+static int get_parent(char *name, unsigned char *result, int idx)
+{
+ unsigned char sha1[20];
+ int ret = get_extended_sha1(name, sha1);
+ struct commit *commit;
+ struct commit_list *p;
+
+ if (ret)
+ return ret;
+ commit = lookup_commit_reference(sha1);
+ if (!commit)
+ return -1;
+ if (parse_commit(commit))
+ return -1;
+ p = commit->parents;
+ while (p) {
+ if (!--idx) {
+ memcpy(result, p->item->object.sha1, 20);
+ return 0;
+ }
+ p = p->next;
+ }
+ return -1;
+}
+
+/*
+ * This is like "get_sha1()", except it allows "sha1 expressions",
+ * notably "xyz^" for "parent of xyz"
+ */
+static int get_extended_sha1(char *name, unsigned char *sha1)
+{
+ int parent;
+ int len = strlen(name);
+
+ parent = 1;
+ if (len > 2 && name[len-1] >= '1' && name[len-1] <= '9') {
+ parent = name[len-1] - '0';
+ len--;
+ }
+ if (len > 1 && name[len-1] == '^') {
+ int ret;
+ name[len-1] = 0;
+ ret = get_parent(name, sha1, parent);
+ name[len-1] = '^';
+ if (!ret)
+ return 0;
+ }
+ return get_sha1(name, sha1);
+}
+