graph: add a 2 commit buffer for lookahead

In a subsequent commit the graph renderer needs to know if the next
commit is a visual root or if it is the last commit to be shown. This
requires peeking 2 commits ahead.

Commits are pre-fetched in get_revision() through next_commit_to_show()
where they are also marked as SHOWN, regardless the source they come
from.

Update graph_is_interesting() so it considers commits inside the
lookahead buffer as interesting as well.

Helped-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -315,6 +315,14 @@ struct git_graph {
 	 * diff_output_prefix_callback().
 	 */
 	struct strbuf prefix_buf;
+
+	/*
+	 * Lookahead buffer: up to 2 pre-fetched commits that will be shown.
+	 * Populated by get_revision() so graph_peek_next_visible() can use
+	 * actual walk results instead of peeking at rev_info internals.
+	 */
+	struct commit *lookahead[2];
+	int lookahead_nr;
 };
 
 static inline int graph_needs_truncation(struct git_graph *graph, int lane)
@@ -388,6 +396,9 @@ struct git_graph *graph_init(struct rev_info *opt)
 	graph->num_columns = 0;
 	graph->num_new_columns = 0;
 	graph->mapping_size = 0;
+	graph->lookahead[0] = NULL;
+	graph->lookahead[1] = NULL;
+	graph->lookahead_nr = 0;
 	/*
 	 * Start the column color at the maximum value, since we'll
 	 * always increment it for the first commit we output.
@@ -456,6 +467,15 @@ static void graph_ensure_capacity(struct git_graph *graph, int num_columns)
  */
 static int graph_is_interesting(struct git_graph *graph, struct commit *commit)
 {
+	/*
+	 * Commits in the lookahead buffer have been pre-fetched by
+	 * get_revision() and will be shown in the future. They already have
+	 * the SHOWN flag set when they were pre-fetched but the graph still
+	 * needs to treat them as interesting parents.
+	 */
+	for (int i = 0; i < graph->lookahead_nr; i++)
+		if (graph->lookahead[i] == commit)
+			return 1;
 	/*
 	 * If revs->boundary is set, commits whose children have
 	 * been shown are always interesting, even if they have the
@@ -763,6 +783,37 @@ static int graph_needs_pre_commit_line(struct git_graph *graph)
 	       graph->expansion_row < graph_num_expansion_rows(graph);
 }
 
+struct commit *graph_pop_lookahead(struct git_graph *graph)
+{
+	struct commit *c;
+
+	if (!graph->lookahead_nr)
+		return NULL;
+
+	c = graph->lookahead[0];
+	if (!c)
+		BUG("lookahead buffer has %d entries but the first one is NULL",
+		    graph->lookahead_nr);
+
+	graph->lookahead[0] = graph->lookahead[1];
+	graph->lookahead[1] = NULL;
+	graph->lookahead_nr--;
+	return c;
+}
+
+int graph_get_lookahead_room(struct git_graph *graph)
+{
+	return (int)ARRAY_SIZE(graph->lookahead) - graph->lookahead_nr;
+}
+
+void graph_push_lookahead(struct git_graph *graph, struct commit *c)
+{
+	if (!graph_get_lookahead_room(graph))
+		BUG("pushing into lookahead buffer when it is already full");
+
+	graph->lookahead[graph->lookahead_nr++] = c;
+}
+
 void graph_update(struct git_graph *graph, struct commit *commit)
 {
 	struct commit_list *parent;

graph.h

@@ -262,4 +262,21 @@ void graph_show_commit_msg(struct git_graph *graph,
 			   FILE *file,
 			   struct strbuf const *sb);
 
+/*
+ * Pop the first commit from the graph's lookahead buffer.
+ * Returns NULL if the buffer is empty.
+ */
+struct commit *graph_pop_lookahead(struct git_graph *graph);
+
+/*
+ * Returns how many more commits can be added to the lookahead buffer.
+ */
+int graph_get_lookahead_room(struct git_graph *graph);
+
+/*
+ * Push a commit into the lookahead buffer. Must only be called when
+ * graph_get_lookahead_room() returns > 0.
+ */
+void graph_push_lookahead(struct git_graph *graph, struct commit *c);
+
 #endif /* GRAPH_H */

revision.c

@@ -4715,10 +4715,24 @@ struct commit *get_revision(struct rev_info *revs)
 		return c;
 	}
 
-	c = next_commit_to_show(revs);
+	if (revs->graph) {
+		c = graph_pop_lookahead(revs->graph);
+		if (!c)
+			c = next_commit_to_show(revs);
+	} else {
+		c = next_commit_to_show(revs);
+	}
 
-	if (c && revs->graph)
+	if (c && revs->graph) {
+		while (graph_get_lookahead_room(revs->graph)) {
+			struct commit *next = next_commit_to_show(revs);
+			if (!next)
+				break;
+			graph_push_lookahead(revs->graph, next);
+		}
 		graph_update(revs->graph, c);
+	}
+
 	if (!c) {
 		free_saved_parents(revs);
 		commit_list_free(revs->previous_parents);