diff: consolidate diff algorithm option parsing

The diff option parsing for --minimal, --patience, --histgoram can all
be consolidated into one function. This is a preparatory step for the
subsequent commit which teaches diff to keep track of whether or not a
diff algorithm has been set via the command line.

While we're at it, the logic that sets the diff algorithm in
diff_opt_diff_algorithm() can be refactored into a helper that will
allow multiple callsites to set the diff algorithm.

Signed-off-by: John Cai <johncai86@gmail.com>

diff.c

@@ -3437,6 +3437,22 @@ static int diff_filepair_is_phoney(struct diff_filespec *one,
 	return !DIFF_FILE_VALID(one) && !DIFF_FILE_VALID(two);
 }
 
+static int set_diff_algorithm(struct diff_options *opts,
+			      const char *alg)
+{
+	long value = parse_algorithm_value(alg);
+
+	if (value < 0)
+		return 1;
+
+	/* clear out previous settings */
+	DIFF_XDL_CLR(opts, NEED_MINIMAL);
+	opts->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
+	opts->xdl_opts |= value;
+
+	return 0;
+}
+
 static void builtin_diff(const char *name_a,
 			 const char *name_b,
 			 struct diff_filespec *one,
@@ -4440,15 +4456,11 @@ static void run_diff_cmd(const char *pgm,
 	const char *xfrm_msg = NULL;
 	int complete_rewrite = (p->status == DIFF_STATUS_MODIFIED) && p->score;
 	int must_show_header = 0;
+	struct userdiff_driver *drv = userdiff_find_by_path(o->repo->index, attr_path);
 
-
-	if (o->flags.allow_external) {
-		struct userdiff_driver *drv;
-
-		drv = userdiff_find_by_path(o->repo->index, attr_path);
+	if (o->flags.allow_external)
 		if (drv && drv->external)
 			pgm = drv->external;
-	}
 
 	if (msg) {
 		/*
@@ -4465,12 +4477,17 @@ static void run_diff_cmd(const char *pgm,
 		run_external_diff(pgm, name, other, one, two, xfrm_msg, o);
 		return;
 	}
-	if (one && two)
+	if (one && two) {
+		if (!o->xdl_opts_command_line)
+			if (drv && drv->algorithm)
+				set_diff_algorithm(o, drv->algorithm);
+
 		builtin_diff(name, other ? other : name,
 			     one, two, xfrm_msg, must_show_header,
 			     o, complete_rewrite);
-	else
+	} else {
 		fprintf(o->file, "* Unmerged path %s\n", name);
+	}
 }
 
 static void diff_fill_oid_info(struct diff_filespec *one, struct index_state *istate)
@@ -4567,6 +4584,10 @@ static void run_diffstat(struct diff_filepair *p, struct diff_options *o,
 	const char *name;
 	const char *other;
 
+	struct userdiff_driver *drv = userdiff_find_by_path(o->repo->index, p->one->path);
+	if (drv && drv->algorithm)
+		set_diff_algorithm(o, drv->algorithm);
+
 	if (DIFF_PAIR_UNMERGED(p)) {
 		/* unmerged */
 		builtin_diffstat(p->one->path, NULL, NULL, NULL,
@@ -5107,17 +5128,44 @@ static int diff_opt_diff_algorithm(const struct option *opt,
 				   const char *arg, int unset)
 {
 	struct diff_options *options = opt->value;
-	long value = parse_algorithm_value(arg);
 
 	BUG_ON_OPT_NEG(unset);
-	if (value < 0)
+
+	if (set_diff_algorithm(options, arg))
 		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;
+	options->xdl_opts_command_line = 1;
+
+	return 0;
+}
+
+static int diff_opt_diff_algorithm_no_arg(const struct option *opt,
+				   const char *arg, int unset)
+{
+	struct diff_options *options = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+	BUG_ON_OPT_ARG(arg);
+
+	if (!strcmp(opt->long_name, "patience")) {
+		size_t i;
+		/*
+		 * Both --patience and --anchored use PATIENCE_DIFF
+		 * internally, so remove any anchors previously
+		 * specified.
+		 */
+		for (i = 0; i < options->anchors_nr; i++)
+			free(options->anchors[i]);
+		options->anchors_nr = 0;
+	}
+
+	if (set_diff_algorithm(options, opt->long_name))
+		BUG("available diff algorithms include \"myers\", "
+			       "\"minimal\", \"patience\" and \"histogram\"");
+
+	options->xdl_opts_command_line = 1;
+
 	return 0;
 }
 
@@ -5242,26 +5290,6 @@ static enum parse_opt_result diff_opt_output(struct parse_opt_ctx_t *ctx,
 	return 0;
 }
 
-static int diff_opt_patience(const struct option *opt,
-			     const char *arg, int unset)
-{
-	struct diff_options *options = opt->value;
-	int i;
-
-	BUG_ON_OPT_NEG(unset);
-	BUG_ON_OPT_ARG(arg);
-	options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
-	/*
-	 * Both --patience and --anchored use PATIENCE_DIFF
-	 * internally, so remove any anchors previously
-	 * specified.
-	 */
-	for (i = 0; i < options->anchors_nr; i++)
-		free(options->anchors[i]);
-	options->anchors_nr = 0;
-	return 0;
-}
-
 static int diff_opt_ignore_regex(const struct option *opt,
 				 const char *arg, int unset)
 {
@@ -5562,9 +5590,10 @@ struct option *add_diff_options(const struct option *opts,
 			    N_("prevent rename/copy detection if the number of rename/copy targets exceeds given limit")),
 
 		OPT_GROUP(N_("Diff algorithm options")),
-		OPT_BIT(0, "minimal", &options->xdl_opts,
-			N_("produce the smallest possible diff"),
-			XDF_NEED_MINIMAL),
+		OPT_CALLBACK_F(0, "minimal", options, NULL,
+			       N_("produce the smallest possible diff"),
+			       PARSE_OPT_NONEG | PARSE_OPT_NOARG,
+			       diff_opt_diff_algorithm_no_arg),
 		OPT_BIT_F('w', "ignore-all-space", &options->xdl_opts,
 			  N_("ignore whitespace when comparing lines"),
 			  XDF_IGNORE_WHITESPACE, PARSE_OPT_NONEG),
@@ -5589,10 +5618,11 @@ struct option *add_diff_options(const struct option *opts,
 		OPT_CALLBACK_F(0, "patience", options, NULL,
 			       N_("generate diff using the \"patience diff\" algorithm"),
 			       PARSE_OPT_NONEG | PARSE_OPT_NOARG,
-			       diff_opt_patience),
-		OPT_BITOP(0, "histogram", &options->xdl_opts,
-			  N_("generate diff using the \"histogram diff\" algorithm"),
-			  XDF_HISTOGRAM_DIFF, XDF_DIFF_ALGORITHM_MASK),
+			       diff_opt_diff_algorithm_no_arg),
+		OPT_CALLBACK_F(0, "histogram", options, NULL,
+			       N_("generate diff using the \"histogram diff\" algorithm"),
+			       PARSE_OPT_NONEG | PARSE_OPT_NOARG,
+			       diff_opt_diff_algorithm_no_arg),
 		OPT_CALLBACK_F(0, "diff-algorithm", options, N_("<algorithm>"),
 			       N_("choose a diff algorithm"),
 			       PARSE_OPT_NONEG, diff_opt_diff_algorithm),