code: private void _advance(int count) {
          if (buf != null) {
              bufPos += count;
              if (bufPos == buf.length) {
                  buf = null;
              }
          } else {
              pos.setIndex(pos.getIndex() + count);
              if (pos.getIndex() > text.length()) {
                  pos.setIndex(text.length());
              }
          }
      }
docs: Advances the position by the given amount.
  @param count the number of 16-bit code units to advance past