log: fix coloring of certain octopus merge shapes

For octopus merges where the first parent edge immediately merges into
the next column to the left, the number of columns should be one less
than the usual case.

First parent to the left case:

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

The usual case:

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

Also refactor the code to iterate over columns rather than dashes,
building from an initial patch suggested by Jeff King.

Signed-off-by: Noam Postavsky <npostavs@users.sourceforge.net>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 43 insertions, 15 deletions

diff --git a/graph.c b/graph.c
index e7e2065..61e046e 100644
--- a/graph.c
+++ b/graph.c
@@ -846,27 +846,55 @@ static void graph_output_commit_char(struct git_graph *graph, struct strbuf *sb)
 }
 
 /*
- * Draw an octopus merge and return the number of characters written.
+ * Draw the horizontal dashes of an octopus merge and return the number of
+ * characters written.
  */
 static int graph_draw_octopus_merge(struct git_graph *graph,
 				    struct strbuf *sb)
 {
 	/*
-	 * Here dashless_commits represents the number of parents
-	 * which don't need to have dashes (because their edges fit
-	 * neatly under the commit).
-	 */
-	const int dashless_commits = 2;
-	int col_num, i;
-	int num_dashes =
-		((graph->num_parents - dashless_commits) * 2) - 1;
-	for (i = 0; i < num_dashes; i++) {
-		col_num = (i / 2) + dashless_commits + graph->commit_index;
-		strbuf_write_column(sb, &graph->new_columns[col_num], '-');
+	 * Here dashless_parents represents the number of parents which don't
+	 * need to have dashes (the edges labeled "0" and "1").  And
+	 * dashful_parents are the remaining ones.
+	 *
+	 * | *---.
+	 * | |\ \ \
+	 * | | | | |
+	 * x 0 1 2 3
+	 *
+	 */
+	const int dashless_parents = 2;
+	int dashful_parents = graph->num_parents - dashless_parents;
+
+	/*
+	 * Usually, we add one new column for each parent (like the diagram
+	 * above) but sometimes the first parent goes into an existing column,
+	 * like this:
+	 *
+	 * | *---.
+	 * | |\ \ \
+	 * |/ / / /
+	 * x 0 1 2
+	 *
+	 * In which case the number of parents will be one greater than the
+	 * number of added columns.
+	 */
+	int added_cols = (graph->num_new_columns - graph->num_columns);
+	int parent_in_old_cols = graph->num_parents - added_cols;
+
+	/*
+	 * In both cases, commit_index corresponds to the edge labeled "0".
+	 */
+	int first_col = graph->commit_index + dashless_parents
+	    - parent_in_old_cols;
+
+	int i;
+	for (i = 0; i < dashful_parents; i++) {
+		strbuf_write_column(sb, &graph->new_columns[i+first_col], '-');
+		strbuf_write_column(sb, &graph->new_columns[i+first_col],
+				    i == dashful_parents-1 ? '.' : '-');
 	}
-	col_num = (i / 2) + dashless_commits + graph->commit_index;
-	strbuf_write_column(sb, &graph->new_columns[col_num], '.');
-	return num_dashes + 1;
+	return 2 * dashful_parents;
 }
 
 static void graph_output_commit_line(struct git_graph *graph, struct strbuf *sb)