graph: flatten edges that fuse with their right neighbor

When a merge commit is printed and its final parent is the same commit
that occupies the column to the right of the merge, this results in a
kink in the displayed edges:

        * |
        |\ \
        | |/
        | *

Graphs containing these shapes can be hard to read, as the expansion to
the right followed immediately by collapsing back to the left creates a
lot of zig-zagging edges, especially when many columns are present.

We can improve this by eliminating the zig-zag and having the merge's
final parent edge fuse immediately with its neighbor:

        * |
        |\|
        | *

This reduces the horizontal width for the current commit by 2, and
requires one less row, making the graph display more compact. Taken in
combination with other graph-smoothing enhancements, it greatly
compresses the space needed to display certain histories:

        *
        |\
        | *                       *
        | |\                      |\
        | | *                     | *
        | | |                     | |\
        | |  \                    | | *
        | *-. \                   | * |
        | |\ \ \        =>        |/|\|
        |/ / / /                  | | *
        | | | /                   | * |
        | | |/                    | |/
        | | *                     * /
        | * |                     |/
        | |/                      *
        * |
        |/
        *

One of the test cases here cannot be correctly rendered in Git v2.23.0;
it produces this output following commit E:

        | | *-. \   5_E
        | | |\ \ \
        | |/ / / /
        | | | / _
        | |_|/
        |/| |

The new implementation makes sure that the rightmost edge in this
history is not left dangling as above.

Signed-off-by: James Coglan <jcoglan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -557,8 +557,24 @@ static void graph_insert_into_new_columns(struct git_graph *graph,
 		shift = (dist > 1) ? 2 * dist - 3 : 1;
 
 		graph->merge_layout = (dist > 0) ? 0 : 1;
+		graph->edges_added = graph->num_parents + graph->merge_layout  - 2;
+
 		mapping_idx = graph->width + (graph->merge_layout - 1) * shift;
 		graph->width += 2 * graph->merge_layout;
+
+	} else if (graph->edges_added > 0 && i == graph->mapping[graph->width - 2]) {
+		/*
+		 * If some columns have been added by a merge, but this commit
+		 * was found in the last existing column, then adjust the
+		 * numbers so that the two edges immediately join, i.e.:
+		 *
+		 *		* |		* |
+		 *		|\ \	=>	|\|
+		 *		| |/		| *
+		 *		| *
+		 */
+		mapping_idx = graph->width - 2;
+		graph->edges_added = -1;
 	} else {
 		mapping_idx = graph->width;
 		graph->width += 2;
@@ -604,6 +620,8 @@ static void graph_update_columns(struct git_graph *graph)
 		graph->mapping[i] = -1;
 
 	graph->width = 0;
+	graph->prev_edges_added = graph->edges_added;
+	graph->edges_added = 0;
 
 	/*
 	 * Populate graph->new_columns and graph->mapping
@@ -731,9 +749,6 @@ void graph_update(struct git_graph *graph, struct commit *commit)
 	 */
 	graph_update_columns(graph);
 
-	graph->prev_edges_added = graph->edges_added;
-	graph->edges_added = graph->num_parents + graph->merge_layout - 2;
-
 	graph->expansion_row = 0;
 
 	/*
@@ -1041,7 +1056,7 @@ const char merge_chars[] = {'/', '|', '\\'};
 static void graph_output_post_merge_line(struct git_graph *graph, struct graph_line *line)
 {
 	int seen_this = 0;
-	int i;
+	int i, j;
 
 	struct commit_list *first_parent = first_interesting_parent(graph);
 	int seen_parent = 0;
@@ -1073,16 +1088,19 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 			char c;
 			seen_this = 1;
 
-			for (; parents; parents = next_interesting_parent(graph, parents)) {
+			for (j = 0; j < graph->num_parents; j++) {
 				par_column = graph_find_new_column_by_commit(graph, parents->item);
 				assert(par_column >= 0);
 
 				c = merge_chars[idx];
 				graph_line_write_column(line, &graph->new_columns[par_column], c);
-				if (idx == 2)
+				if (idx == 2) {
-					graph_line_addch(line, ' ');
+					if (graph->edges_added > 0 || j < graph->num_parents - 1)
-				else
+						graph_line_addch(line, ' ');
+				} else {
 					idx++;
+				}
+				parents = next_interesting_parent(graph, parents);
 			}
 			if (graph->edges_added == 0)
 				graph_line_addch(line, ' ');

t/t4215-log-skewed-merges.sh

@@ -11,9 +11,8 @@ test_expect_success 'log --graph with merge fusing with its left and right neigh
 	| *   G
 	| |\
 	| | * F
-	| * |   E
+	| * | E
-	|/|\ \
+	|/|\|
-	| | |/
 	| | * D
 	| * | C
 	| |/
@@ -43,9 +42,9 @@ test_expect_success 'log --graph with left-skewed merge' '
 	| | | | * 0_G
 	| |_|_|/|
 	|/| | | |
-	| | | * |   0_F
+	| | | * | 0_F
-	| |_|/|\ \
+	| |_|/|\|
-	|/| | | |/
+	|/| | | |
 	| | | | * 0_E
 	| |_|_|/
 	|/| | |
@@ -153,9 +152,8 @@ test_expect_success 'log --graph with nested right-skewed merge following left-s
 	| | * 3_G
 	| * | 3_F
 	|/| |
-	| * |   3_E
+	| * | 3_E
-	| |\ \
+	| |\|
-	| | |/
 	| | * 3_D
 	| * | 3_C
 	| |/
@@ -216,4 +214,44 @@ test_expect_success 'log --graph with right-skewed merge following a left-skewed
 	test_cmp expect actual
 '
 
