log --graph: customize the graph lines with config log.graphColors

If you have a 256 colors terminal (or one with true color support), then
the predefined 12 colors seem limited. On the other hand, you don't want
to draw graph lines with every single color in this mode because the two
colors could look extremely similar. This option allows you to hand pick
the colors you want.

Even with standard terminal, if your background color is neither black
or white, then the graph line may match your background and become
hidden. You can exclude your background color (or simply the colors you
hate) with this.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2003,6 +2003,10 @@ log.follow::
 	i.e. it cannot be used to follow multiple files and does not work well
 	on non-linear history.
 
+log.graphColors::
+	A list of colors, separated by commas, that can be used to draw
+	history lines in `git log --graph`.
+
 log.showRoot::
 	If true, the initial commit will be shown as a big creation event.
 	This is equivalent to a diff against an empty tree.

graph.c

@@ -4,6 +4,7 @@
 #include "graph.h"
 #include "diff.h"
 #include "revision.h"
+#include "argv-array.h"
 
 /* Internal API */
 
@@ -62,6 +63,26 @@ enum graph_state {
 static const char **column_colors;
 static unsigned short column_colors_max;
 
+static void parse_graph_colors_config(struct argv_array *colors, const char *string)
+{
+	const char *end, *start;
+
+	start = string;
+	end = string + strlen(string);
+	while (start < end) {
+		const char *comma = strchrnul(start, ',');
+		char color[COLOR_MAXLEN];
+
+		if (!color_parse_mem(start, comma - start, color))
+			argv_array_push(colors, color);
+		else
+			warning(_("ignore invalid color '%.*s' in log.graphColors"),
+				(int)(comma - start), start);
+		start = comma + 1;
+	}
+	argv_array_push(colors, GIT_COLOR_RESET);
+}
+
 void graph_set_column_colors(const char **colors, unsigned short colors_max)
 {
 	column_colors = colors;
@@ -207,9 +228,22 @@ struct git_graph *graph_init(struct rev_info *opt)
 {
 	struct git_graph *graph = xmalloc(sizeof(struct git_graph));
 
-	if (!column_colors)
-		graph_set_column_colors(column_colors_ansi,
-					column_colors_ansi_max);
+	if (!column_colors) {
+		char *string;
+		if (git_config_get_string("log.graphcolors", &string)) {
+			/* not configured -- use default */
+			graph_set_column_colors(column_colors_ansi,
+						column_colors_ansi_max);
+		} else {
+			static struct argv_array custom_colors = ARGV_ARRAY_INIT;
+			argv_array_clear(&custom_colors);
+			parse_graph_colors_config(&custom_colors, string);
+			free(string);
+			/* graph_set_column_colors takes a max-index, not a count */
+			graph_set_column_colors(custom_colors.argv,
+						custom_colors.argc - 1);
+		}
+	}
 
 	graph->commit = NULL;
 	graph->revs = opt;

t/t4202-log.sh

@@ -313,6 +313,28 @@ test_expect_success 'log --graph with merge' '
 	test_cmp expect actual
 '
 
+cat > expect.colors <<\EOF
+*   Merge branch 'side'
+<BLUE>|<RESET><CYAN>\<RESET>
+<BLUE>|<RESET> * side-2
+<BLUE>|<RESET> * side-1
+* <CYAN>|<RESET> Second
+* <CYAN>|<RESET> sixth
+* <CYAN>|<RESET> fifth
+* <CYAN>|<RESET> fourth
+<CYAN>|<RESET><CYAN>/<RESET>
+* third
+* second
+* initial
+EOF
+
+test_expect_success 'log --graph with merge with log.graphColors' '
+	test_config log.graphColors ",, blue,invalid-color, cyan, red  , " &&
+	git log --color=always --graph --date-order --pretty=tformat:%s |
+		test_decode_color | sed "s/ *\$//" >actual &&
+	test_cmp expect.colors actual
+'
+
 test_expect_success 'log --raw --graph -m with merge' '
 	git log --raw --graph --oneline -m master | head -n 500 >actual &&
 	grep "initial" actual