+static int get_nth_ancestor(const char *name, int len,
+ unsigned char *result, int generation)
+{
+ unsigned char sha1[20];
+ int ret = get_sha1_1(name, len, sha1);
+ if (ret)
+ return ret;
+
+ while (generation--) {
+ struct commit *commit = lookup_commit_reference(sha1);
+
+ if (!commit || parse_commit(commit) || !commit->parents)
+ return -1;
+ memcpy(sha1, commit->parents->item->object.sha1, 20);
+ }
+ memcpy(result, sha1, 20);
+ return 0;
+}
+
+static int peel_onion(const char *name, int len, unsigned char *sha1)
+{
+ unsigned char outer[20];
+ const char *sp;
+ const char *type_string = NULL;
+ struct object *o;
+
+ /*
+ * "ref^{type}" dereferences ref repeatedly until you cannot
+ * dereference anymore, or you get an object of given type,
+ * whichever comes first. "ref^{}" means just dereference
+ * tags until you get a non-tag. "ref^0" is a shorthand for
+ * "ref^{commit}". "commit^{tree}" could be used to find the
+ * top-level tree of the given commit.
+ */
+ if (len < 4 || name[len-1] != '}')
+ return -1;
+
+ for (sp = name + len - 1; name <= sp; sp--) {
+ int ch = *sp;
+ if (ch == '{' && name < sp && sp[-1] == '^')
+ break;
+ }
+ if (sp <= name)
+ return -1;
+
+ sp++; /* beginning of type name, or closing brace for empty */
+ if (!strncmp(commit_type, sp, 6) && sp[6] == '}')
+ type_string = commit_type;
+ else if (!strncmp(tree_type, sp, 4) && sp[4] == '}')
+ type_string = tree_type;
+ else if (!strncmp(blob_type, sp, 4) && sp[4] == '}')
+ type_string = blob_type;
+ else if (sp[0] == '}')
+ type_string = NULL;
+ else
+ return -1;
+
+ if (get_sha1_1(name, sp - name - 2, outer))
+ return -1;
+
+ o = parse_object(outer);
+ if (!o)
+ return -1;
+ if (!type_string) {
+ o = deref_tag(o, name, sp - name - 2);
+ if (!o || (!o->parsed && !parse_object(o->sha1)))
+ return -1;
+ memcpy(sha1, o->sha1, 20);
+ }
+ else {
+ /* At this point, the syntax look correct, so
+ * if we do not get the needed object, we should
+ * barf.
+ */
+
+ while (1) {
+ if (!o || (!o->parsed && !parse_object(o->sha1)))
+ return -1;
+ if (o->type == type_string) {
+ memcpy(sha1, o->sha1, 20);
+ return 0;
+ }
+ if (o->type == tag_type)
+ o = ((struct tag*) o)->tagged;
+ else if (o->type == commit_type)
+ o = &(((struct commit *) o)->tree->object);
+ else
+ return error("%.*s: expected %s type, but the object dereferences to %s type",
+ len, name, type_string,
+ o->type);
+ if (!o->parsed)
+ parse_object(o->sha1);
+ }
+ }
+ return 0;
+}
+