paint_down_to_common: use prio_queue

When we are traversing to find merge bases, we keep our
usual commit_list of commits to process, sorted by their
commit timestamp. As we add each parent to the list, we have
to spend "O(width of history)" to do the insertion, where
the width of history is the number of simultaneous lines of
development.

If we instead use a heap-based priority queue, we can do
these insertions in "O(log width)" time. This provides minor
speedups to merge-base calculations (timings in linux.git,
warm cache, best-of-five):

  [before]
  $ git merge-base HEAD v2.6.12
  real    0m3.251s
  user    0m3.148s
  sys     0m0.104s

  [after]
  $ git merge-base HEAD v2.6.12
  real    0m3.234s
  user    0m3.108s
  sys     0m0.128s

That's only an 0.5% speedup, but it does help protect us
against pathological cases.

While we are munging the "interesting" function, we also
take the opportunity to give it a more descriptive name, and
convert the return value to an int (we returned the first
interesting commit, but nobody ever looked at it).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -786,45 +786,41 @@ void sort_in_topological_order(struct commit_list **list, enum rev_sort_order so
 
 static const unsigned all_flags = (PARENT1 | PARENT2 | STALE | RESULT);
 
-static struct commit *interesting(struct commit_list *list)
+static int queue_has_nonstale(struct prio_queue *queue)
 {
-	while (list) {
-		struct commit *commit = list->item;
-		list = list->next;
-		if (commit->object.flags & STALE)
-			continue;
-		return commit;
+	int i;
+	for (i = 0; i < queue->nr; i++) {
+		struct commit *commit = queue->array[i].data;
+		if (!(commit->object.flags & STALE))
+			return 1;
 	}
-	return NULL;
+	return 0;
 }
 
 /* all input commits in one and twos[] must have been parsed! */
 static struct commit_list *paint_down_to_common(struct commit *one, int n, struct commit **twos)
 {
-	struct commit_list *list = NULL;
+	struct prio_queue queue = { compare_commits_by_commit_date };
 	struct commit_list *result = NULL;
 	int i;
 
 	one->object.flags |= PARENT1;
-	commit_list_insert_by_date(one, &list);
-	if (!n)
-		return list;
+	if (!n) {
+		commit_list_append(one, &result);
+		return result;
+	}
+	prio_queue_put(&queue, one);
+
 	for (i = 0; i < n; i++) {
 		twos[i]->object.flags |= PARENT2;
-		commit_list_insert_by_date(twos[i], &list);
+		prio_queue_put(&queue, twos[i]);
 	}
 
-	while (interesting(list)) {
-		struct commit *commit;
+	while (queue_has_nonstale(&queue)) {
+		struct commit *commit = prio_queue_get(&queue);
 		struct commit_list *parents;
-		struct commit_list *next;
 		int flags;
 
-		commit = list->item;
-		next = list->next;
-		free(list);
-		list = next;
-
 		flags = commit->object.flags & (PARENT1 | PARENT2 | STALE);
 		if (flags == (PARENT1 | PARENT2)) {
 			if (!(commit->object.flags & RESULT)) {
@@ -843,11 +839,11 @@ static struct commit_list *paint_down_to_common(struct commit *one, int n, struc
 			if (parse_commit(p))
 				return NULL;
 			p->object.flags |= flags;
-			commit_list_insert_by_date(p, &list);
+			prio_queue_put(&queue, p);
 		}
 	}
 
-	free_commit_list(list);
+	clear_prio_queue(&queue);
 	return result;
 }