docs: private static void shuffle(int[] a) {
          Random rnd = ThreadLocalRandom.current();
          for (int i = a.length - 1; i > 0; i--) {
              int index = rnd.nextInt(i + 1);
              int e = a[index];
              a[index] = a[i];
              a[i] = e;
          }
      }