graph: commit and post-merge lines for left-skewed merges

Following the introduction of "left-skewed" merges, which are merges
whose first parent fuses with another edge to its left, we have some
more edge cases to deal with in the display of commit and post-merge
lines.

The current graph code handles the following cases for edges appearing
to the right of the commit (*) on commit lines. A 2-way merge is usually
followed by vertical lines:

        | | |
        | * |
        | |\ \

An octopus merge (more than two parents) is always followed by edges
sloping to the right:

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

A 2-way merge is followed by a right-sloping edge if the commit line
immediately follows a post-merge line for a commit that appears in the
same column as the current commit, or any column to the left of that:

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

This commit introduces the following new cases for commit lines. If a
2-way merge skews to the left, then the edges to its right are always
vertical lines, even if the commit follows a post-merge line:

        | | |           | |\
        | * |           | * |
        |/| |           |/| |

A commit with 3 parents that skews left is followed by vertical edges:

        | | |
        | * |
        |/|\ \

If a 3-way left-skewed merge commit appears immediately after a
post-merge line, then it may be followed the right-sloping edges, just
like a 2-way merge that is not skewed.

        | |\
        | * \
        |/|\ \

Octopus merges with 4 or more parents that skew to the left will always
be followed by right-sloping edges, because the existing columns need to
expand around the merge.

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

On post-merge lines, usually all edges following the current commit
slope to the right:

        | * | |
        | |\ \ \

However, if the commit is a left-skewed 2-way merge, the edges to its
right remain vertical. We also need to display a space after the
vertical line descending from the commit marker, whereas this line would
normally be followed by a backslash.

        | * | |
        |/| | |

If a left-skewed merge has more than 2 parents, then the edges to its
right are still sloped as they bend around the edges introduced by the
merge.

        | * | |
        |/|\ \ \

To handle these new cases, we need to know not just how many parents
each commit has, but how many new columns it adds to the display; this
quantity is recorded in the `edges_added` field for the current commit,
and `prev_edges_added` field for the previous commit.

Here, "column" refers to visual columns, not the logical columns of the
`columns` array. This is because even if all the commit's parents end up
fusing with existing edges, they initially introduce distinct edges in
the commit and post-merge lines before those edges collapse. For
example, a 3-way merge whose 2nd and 3rd parents fuse with existing
edges still introduces 2 visual columns that affect the display of edges
to their right.

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

This merge does not introduce any *logical* columns; there are 4 edges
before and after this commit once all edges have collapsed. But it does
initially introduce 2 new edges that need to be accommodated by the
edges to their right.

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

1 files changed, 58 insertions, 5 deletions

diff --git a/graph.c b/graph.c
index e37127f..21edad8 100644
--- a/graph.c
+++ b/graph.c
@@ -217,6 +217,46 @@ struct git_graph {
 	 */
 	int merge_layout;
 	/*
+	 * The number of columns added to the graph by the current commit. For
+	 * 2-way and octopus merges, this is is usually one less than the
+	 * number of parents:
+	 *
+	 * 		| | |			| |    \
+	 *		| * |			| *---. \
+	 *		| |\ \			| |\ \ \ \
+	 *		| | | |         	| | | | | |
+	 *
+	 *		num_parents: 2		num_parents: 4
+	 *		edges_added: 1		edges_added: 3
+	 *
+	 * For left-skewed merges, the first parent fuses with its neighbor and
+	 * so one less column is added:
+	 *
+	 *		| | |			| |  \
+	 *		| * |			| *-. \
+	 *		|/| |			|/|\ \ \
+	 *		| | |			| | | | |
+	 *
+	 *		num_parents: 2		num_parents: 4
+	 *		edges_added: 0		edges_added: 2
+	 *
+	 * This number determines how edges to the right of the merge are
+	 * displayed in commit and post-merge lines; if no columns have been
+	 * added then a vertical line should be used where a right-tracking
+	 * line would otherwise be used.
+	 *
+	 *		| * \			| * |
+	 *		| |\ \			|/| |
+	 *		| | * \			| * |
+	 */
+	int edges_added;
+	/*
+	 * The number of columns added by the previous commit, which is used to
+	 * smooth edges appearing to the right of a commit in a commit line
+	 * following a post-merge line.
+	 */
+	int prev_edges_added;
+	/*
 	 * The maximum number of columns that can be stored in the columns
 	 * and new_columns arrays.  This is also half the number of entries
 	 * that can be stored in the mapping and new_mapping arrays.
@@ -328,6 +368,8 @@ struct git_graph *graph_init(struct rev_info *opt)
 	graph->commit_index = 0;
 	graph->prev_commit_index = 0;
 	graph->merge_layout = 0;
+	graph->edges_added = 0;
+	graph->prev_edges_added = 0;
 	graph->num_columns = 0;
 	graph->num_new_columns = 0;
 	graph->mapping_size = 0;
@@ -689,6 +731,9 @@ 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;
 
 	/*
@@ -947,12 +992,13 @@ static void graph_output_commit_line(struct git_graph *graph, struct graph_line
 
 			if (graph->num_parents > 2)
 				graph_draw_octopus_merge(graph, line);
-		} else if (seen_this && (graph->num_parents > 2)) {
+		} else if (seen_this && (graph->edges_added > 1)) {
 			graph_line_write_column(line, col, '\\');
-		} else if (seen_this && (graph->num_parents == 2)) {
+		} else if (seen_this && (graph->edges_added == 1)) {
 			/*
-			 * This is a 2-way merge commit.
-			 * There is no GRAPH_PRE_COMMIT stage for 2-way
+			 * This is either a right-skewed 2-way merge
+			 * commit, or a left-skewed 3-way merge.
+			 * There is no GRAPH_PRE_COMMIT stage for such
 			 * merges, so this is the first line of output
 			 * for this commit.  Check to see what the previous
 			 * line of output was.
@@ -964,6 +1010,7 @@ static void graph_output_commit_line(struct git_graph *graph, struct graph_line
 			 * makes the output look nicer.
 			 */
 			if (graph->prev_state == GRAPH_POST_MERGE &&
+			    graph->prev_edges_added > 0 &&
 			    graph->prev_commit_index < i)
 				graph_line_write_column(line, col, '\\');
 			else
@@ -1033,8 +1080,14 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 				else
 					idx++;
 			}
+			if (graph->edges_added == 0)
+				graph_line_addch(line, ' ');
+
 		} else if (seen_this) {
-			graph_line_write_column(line, col, '\\');
+			if (graph->edges_added > 0)
+				graph_line_write_column(line, col, '\\');
+			else
+				graph_line_write_column(line, col, '|');
 			graph_line_addch(line, ' ');
 		} else {
 			graph_line_write_column(line, col, '|');