Subject: [GSoC PATCH v6 2/3] graph: add --graph-lane-limit option

From: Pablo Sabater <[email-masked]>
Date: Sat, 28 Mar 2026 01:11:12 +0100

Replace the hard-coded lane limit with a user-facing
option '--graph-lane-limit=<n>'. It caps the number of
visible lanes to n. This option requires '--graph', without
it, limiting the graph has no meaning, in this case error out.

Zero and negative values are valid inputs but silently
ignored treating them as "no limit", the same as not using
the option. This follows what '--max-parents' does with
negative values.

The default is 0, same as not being used.

Signed-off-by: Pablo Sabater <[email-masked]>
---
 Documentation/rev-list-options.adoc |   5 +
 graph.c                             |  53 +++++-----
 revision.c                          |   6 ++
 revision.h                          |   1 +
 t/t4215-log-skewed-merges.sh        | 144 ++++++++++++++++++++++++++++
 5 files changed, 186 insertions(+), 23 deletions(-)

diff --git a/Documentation/rev-list-options.adoc b/Documentation/rev-list-options.adoc
index 2d195a1474..1b6ea89a63 100644
--- a/Documentation/rev-list-options.adoc
+++ b/Documentation/rev-list-options.adoc
@@ -1259,6 +1259,11 @@ This implies the `--topo-order` option by default, but the
 	in between them in that case. If _<barrier>_ is specified, it
 	is the string that will be shown instead of the default one.
 
+`--graph-lane-limit=<n>`::
+	When `--graph` is used, limit the number of graph lanes to be shown.
+	Lanes over the limit are not shown. By default it is set to 0 
+	(no limit), zero and negative values are ignored and treated as no limit.
+
 ifdef::git-rev-list[]
 `--count`::
 	Print a number stating how many commits would have been
diff --git a/graph.c b/graph.c
index 70458cf323..ee1f9e2d2d 100644
--- a/graph.c
+++ b/graph.c
@@ -82,8 +82,6 @@ static void graph_show_line_prefix(const struct diff_options *diffopt)
 static const char **column_colors;
 static unsigned short column_colors_max;
 
-static unsigned int max_lanes = 15;
-
 static void parse_graph_colors_config(struct strvec *colors, const char *string)
 {
 	const char *end, *start;
@@ -319,9 +317,13 @@ struct git_graph {
 	struct strbuf prefix_buf;
 };
 
-static inline int graph_needs_truncation(int lane)
+static inline int graph_needs_truncation(struct git_graph *graph, int lane)
 {
-	return lane >= max_lanes;
+	int max = graph->revs->graph_max_lanes;
+	/*
+	 * Ignore values <= 0, meaning no limit.
+	 */
+	return max > 0 && lane >= max;
 }
 
 static const char *diff_output_prefix_callback(struct diff_options *opt, void *data)
@@ -614,7 +616,7 @@ static void graph_update_columns(struct git_graph *graph)
 {
 	struct commit_list *parent;
 	int max_new_columns;
-	int i, seen_this, is_commit_in_columns, max;
+	int i, seen_this, is_commit_in_columns;
 
 	/*
 	 * Swap graph->columns with graph->new_columns
@@ -704,12 +706,17 @@ static void graph_update_columns(struct git_graph *graph)
 	}
 
 	/*
-	 * Cap to the hard-coded limit.
-	 * Allow commits from merges to align to the merged lane.
+	 *  If graph_max_lanes is set, cap the width
 	 */
-	max = max_lanes * 2 + 2;
-	if (graph->width > max)
-		graph->width = max;
+	if (graph->revs->graph_max_lanes > 0) {
+		/*
+		 * Width is column index while a lane is half that.
+		 * Allow commits from merges to align to the merged lane.
+		 */
+		int max_width = graph->revs->graph_max_lanes * 2 + 2;
+		if (graph->width > max_width)
+			graph->width = max_width;
+	}
 
 	/*
 	 * Shrink mapping_size to be the minimum necessary
@@ -861,7 +868,7 @@ static void graph_output_padding_line(struct git_graph *graph,
 	 * Output a padding row, that leaves all branch lines unchanged
 	 */
 	for (i = 0; i < graph->num_new_columns; i++) {
-		if (graph_needs_truncation(i))
+		if (graph_needs_truncation(graph, i))
 			break;
 		graph_line_write_column(line, &graph->new_columns[i], '|');
 		graph_line_addch(line, ' ');
@@ -920,7 +927,7 @@ static void graph_output_pre_commit_line(struct git_graph *graph,
 			seen_this = 1;
 			graph_line_write_column(line, col, '|');
 			graph_line_addchars(line, ' ', graph->expansion_row);
-		} else if (seen_this && graph_needs_truncation(i)) {
+		} else if (seen_this && graph_needs_truncation(graph, i)) {
 			break;
 		} else if (seen_this && (graph->expansion_row == 0)) {
 			/*
@@ -1018,7 +1025,7 @@ static void graph_draw_octopus_merge(struct git_graph *graph, struct graph_line
 		 * Commit is at commit_index, each iteration move one lane to
 		 * the right from the commit.
 		 */
-		if (graph_needs_truncation(graph->commit_index + 1 + i))
+		if (graph_needs_truncation(graph, graph->commit_index + 1 + i))
 			break;
 
 		graph_line_write_column(line, col, (i == dashed_parents - 1) ? '.' : '-');
@@ -1055,14 +1062,14 @@ static void graph_output_commit_line(struct git_graph *graph, struct graph_line
 			seen_this = 1;
 			graph_output_commit_char(graph, line);
 
