graph: limit the graph width to a hard-coded max

Repositories that have many active branches at the same time
produce wide graphs. A lane consists of two columns, the edge and the
padding (or another edge), each branch takes a lane in the graph and
there is no way to limit how many can be shown.

Limit the graph engine to draw at most 15 lanes. Lanes over the limit are
not rendered.

On the commit line, if the commit lives on a visible lane, show the
normal commit mark and stop rendering. If the commit lives on the
first hidden lane, show the "*" commit mark so it is known that
this commit lives in the first hidden lane. Commits on deeper lanes
aren't rendered, but the commit subject will always remain.

For merges, the post-merge lane is only needed when the commit or
the first parent lives on a visible lane (to draw the connection
between them), when both are on hidden lanes, post-merge carries
no useful information, skip it and go to collapsing or padding state.

Also fix a pre-existing indentation issue.

The hard-coded limit will be replaced by a user-facing option
on a subsequent commit.

Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pabloosabaterr

graph.c

@@ -82,6 +82,8 @@ 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;
@@ -317,6 +319,11 @@ struct git_graph {
 	struct strbuf prefix_buf;
 };
 
+static inline int graph_needs_truncation(int lane)
+{
+	return lane >= max_lanes;
+}
+
 static const char *diff_output_prefix_callback(struct diff_options *opt, void *data)
 {
 	struct git_graph *graph = data;
@@ -607,7 +614,7 @@ 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;
+	int i, seen_this, is_commit_in_columns, max;
 
 	/*
 	 * Swap graph->columns with graph->new_columns
@@ -696,6 +703,14 @@ 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.
+	 */
+	max = max_lanes * 2 + 2;
+	if (graph->width > max)
+		graph->width = max;
+
 	/*
 	 * Shrink mapping_size to be the minimum necessary
 	 */
@@ -846,6 +861,8 @@ 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))
+			break;
 		graph_line_write_column(line, &graph->new_columns[i], '|');
 		graph_line_addch(line, ' ');
 	}
@@ -903,6 +920,8 @@ 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)) {
+			break;
 		} else if (seen_this && (graph->expansion_row == 0)) {
 			/*
 			 * This is the first line of the pre-commit output.
@@ -994,6 +1013,14 @@ static void graph_draw_octopus_merge(struct git_graph *graph, struct graph_line
 		col = &graph->new_columns[j];
 
 		graph_line_write_column(line, col, '-');
+
+		/*
+		 * 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))
+			break;
+
 		graph_line_write_column(line, col, (i == dashed_parents - 1) ? '.' : '-');
 	}
 
@@ -1028,8 +1055,16 @@ 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)) {
+				graph_line_addch(line, ' ');
+				break;
+			}
+
 			if (graph->num_parents > 2)
 				graph_draw_octopus_merge(graph, line);
+		} else if (graph_needs_truncation(i)) {
+			seen_this = 1;
+			break;
 		} else if (seen_this && (graph->edges_added > 1)) {
 			graph_line_write_column(line, col, '\\');
 		} else if (seen_this && (graph->edges_added == 1)) {
@@ -1065,13 +1100,46 @@ static void graph_output_commit_line(struct git_graph *graph, struct graph_line
 
 	/*
 	 * Update graph->state
+	 *
+	 * If the commit is a merge and the first parent is in a visible lane,
+	 * then the GRAPH_POST_MERGE is needed to draw the merge lane.
+	 *
+	 * If the commit is over the truncation limit, but the first parent is on
+	 * a visible lane, then we still need the merge lane but truncated.
+	 *
+	 * If both commit and first parent are over the truncation limit, then
+	 * there's no need to draw the merge lane because it would work as a
+	 * padding lane.
 	 */
-	if (graph->num_parents > 1)
-		graph_update_state(graph, GRAPH_POST_MERGE);
-	else if (graph_is_mapping_correct(graph))
+	if (graph->num_parents > 1) {
+		if (!graph_needs_truncation(graph->commit_index)) {
+			graph_update_state(graph, GRAPH_POST_MERGE);
+		} else {
+			struct commit_list *p = first_interesting_parent(graph);
+			int lane;
+
+			/*
+			 * graph->num_parents are found using first_interesting_parent
+			 * and next_interesting_parent so it can't be a scenario
+			 * where num_parents > 1 and there are no interesting parents
+			 */
+			if (!p)
+				BUG("num_parents > 1 but no interesting parent");
+
+			lane = graph_find_new_column_by_commit(graph, p->item);
+
+			if (!graph_needs_truncation(lane))
+				graph_update_state(graph, GRAPH_POST_MERGE);
+			else if (graph_is_mapping_correct(graph))
+				graph_update_state(graph, GRAPH_PADDING);
+			else
+				graph_update_state(graph, GRAPH_COLLAPSING);
+		}
+	} else if (graph_is_mapping_correct(graph)) {
 		graph_update_state(graph, GRAPH_PADDING);
-	else
+	} else {
 		graph_update_state(graph, GRAPH_COLLAPSING);
+	}
 }
 
