code: public static void sortIndexAsc(int[] index, long[] values, int top) {
          assert top <= index.length;
          quicksortIndex(index, values, 0, top - 1, true);
      }
docs: Performs an in-place quick sort on {@code index} on the positions up to but not
  including {@code top}. All the sorting operations on {@code index} are mirrored in {@code values}.
  Sorts in ascending order.
  @return {@code index} - sorted.