graph API: Use horizontal lines for more compact graphs

Use horizontal lines instead of long diagonal lines during the
collapsing state of graph rendering.  For example what used to be:

    | | | | |
    | | | |/
    | | |/|
    | |/| |
    |/| | |
    | | | |

is now

    | | | | |
    | |_|_|/
    |/| | |
    | | | |

This results in more compact and legible graphs.

Signed-off-by: Allan Caffee <allan.caffee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -47,20 +47,6 @@ static void graph_show_strbuf(struct git_graph *graph, struct strbuf const *sb);
  * - Limit the number of columns, similar to the way gitk does.
  *   If we reach more than a specified number of columns, omit
  *   sections of some columns.
- *
- * - The output during the GRAPH_PRE_COMMIT and GRAPH_COLLAPSING states
- *   could be made more compact by printing horizontal lines, instead of
- *   long diagonal lines.  For example, during collapsing, something like
- *   this:          instead of this:
- *   | | | | |      | | | | |
- *   | |_|_|/       | | | |/
- *   |/| | |        | | |/|
- *   | | | |        | |/| |
- *                  |/| | |
- *                  | | | |
- *
- *   If there are several parallel diagonal lines, they will need to be
- *   replaced with horizontal lines on subsequent rows.
  */
 
 struct column {
@@ -982,6 +968,9 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
 {
 	int i;
 	int *tmp_mapping;
+	short used_horizontal = 0;
+	int horizontal_edge = -1;
+	int horizontal_edge_target = -1;
 
 	/*
 	 * Clear out the new_mapping array
@@ -1019,6 +1008,23 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
 			 * Move to the left by one
 			 */
 			graph->new_mapping[i - 1] = target;
+			/*
+			 * If there isn't already an edge moving horizontally
+			 * select this one.
+			 */
+			if (horizontal_edge == -1) {
+				int j;
+				horizontal_edge = i;
+				horizontal_edge_target = target;
+				/*
+				 * The variable target is the index of the graph
+				 * column, and therefore target*2+3 is the
+				 * actual screen column of the first horizontal
+				 * line.
+				 */
+				for (j = (target * 2)+3; j < (i - 2); j += 2)
+					graph->new_mapping[j] = target;
+			}
 		} else if (graph->new_mapping[i - 1] == target) {
 			/*
 			 * There is a branch line to our left
@@ -1039,10 +1045,21 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
 			 *
 			 * The space just to the left of this
 			 * branch should always be empty.
+			 *
+			 * The branch to the left of that space
+			 * should be our eventual target.
 			 */
 			assert(graph->new_mapping[i - 1] > target);
 			assert(graph->new_mapping[i - 2] < 0);
+			assert(graph->new_mapping[i - 3] == target);
 			graph->new_mapping[i - 2] = target;
+			/*
+			 * Mark this branch as the horizontal edge to
+			 * prevent any other edges from moving
+			 * horizontally.
+			 */
+			if (horizontal_edge == -1)
+				horizontal_edge = i;
 		}
 	}
 
@@ -1061,8 +1078,23 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct strbuf
 			strbuf_addch(sb, ' ');
 		else if (target * 2 == i)
 			strbuf_write_column(sb, &graph->new_columns[target], '|');
-		else
+		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->new_mapping[i] = -1;
+				used_horizontal = 1;
+			strbuf_write_column(sb, &graph->new_columns[target], '_');
+		} else {
+			if (used_horizontal && i < horizontal_edge)
+				graph->new_mapping[i] = -1;
 			strbuf_write_column(sb, &graph->new_columns[target], '/');
+
+		}
 	}
 
 	graph_pad_horizontally(graph, sb, graph->mapping_size);

t/t4202-log.sh

@@ -298,14 +298,12 @@ cat > expect <<\EOF
 * | | |   Merge branch 'side'
 |\ \ \ \
 | * | | | side-2
-| | | |/
-| | |/|
+| | |_|/
 | |/| |
 | * | | side-1
 * | | | Second
 * | | | sixth
-| | |/
-| |/|
+| |_|/
 |/| |
 * | | fifth
 * | | fourth