPT_ARG(arg);
+
+	*opt &= ~XDF_DIFF_ALGORITHM_MASK;
+	if (!unset)
+		*opt |= XDF_NEED_MINIMAL;
+
+	return 0;
+}
+
+static int blame_diff_algorithm_callback(const struct option *option,
+					 const char *arg, int unset)
+{
+	int *opt = option->value;
+	long value = parse_algorithm_value(arg);
+
+	BUG_ON_OPT_NEG(unset);
+
+	if (value < 0)
+		return error(_("option diff-algorithm accepts \"myers\", "
+			       "\"minimal\", \"patience\" and \"histogram\""));
+
+	*opt &= ~XDF_DIFF_ALGORITHM_MASK;
+	*opt |= value;
+
+	return 0;
+}
+
 static int is_a_rev(const char *name)
 {
 	struct object_id oid;
@@ -915,11 +960,16 @@ int cmd_blame(int argc,
 		OPT_BIT('s', NULL, &output_option, N_("suppress author name and timestamp (Default: off)"), OUTPUT_NO_AUTHOR),
 		OPT_BIT('e', "show-email", &output_option, N_("show author email instead of name (Default: off)"), OUTPUT_SHOW_EMAIL),
 		OPT_BIT('w', NULL, &xdl_opts, N_("ignore whitespace differences"), XDF_IGNORE_WHITESPACE),
+		OPT_CALLBACK_F(0, "diff-algorithm", &xdl_opts, N_("<algorithm>"),
+			       N_("choose a diff algorithm"),
+			       PARSE_OPT_NONEG, blame_diff_algorithm_callback),
 		OPT_STRING_LIST(0, "ignore-rev", &ignore_rev_list, N_("rev"), N_("ignore <rev> when blaming")),
 		OPT_STRING_LIST(0, "ignore-revs-file", &ignore_revs_file_list, N_("file"), N_("ignore revisions from <file>")),
 		OPT_BIT(0, "color-lines", &output_option, N_("color redundant metadata from previous line differently"), OUTPUT_COLOR_LINE),
 		OPT_BIT(0, "color-by-age", &output_option, N_("color lines by age"), OUTPUT_SHOW_AGE_WITH_COLOR),
-		OPT_BIT(0, "minimal", &xdl_opts, N_("spend extra cycles to find better match"), XDF_NEED_MINIMAL),
+		OPT_CALLBACK_F(0, "minimal", &xdl_opts, NULL,
+			       N_("spend extra cycles to find a better match"),
+			       PARSE_OPT_NOARG | PARSE_OPT_HIDDEN, blame_diff_algorithm_minimal),
 		OPT_STRING('S', NULL, &revs_file, N_("file"), N_("use revisions from <file> instead of calling git-rev-list")),
 		OPT_STRING(0, "contents", &contents_from, N_("file"), N_("use <file>'s contents as the final image")),
 		OPT_CALLBACK_F('C', NULL, &opt, N_("score"), N_("find line copies within and across files"), PARSE_OPT_OPTARG, blame_copy_callback),