+/** @param {!string} fen
+ * @return {!number}
+ *
+ * Return the half-move clock, starting from 0 (and never resetting).
+ */
+function find_halfmove_num(fen) {
+ let move_num = find_move_num(fen);
+ let toplay = find_toplay(fen);
+ return (move_num - 1) * 2 + (toplay === 'w' ? 0 : 1);
+}
+