+test_expect_success 'log --graph with octopus merge with column joining its penultimate parent' '
+	cat >expect <<-\EOF &&
+	*   5_H
+	|\
+	| *-.   5_G
+	| |\ \
+	| | | * 5_F
+	| | * |   5_E
+	| |/|\ \
+	| |_|/ /
+	|/| | /
+	| | |/
+	* | | 5_D
+	| | * 5_C
+	| |/
+	|/|
+	| * 5_B
+	|/
+	* 5_A
+	EOF
+
+	git checkout --orphan 5_p &&
+	test_commit 5_A &&
+	git branch 5_q &&
+	git branch 5_r &&
+	test_commit 5_B &&
+	git checkout 5_q && test_commit 5_C &&
+	git checkout 5_r && test_commit 5_D &&
+	git checkout 5_p &&
+	git merge --no-ff 5_q 5_r -m 5_E &&
+	git checkout 5_q && test_commit 5_F &&
+	git checkout -b 5_s 5_p^ &&
+	git merge --no-ff 5_p 5_q -m 5_G &&
+	git checkout 5_r &&
+	git merge --no-ff 5_s -m 5_H &&
+
+	git log --graph --pretty=tformat:%s | sed "s/ *$//" >actual &&
+	test_cmp expect actual
+'
+
 test_done

t/t6016-rev-list-graph-simplify-history.sh

@@ -67,11 +67,10 @@ test_expect_success '--graph --all' '
 	echo "| * $C4" >> expected &&
 	echo "| * $C3" >> expected &&
 	echo "* | $A5" >> expected &&
-	echo "| |     " >> expected &&
+	echo "| |   " >> expected &&
-	echo "|  \\    " >> expected &&
+	echo "|  \\  " >> expected &&
-	echo "*-. \\   $A4" >> expected &&
+	echo "*-. | $A4" >> expected &&
-	echo "|\\ \\ \\  " >> expected &&
+	echo "|\\ \\| " >> expected &&
-	echo "| | |/  " >> expected &&
 	echo "| | * $C2" >> expected &&
 	echo "| | * $C1" >> expected &&
 	echo "| * | $B2" >> expected &&
@@ -97,11 +96,10 @@ test_expect_success '--graph --simplify-by-decoration' '
 	echo "| * $C4" >> expected &&
 	echo "| * $C3" >> expected &&
 	echo "* | $A5" >> expected &&
-	echo "| |     " >> expected &&
+	echo "| |   " >> expected &&
-	echo "|  \\    " >> expected &&
+	echo "|  \\  " >> expected &&
-	echo "*-. \\   $A4" >> expected &&
+	echo "*-. | $A4" >> expected &&
-	echo "|\\ \\ \\  " >> expected &&
+	echo "|\\ \\| " >> expected &&
-	echo "| | |/  " >> expected &&
 	echo "| | * $C2" >> expected &&
 	echo "| | * $C1" >> expected &&
 	echo "| * | $B2" >> expected &&
@@ -131,9 +129,8 @@ test_expect_success '--graph --simplify-by-decoration prune branch B' '
 	echo "| * $C4" >> expected &&
 	echo "| * $C3" >> expected &&
 	echo "* | $A5" >> expected &&
-	echo "* |   $A4" >> expected &&
+	echo "* | $A4" >> expected &&
-	echo "|\\ \\  " >> expected &&
+	echo "|\\| " >> expected &&
-	echo "| |/  " >> expected &&
 	echo "| * $C2" >> expected &&
 	echo "| * $C1" >> expected &&
 	echo "* | $A3" >> expected &&
@@ -151,10 +148,9 @@ test_expect_success '--graph --full-history -- bar.txt' '
 	echo "|\\  " >> expected &&
 	echo "| * $C4" >> expected &&
 	echo "* | $A5" >> expected &&
-	echo "* |   $A4" >> expected &&
+	echo "* | $A4" >> expected &&
-	echo "|\\ \\  " >> expected &&
+	echo "|\\| " >> expected &&
-	echo "| |/  " >> expected &&
+	echo "* | $A3" >> expected &&
-	echo "* / $A3" >> expected &&
 	echo "|/  " >> expected &&
 	echo "* $A2" >> expected &&
 	git rev-list --graph --full-history --all -- bar.txt > actual &&