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>

1 files changed, 13 insertions, 4 deletions

diff --git a/graph.c b/graph.c
index 2315f36..63f8d18 100644
--- a/graph.c
+++ b/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, '|');
 		}
@@ -1212,6 +1216,11 @@ 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
 	 */
 	if (graph->mapping[graph->mapping_size - 1] < 0)