revwalk: use a priority queue for calculating merge bases

As parents are older than their children, we're appending to the
commit list most of the time, which makes an ordered linked list quite
inefficient.

While we're there, don't sort the results list in the main loop, as
we're sorting them afterwards and it creates extra work.

src/revwalk.c

@@ -268,18 +268,16 @@ static int commit_parse(git_revwalk *walk, commit_object *commit)
 	return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to parse commit");
 }
 
-static commit_object *interesting(commit_list *list)
+static int interesting(git_pqueue *list)
 {
-	while (list) {
-		commit_object *commit = list->item;
-		list = list->next;
-		if (commit->flags & STALE)
-			continue;
-
-		return commit;
+	unsigned int i;
+	for (i = 1; i < git_pqueue_size(list); i++) {
+		commit_object *commit = list->d[i];
+		if ((commit->flags & STALE) == 0)
+			return 1;
 	}
 
-	return NULL;
+	return 0;
 }
 
 static int merge_bases_many(commit_list **out, git_revwalk *walk, commit_object *one, git_vector *twos)
@@ -287,44 +285,45 @@ static int merge_bases_many(commit_list **out, git_revwalk *walk, commit_object
 	int error;
 	unsigned int i;
 	commit_object *two;
-	commit_list *list = NULL, *result = NULL;
+	commit_list *result = NULL, *tmp = NULL;
+	git_pqueue list;
 
 	/* if the commit is repeated, we have a our merge base already */
 	git_vector_foreach(twos, i, two) {
 		if (one == two)
-			return commit_list_insert(one, out) ? GIT_SUCCESS : GIT_ENOMEM;
+			return commit_list_insert(one, out) ? 0 : -1;
 	}
 
-	if ((error = commit_parse(walk, one)) < GIT_SUCCESS)
-	    return error;
+	if (git_pqueue_init(&list, twos->length * 2, commit_time_cmp) < 0)
+		return -1;
+
+	if (commit_parse(walk, one) < 0)
+	    return -1;
 
 	one->flags |= PARENT1;
-	if (commit_list_insert(one, &list) == NULL)
-		return GIT_ENOMEM;
+	if (git_pqueue_insert(&list, one) < 0)
+		return -1;
 
 	git_vector_foreach(twos, i, two) {
 		commit_parse(walk, two);
 		two->flags |= PARENT2;
-		if (commit_list_insert_by_date(two, &list) == NULL)
-			return GIT_ENOMEM;
+		if (git_pqueue_insert(&list, two) < 0)
+			return -1;
 	}
 
 	/* as long as there are non-STALE commits */
-	while (interesting(list)) {
-		commit_object *commit = list->item;
-		commit_list *next;
+	while (interesting(&list)) {
+		commit_object *commit;
 		int flags;
 
-		next = list->next;
-		git__free(list);
-		list = next;
+		commit = git_pqueue_pop(&list);
 
 		flags = commit->flags & (PARENT1 | PARENT2 | STALE);
 		if (flags == (PARENT1 | PARENT2)) {
 			if (!(commit->flags & RESULT)) {
 				commit->flags |= RESULT;
-				if (commit_list_insert_by_date(commit, &result) == NULL)
-					return GIT_ENOMEM;
+				if (commit_list_insert(commit, &result) == NULL)
+					return -1;
 			}
 			/* we mark the parents of a merge stale */
 			flags |= STALE;
@@ -339,29 +338,29 @@ static int merge_bases_many(commit_list **out, git_revwalk *walk, commit_object
 				return error;
 
 			p->flags |= flags;
-			if (commit_list_insert_by_date(p, &list) == NULL)
-				return GIT_ENOMEM;
+			if (git_pqueue_insert(&list, p) < 0)
+				return -1;
 		}
 	}
 
-	commit_list_free(&list);
+	git_pqueue_free(&list);
 
 	/* filter out any stale commits in the results */
-	list = result;
+	tmp = result;
 	result = NULL;
 
-	while (list) {
-		struct commit_list *next = list->next;
-		if (!(list->item->flags & STALE))
-			if (commit_list_insert_by_date(list->item, &result) == NULL)
-				return GIT_ENOMEM;
+	while (tmp) {
+		struct commit_list *next = tmp->next;
+		if (!(tmp->item->flags & STALE))
+			if (commit_list_insert_by_date(tmp->item, &result) == NULL)
+				return -1;
 
-		    free(list);
-		    list = next;
+		git__free(tmp);
+		tmp = next;
 	}
 
 	*out = result;
-	return GIT_SUCCESS;
+	return 0;
 }
 
 int git_merge_base(git_oid *out, git_repository *repo, git_oid *one, git_oid *two)