object: introduce git_object_peel()

Partially fix #530

1 files changed, 69 insertions, 0 deletions

diff --git a/src/object.c b/src/object.c
index 14d64befe..3ff894212 100644
--- a/src/object.c
+++ b/src/object.c
@@ -333,3 +333,72 @@ int git_object__resolve_to_type(git_object **obj, git_otype type)
 	*obj = scan;
 	return error;
 }
+
+static int dereference_object(git_object **dereferenced, git_object *obj)
+{
+	git_otype type = git_object_type(obj);
+
+	switch (type) {
+	case GIT_OBJ_COMMIT:
+		return git_commit_tree((git_tree **)dereferenced, (git_commit*)obj);
+		break;
+
+	case GIT_OBJ_TAG:
+		return git_tag_target(dereferenced, (git_tag*)obj);
+		break;
+
+	default:
+		return GIT_ENOTFOUND;
+		break;
+	}
+}
+
+static int peel_error(int error, const char* msg)
+{
+	giterr_set(GITERR_INVALID, "The given object cannot be peeled - %s", msg);
+	return error;
+}
+
+int git_object_peel(
+		git_object **peeled,
+		git_object *object,
+		git_otype target_type)
+{
+	git_object *source, *deref = NULL;
+
+	assert(object);
+
+	if (git_object_type(object) == target_type)
+		return git_object__dup(peeled, object);
+
+	if (target_type == GIT_OBJ_BLOB
+		|| target_type == GIT_OBJ_ANY)
+		return peel_error(GIT_EAMBIGUOUS, "Ambiguous target type");
+
+	if (git_object_type(object) == GIT_OBJ_BLOB)
+		return peel_error(GIT_ERROR, "A blob cannot be dereferenced");
+
+	source = object;
+
+	while (true) {
+		if (dereference_object(&deref, source) < 0)
+			goto cleanup;
+
+		if (source != object)
+			git_object_free(source);
+
+		if (git_object_type(deref) == target_type) {
+			*peeled = deref;
+			return 0;
+		}
+
+		source = deref;
+		deref = NULL;
+	}
+
+cleanup:
+	if (source != object)
+		git_object_free(source);
+	git_object_free(deref);
+	return -1;
+}