+/*
+ * This reads short-hand syntax that not only evaluates to a commit
+ * object name, but also can act as if the end user spelled the name
+ * of the branch from the command line.
+ *
+ * - "@{-N}" finds the name of the Nth previous branch we were on, and
+ * places the name of the branch in the given buf and returns the
+ * number of characters parsed if successful.
+ *
+ * - "<branch>@{upstream}" finds the name of the other ref that
+ * <branch> is configured to merge with (missing <branch> defaults
+ * to the current branch), and places the name of the branch in the
+ * given buf and returns the number of characters parsed if
+ * successful.
+ *
+ * If the input is not of the accepted format, it returns a negative
+ * number to signal an error.
+ *
+ * If the input was ok but there are not N branch switches in the
+ * reflog, it returns 0.
+ */
+int interpret_branch_name(const char *name, struct strbuf *buf)
+{
+ char *cp;
+ struct branch *upstream;
+ int namelen = strlen(name);
+ int len = interpret_nth_prior_checkout(name, buf);
+ int tmp_len;
+
+ if (!len)
+ return len; /* syntax Ok, not enough switches */
+ if (0 < len && len == namelen)
+ return len; /* consumed all */
+ else if (0 < len) {
+ /* we have extra data, which might need further processing */
+ struct strbuf tmp = STRBUF_INIT;
+ int used = buf->len;
+ int ret;
+
+ strbuf_add(buf, name + len, namelen - len);
+ ret = interpret_branch_name(buf->buf, &tmp);
+ /* that data was not interpreted, remove our cruft */
+ if (ret < 0) {
+ strbuf_setlen(buf, used);
+ return len;
+ }
+ strbuf_reset(buf);
+ strbuf_addbuf(buf, &tmp);
+ strbuf_release(&tmp);
+ /* tweak for size of {-N} versus expanded ref name */
+ return ret - used + len;
+ }
+
+ cp = strchr(name, '@');
+ if (!cp)
+ return -1;
+ tmp_len = upstream_mark(cp, namelen - (cp - name));
+ if (!tmp_len)
+ return -1;
+ len = cp + tmp_len - name;
+ cp = xstrndup(name, cp - name);
+ upstream = branch_get(*cp ? cp : NULL);
+ if (!upstream
+ || !upstream->merge
+ || !upstream->merge[0]->dst)
+ return error("No upstream branch found for '%s'", cp);
+ free(cp);
+ cp = shorten_unambiguous_ref(upstream->merge[0]->dst, 0);
+ strbuf_reset(buf);
+ strbuf_addstr(buf, cp);
+ free(cp);
+ return len;
+}
+