-			if (graph_needs_truncation(i)) {
+			if (graph_needs_truncation(graph, i)) {
 				graph_line_addch(line, ' ');
 				break;
 			}
 
 			if (graph->num_parents > 2)
 				graph_draw_octopus_merge(graph, line);
-		} else if (graph_needs_truncation(i)) {
+		} else if (graph_needs_truncation(graph, i)) {
 			seen_this = 1;
 			break;
 		} else if (seen_this && (graph->edges_added > 1)) {
@@ -1112,7 +1119,7 @@ static void graph_output_commit_line(struct git_graph *graph, struct graph_line
 	 * padding lane.
 	 */
 	if (graph->num_parents > 1) {
-		if (!graph_needs_truncation(graph->commit_index)) {
+		if (!graph_needs_truncation(graph, graph->commit_index)) {
 			graph_update_state(graph, GRAPH_POST_MERGE);
 		} else {
 			struct commit_list *p = first_interesting_parent(graph);
@@ -1128,7 +1135,7 @@ static void graph_output_commit_line(struct git_graph *graph, struct graph_line
 
 			lane = graph_find_new_column_by_commit(graph, p->item);
 
-			if (!graph_needs_truncation(lane))
+			if (!graph_needs_truncation(graph, lane))
 				graph_update_state(graph, GRAPH_POST_MERGE);
 			else if (graph_is_mapping_correct(graph))
 				graph_update_state(graph, GRAPH_PADDING);
@@ -1192,7 +1199,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 				 * comparable with i. Don't truncate if there are
 				 * no more lanes to print (end of the lane)
 				 */
-				if (graph_needs_truncation(j / 2 + i) &&
+				if (graph_needs_truncation(graph, j / 2 + i) &&
 				    j / 2 + i <= graph->num_columns) {
 					if ((j + i * 2) % 2 != 0)
 						graph_line_addch(line, ' ');
@@ -1205,7 +1212,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 					 * Check if the next lane needs truncation
 					 * to avoid having the padding doubled
 					 */
-					if (graph_needs_truncation((j + 1) / 2 + i) &&
+					if (graph_needs_truncation(graph, (j + 1) / 2 + i) &&
 					    j < graph->num_parents - 1) {
 						truncated = 1;
 						break;
@@ -1220,7 +1227,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 				break;
 			if (graph->edges_added == 0)
 				graph_line_addch(line, ' ');
-		} else if (graph_needs_truncation(i)) {
+		} else if (graph_needs_truncation(graph, i)) {
 			break;
 		} else if (seen_this) {
 			if (graph->edges_added > 0)
@@ -1231,7 +1238,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 			 * If it's between two lanes and next would be truncated,
 			 * don't add space padding.
 			 */
-			if (!graph_needs_truncation(i + 1))
+			if (!graph_needs_truncation(graph, i + 1))
 				graph_line_addch(line, ' ');
 		} else {
 			graph_line_write_column(line, col, '|');
@@ -1380,7 +1387,7 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct graph_l
 	for (i = 0; i < graph->mapping_size; i++) {
 		int target = graph->mapping[i];
 
-		if (!truncated && graph_needs_truncation(i / 2)) {
+		if (!truncated && graph_needs_truncation(graph, i / 2)) {
 			truncated = 1;
 		}
 
@@ -1480,7 +1487,7 @@ static void graph_padding_line(struct git_graph *graph, struct strbuf *sb)
 	for (i = 0; i < graph->num_columns; i++) {
 		struct column *col = &graph->columns[i];
 
-		if (graph_needs_truncation(i))
+		if (graph_needs_truncation(graph, i))
 			break;
 
 		graph_line_write_column(&line, col, '|');
diff --git a/revision.c b/revision.c
index 31808e3df0..81b67682a8 100644
--- a/revision.c
+++ b/revision.c
@@ -2605,6 +2605,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	} else if (!strcmp(arg, "--no-graph")) {
 		graph_clear(revs->graph);
 		revs->graph = NULL;
+	} else if (skip_prefix(arg, "--graph-lane-limit=", &optarg)) {
+		revs->graph_max_lanes = parse_count(optarg);
 	} else if (!strcmp(arg, "--encode-email-headers")) {
 		revs->encode_email_headers = 1;
 	} else if (!strcmp(arg, "--no-encode-email-headers")) {
@@ -3172,6 +3174,10 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 
 	if (revs->no_walk && revs->graph)
 		die(_("options '%s' and '%s' cannot be used together"), "--no-walk", "--graph");
+
+	if (revs->graph_max_lanes > 0 && !revs->graph)
+		die(_("the option '%s' requires '%s'"), "--graph-lane-limit", "--graph");
+
 	if (!revs->reflog_info && revs->grep_filter.use_reflog_filter)
 		die(_("the option '%s' requires '%s'"), "--grep-reflog", "--walk-reflogs");
 
diff --git a/revision.h b/revision.h
index 69242ecb18..874ccce625 100644
--- a/revision.h
+++ b/revision.h
@@ -304,6 +304,7 @@ struct rev_info {
 
