graph: smooth appearance of collapsing edges on commit lines

When a graph contains edges that are in the process of collapsing to the
left, but those edges cross a commit line, the effect is that the edges
have a jagged appearance:

        *
        |\
        | *
        |  \
        *-. \
        |\ \ \
        | | * |
        | * | |
        | |/ /
        * | |
        |/ /
        * |
        |/
        *

We already takes steps to smooth edges like this when they're expanding;
when an edge appears to the right of a merge commit marker on a
GRAPH_COMMIT line immediately following a GRAPH_POST_MERGE line, we
render it as a `\`:

        * \
        |\ \
        | * \
        | |\ \

We can make a similar improvement to collapsing edges, making them
easier to follow and giving the overall graph a feeling of increased
symmetry:

        *
        |\
        | *
        |  \
        *-. \
        |\ \ \
        | | * |
        | * | |
        | |/ /
        * / /
        |/ /
        * /
        |/
        *

To do this, we introduce a new special case for edges on GRAPH_COMMIT
lines that immediately follow a GRAPH_COLLAPSING line. By retaining a
copy of the `mapping` array used to render the GRAPH_COLLAPSING line in
the `old_mapping` array, we can determine that an edge is collapsing
through the GRAPH_COMMIT line and should be smoothed.

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

graph.c

@@ -297,10 +297,10 @@ struct git_graph {
 	 */
 	int *mapping;
 	/*
-	 * A temporary array for computing the next mapping state
-	 * while we are outputting a mapping line.  This is stored as part
-	 * of the git_graph simply so we don't have to allocate a new
-	 * temporary array each time we have to output a collapsing line.
+	 * A copy of the contents of the mapping array from the last commit,
+	 * which we use to improve the display of columns that are tracking
+	 * from right to left through a commit line.  We also use this to
+	 * avoid allocating a fresh array when we compute the next mapping.
 	 */
 	int *old_mapping;
 	/*
@@ -1015,6 +1015,10 @@ static void graph_output_commit_line(struct git_graph *graph, struct graph_line
 				graph_line_write_column(line, col, '\\');
 			else
 				graph_line_write_column(line, col, '|');
+		} else if (graph->prev_state == GRAPH_COLLAPSING &&
+			   graph->old_mapping[2 * i + 1] == i &&
+			   graph->mapping[2 * i] < i) {
+			graph_line_write_column(line, col, '/');
 		} else {
 			graph_line_write_column(line, col, '|');
 		}
@@ -1211,6 +1215,11 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct graph_l
 		}
 	}
 
+	/*
+	 * Copy the current mapping array into old_mapping
+	 */
+	COPY_ARRAY(graph->old_mapping, graph->mapping, graph->mapping_size);
+
 	/*
 	 * The new mapping may be 1 smaller than the old mapping
 	 */

t/t3430-rebase-merges.sh

@@ -408,7 +408,7 @@ test_expect_success 'octopus merges' '
 	| | * three
 	| * | two
 	| |/
-	* | one
+	* / one
 	|/
 	o before-octopus
 	EOF

t/t4202-log.sh

@@ -667,7 +667,7 @@ cat > expect <<\EOF
 * | | fifth
 * | | fourth
 |/ /
-* | third
+* / third
 |/
 * second
 * initial

