Add initial implementation of ahead-behind count

include/git2/merge.h

@@ -50,6 +50,17 @@ GIT_EXTERN(int) git_merge_base_many(
 	const git_oid input_array[],
 	size_t length);
 
+/**
+ * Count the number of unique commits between two commit objects
+ *
+ * @param ahead number of commits, starting at `one`, unique from commits in `two` 
+ * @param behind number of commits, starting at `two`, unique from commits in `one`
+ * @param repo the repository where the commits exist
+ * @param one one of the commits
+ * @param two the other commit
+ */
+GIT_EXTERN(int) git_count_ahead_behind(int *ahead, int *behind, git_repository *repo, git_oid *one, git_oid *two);
+
 /** @} */
 GIT_END_DECL
 #endif

src/merge.c

@@ -242,3 +242,84 @@ int git_merge__bases_many(git_commit_list **out, git_revwalk *walk, git_commit_l
 	return 0;
 }
 
+static int count_ahead_behind(git_commit_list_node *one, git_commit_list_node *two,
+	int *ahead, int *behind)
+{
+	git_commit_list_node *commit;
+	git_pqueue pq;
+	int i;
+	*ahead = 0;
+	*behind = 0;
+
+	if (git_pqueue_init(&pq, 2, git_commit_list_time_cmp) < 0)
+		return -1;
+	if (git_pqueue_insert(&pq, one) < 0)
+		return -1;
+	if (git_pqueue_insert(&pq, two) < 0)
+		return -1;
+
+	while((commit = git_pqueue_pop(&pq)) != NULL) {
+		if (commit->flags & RESULT ||
+			(commit->flags & (PARENT1 | PARENT2)) == (PARENT1 | PARENT2))
+			continue;
+		else if (commit->flags & PARENT1)
+			(*behind)++;
+		else if (commit->flags & PARENT2)
+			(*ahead)++;
+
+		for (i = 0; i < commit->out_degree; i++) {
+			git_commit_list_node *p = commit->parents[i];
+			if (git_pqueue_insert(&pq, p) < 0)
+				return -1;
+		}
+		commit->flags |= RESULT;
+	}
+
+	return 0;
+}
+
+int git_count_ahead_behind(int *ahead, int *behind, git_repository *repo, git_oid *one,
+	git_oid *two)
+{
+	git_revwalk *walk;
+	git_vector list;
+	struct git_commit_list *result = NULL;
+	git_commit_list_node *commit1, *commit2;
+	void *contents[1];
+
+	if (git_revwalk_new(&walk, repo) < 0)
+		return -1;
+
+	commit2 = commit_lookup(walk, two);
+	if (commit2 == NULL)
+		goto on_error;
+
+	/* This is just one value, so we can do it on the stack */
+	memset(&list, 0x0, sizeof(git_vector));
+	contents[0] = commit2;
+	list.length = 1;
+	list.contents = contents;
+
+	commit1 = commit_lookup(walk, one);
+	if (commit1 == NULL)
+		goto on_error;
+
+	if (git_merge__bases_many(&result, walk, commit1, &list) < 0)
+		goto on_error;
+	if (count_ahead_behind(commit1, commit2, ahead, behind) < 0)
+		goto on_error;
+
+	if (!result) {
+		git_revwalk_free(walk);
+		return GIT_ENOTFOUND;
+	}
+
+	git_commit_list_free(&result);
+	git_revwalk_free(walk);
+
+	return 0;
+
+on_error:
+	git_revwalk_free(walk);
+	return -1;
+}