 	/* Display history graph */
 	struct git_graph *graph;
+	int graph_max_lanes;
 
 	/* special limits */
 	int skip_count;
diff --git a/t/t4215-log-skewed-merges.sh b/t/t4215-log-skewed-merges.sh
index 28d0779a8c..d7524e9366 100755
--- a/t/t4215-log-skewed-merges.sh
+++ b/t/t4215-log-skewed-merges.sh
@@ -370,4 +370,148 @@ test_expect_success 'log --graph with multiple tips' '
 	EOF
 '
 
+test_expect_success 'log --graph --graph-lane-limit=2 limited to two lanes' '
+	check_graph --graph-lane-limit=2 M_7 <<-\EOF
+	*-.   7_M4
+	|\ \
+	| | * 7_G
+	| | * 7_F
+	| *   7_E
+	| *   7_D
+	* |   7_C
+	| |/
+	|/|
+	* | 7_B
+	|/
+	* 7_A
+	EOF
+'
+
+test_expect_success 'log --graph --graph-lane-limit=1 truncate mid octopus merge' '
+	check_graph --graph-lane-limit=1 M_7 <<-\EOF
+	*-  7_M4
+	|\
+	|   7_G
+	|   7_F
+	| * 7_E
+	| * 7_D
+	*   7_C
+	|
+	|/
+	*   7_B
+	|/
+	* 7_A
+	EOF
+'
+
+test_expect_success 'log --graph --graph-lane-limit=3 limited to three lanes' '
+	check_graph --graph-lane-limit=3 M_1 M_3 M_5 M_7 <<-\EOF
+	*   7_M1
+	|\
+	| | *   7_M2
+	| | |\
+	| | | * 7_H
+	| | |   7_M3
+	| | |   7_J
+	| | |   7_I
+	| | |   7_M4
+	| |_|_
+	|/| |
+	| | |_
+	| |/|
+	| | |
+	| | |/
+	| | *   7_G
+	| | |
+	| | |/
+	| | *   7_F
+	| * |   7_E
+	| | |/
+	| |/|
+	| * |   7_D
+	| | |/
+	| |/|
+	* | | 7_C
+	| |/
+	|/|
+	* | 7_B
+	|/
+	* 7_A
+	EOF
+'
+
+test_expect_success 'log --graph --graph-lane-limit=6 check if it only shows first of 3 parent merge' '
+	check_graph --graph-lane-limit=6 M_1 M_3 M_5 M_7 <<-\EOF
+	*   7_M1
+	|\
+	| | *   7_M2
+	| | |\
+	| | | * 7_H
+	| | | | *   7_M3
+	| | | | |\
+	| | | | | * 7_J
+	| | | | * | 7_I
+	| | | | | | * 7_M4
+	| |_|_|_|_|/
+	|/| | | | |/
+	| | |_|_|/|
+	| |/| | | |/
+	| | | |_|/|
+	| | |/| | |
+	| | * | | | 7_G
+	| | | |_|/
+	| | |/| |
+	| | * | | 7_F
+	| * | | | 7_E
+	| | |/ /
+	| |/| |
+	| * | | 7_D
+	| | |/
+	| |/|
+	* | | 7_C
+	| |/
+	|/|
+	* | 7_B
+	|/
+	* 7_A
+	EOF
+'
+
+test_expect_success 'log --graph --graph-lane-limit=7 check if it shows all 3 parent merge' '
+	check_graph --graph-lane-limit=7 M_1 M_3 M_5 M_7 <<-\EOF
+	*   7_M1
+	|\
+	| | *   7_M2
+	| | |\
+	| | | * 7_H
+	| | | | *   7_M3
+	| | | | |\
+	| | | | | * 7_J
+	| | | | * | 7_I
+	| | | | | | *   7_M4
+	| |_|_|_|_|/|\
+	|/| | | | |/ /
+	| | |_|_|/| /
+	| |/| | | |/
+	| | | |_|/|
+	| | |/| | |
+	| | * | | | 7_G
+	| | | |_|/
+	| | |/| |
+	| | * | | 7_F
+	| * | | | 7_E
+	| | |/ /
+	| |/| |
+	| * | | 7_D
+	| | |/
+	| |/|
+	* | | 7_C
+	| |/
+	|/|
+	* | 7_B
+	|/
+	* 7_A
+	EOF
+'
+
 test_done
-- 
2.43.0