t/t4214-log-graph-octopus.sh

@@ -31,9 +31,9 @@ test_expect_success 'log --graph with tricky octopus merge, no color' '
 	| | | * 4
 	| | * | 3
 	| | |/
-	| * | 2
+	| * / 2
 	| |/
-	* | 1
+	* / 1
 	|/
 	* initial
 	EOF
@@ -51,9 +51,9 @@ test_expect_success 'log --graph with tricky octopus merge with colors' '
 	<RED>|<RESET> <YELLOW>|<RESET> <BLUE>|<RESET> * 4
 	<RED>|<RESET> <YELLOW>|<RESET> * <MAGENTA>|<RESET> 3
 	<RED>|<RESET> <YELLOW>|<RESET> <MAGENTA>|<RESET><MAGENTA>/<RESET>
-	<RED>|<RESET> * <MAGENTA>|<RESET> 2
+	<RED>|<RESET> * <MAGENTA>/<RESET> 2
 	<RED>|<RESET> <MAGENTA>|<RESET><MAGENTA>/<RESET>
-	* <MAGENTA>|<RESET> 1
+	* <MAGENTA>/<RESET> 1
 	<MAGENTA>|<RESET><MAGENTA>/<RESET>
 	* initial
 	EOF
@@ -72,9 +72,9 @@ test_expect_success 'log --graph with normal octopus merge, no color' '
 	| | | * 4
 	| | * | 3
 	| | |/
-	| * | 2
+	| * / 2
 	| |/
-	* | 1
+	* / 1
 	|/
 	* initial
 	EOF
@@ -90,9 +90,9 @@ test_expect_success 'log --graph with normal octopus merge with colors' '
 	<RED>|<RESET> <GREEN>|<RESET> <YELLOW>|<RESET> * 4
 	<RED>|<RESET> <GREEN>|<RESET> * <BLUE>|<RESET> 3
 	<RED>|<RESET> <GREEN>|<RESET> <BLUE>|<RESET><BLUE>/<RESET>
-	<RED>|<RESET> * <BLUE>|<RESET> 2
+	<RED>|<RESET> * <BLUE>/<RESET> 2
 	<RED>|<RESET> <BLUE>|<RESET><BLUE>/<RESET>
-	* <BLUE>|<RESET> 1
+	* <BLUE>/<RESET> 1
 	<BLUE>|<RESET><BLUE>/<RESET>
 	* initial
 	EOF
@@ -110,9 +110,9 @@ test_expect_success 'log --graph with normal octopus merge and child, no color'
 	| | | * 4
 	| | * | 3
 	| | |/
-	| * | 2
+	| * / 2
 	| |/
-	* | 1
+	* / 1
 	|/
 	* initial
 	EOF
@@ -129,9 +129,9 @@ test_expect_failure 'log --graph with normal octopus and child merge with colors
 	<GREEN>|<RESET> <YELLOW>|<RESET> <BLUE>|<RESET> * 4
 	<GREEN>|<RESET> <YELLOW>|<RESET> * <MAGENTA>|<RESET> 3
 	<GREEN>|<RESET> <YELLOW>|<RESET> <MAGENTA>|<RESET><MAGENTA>/<RESET>
-	<GREEN>|<RESET> * <MAGENTA>|<RESET> 2
+	<GREEN>|<RESET> * <MAGENTA>/<RESET> 2
 	<GREEN>|<RESET> <MAGENTA>|<RESET><MAGENTA>/<RESET>
-	* <MAGENTA>|<RESET> 1
+	* <MAGENTA>/<RESET> 1
 	<MAGENTA>|<RESET><MAGENTA>/<RESET>
 	* initial
 	EOF
@@ -150,9 +150,9 @@ test_expect_success 'log --graph with tricky octopus merge and its child, no col
 	| | | * 4
 	| | * | 3
 	| | |/
-	| * | 2
+	| * / 2
 	| |/
-	* | 1
+	* / 1
 	|/
 	* initial
 	EOF
@@ -171,9 +171,9 @@ test_expect_failure 'log --graph with tricky octopus merge and its child with co
 	<RED>|<RESET> <BLUE>|<RESET> <MAGENTA>|<RESET> * 4
 	<RED>|<RESET> <BLUE>|<RESET> * <CYAN>|<RESET> 3
 	<RED>|<RESET> <BLUE>|<RESET> <CYAN>|<RESET><CYAN>/<RESET>
-	<RED>|<RESET> * <CYAN>|<RESET> 2
+	<RED>|<RESET> * <CYAN>/<RESET> 2
 	<RED>|<RESET> <CYAN>|<RESET><CYAN>/<RESET>
-	* <CYAN>|<RESET> 1
+	* <CYAN>/<RESET> 1
 	<CYAN>|<RESET><CYAN>/<RESET>
 	* initial
 	EOF

t/t4215-log-skewed-merges.sh

@@ -17,7 +17,7 @@ test_expect_success 'log --graph with merge fusing with its left and right neigh
 	| | * D
 	| * | C
 	| |/
-	* | B
+	* / B
 	|/
 	* A
 	EOF
@@ -85,7 +85,7 @@ test_expect_success 'log --graph with nested left-skewed merge' '
 	| * | 1_D
 	* | | 1_C
 	|/ /
-	* | 1_B
+	* / 1_B
 	|/
 	* 1_A
 	EOF

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

@@ -154,7 +154,7 @@ test_expect_success '--graph --full-history -- bar.txt' '
 	echo "* |   $A4" >> 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 &&
@@ -255,7 +255,7 @@ test_expect_success '--graph --boundary ^C3' '
 	echo "* | | | $A3" >> expected &&
 	echo "o | | | $A2" >> expected &&
 	echo "|/ / /  " >> expected &&
-	echo "o | | $A1" >> expected &&
+	echo "o / / $A1" >> expected &&
 	echo " / /  " >> expected &&
 	echo "| o $C3" >> expected &&
 	echo "|/  " >> expected &&