 static const char merge_chars[] = {'/', '|', '\\'};
@@ -1109,6 +1177,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 			int par_column;
 			int idx = graph->merge_layout;
 			char c;
+			int truncated = 0;
 			seen_this = 1;
 
 			for (j = 0; j < graph->num_parents; j++) {
@@ -1117,23 +1186,53 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 
 				c = merge_chars[idx];
 				graph_line_write_column(line, &graph->new_columns[par_column], c);
+
+				/*
+				 * j counts parents, it needs to be halved to be
+				 * 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) &&
+				    j / 2 + i <= graph->num_columns) {
+					if ((j + i * 2) % 2 != 0)
+						graph_line_addch(line, ' ');
+					truncated = 1;
+					break;
+				}
+
 				if (idx == 2) {
-					if (graph->edges_added > 0 || j < graph->num_parents - 1)
+					/*
+					 * Check if the next lane needs truncation
+					 * to avoid having the padding doubled
+					 */
+					if (graph_needs_truncation((j + 1) / 2 + i) &&
+					    j < graph->num_parents - 1) {
+						truncated = 1;
+						break;
+					} else if (graph->edges_added > 0 || j < graph->num_parents - 1)
 						graph_line_addch(line, ' ');
 				} else {
 					idx++;
 				}
 				parents = next_interesting_parent(graph, parents);
 			}
+			if (truncated)
+				break;
 			if (graph->edges_added == 0)
 				graph_line_addch(line, ' ');
-
+		} else if (graph_needs_truncation(i)) {
+			break;
 		} else if (seen_this) {
 			if (graph->edges_added > 0)
 				graph_line_write_column(line, col, '\\');
 			else
 				graph_line_write_column(line, col, '|');
-			graph_line_addch(line, ' ');
+			/*
+			 * If it's between two lanes and next would be truncated,
+			 * don't add space padding.
+			 */
+			if (!graph_needs_truncation(i + 1))
+				graph_line_addch(line, ' ');
 		} else {
 			graph_line_write_column(line, col, '|');
 			if (graph->merge_layout != 0 || i != graph->commit_index - 1) {
@@ -1164,6 +1263,7 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct graph_l
 	short used_horizontal = 0;
 	int horizontal_edge = -1;
 	int horizontal_edge_target = -1;
+	int truncated = 0;
 
 	/*
 	 * Swap the mapping and old_mapping arrays
@@ -1279,26 +1379,34 @@ 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 (target < 0)
-			graph_line_addch(line, ' ');
-		else if (target * 2 == i)
-			graph_line_write_column(line, &graph->new_columns[target], '|');
-		else if (target == horizontal_edge_target &&
-			 i != horizontal_edge - 1) {
-				/*
-				 * Set the mappings for all but the
-				 * first segment to -1 so that they
-				 * won't continue into the next line.
-				 */
-				if (i != (target * 2)+3)
-					graph->mapping[i] = -1;
-				used_horizontal = 1;
-			graph_line_write_column(line, &graph->new_columns[target], '_');
+
+		if (!truncated && graph_needs_truncation(i / 2)) {
+			truncated = 1;
+		}
+
+		if (target < 0) {
+			if (!truncated)
+				graph_line_addch(line, ' ');
+		} else if (target * 2 == i) {
+			if (!truncated)
+				graph_line_write_column(line, &graph->new_columns[target], '|');
+		} else if (target == horizontal_edge_target &&
+			   i != horizontal_edge - 1) {
+			/*
+			 * Set the mappings for all but the
+			 * first segment to -1 so that they
+			 * won't continue into the next line.
+			 */
+			if (i != (target * 2)+3)
+				graph->mapping[i] = -1;
+			used_horizontal = 1;
+			if (!truncated)
+				graph_line_write_column(line, &graph->new_columns[target], '_');
 		} else {
 			if (used_horizontal && i < horizontal_edge)
 				graph->mapping[i] = -1;
-			graph_line_write_column(line, &graph->new_columns[target], '/');
-
+			if (!truncated)
+				graph_line_write_column(line, &graph->new_columns[target], '/');
 		}
 	}
 
@@ -1372,6 +1480,9 @@ 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))
+			break;
+
 		graph_line_write_column(&line, col, '|');
 
 		if (col->commit == graph->commit && graph->num_parents > 2) {