code: public void addNum(double d) {
      if( isUUID() ) { addNA(); return; }
      if(_id == null || d != 0) {
        if(_ls != null)switch_to_doubles();
        if( _ds == null || _sparseLen >= _ds.length ) {
          append2slowd();
          // call addNum again since append2slow might have flipped to sparse
          addNum(d);
          assert _sparseLen <= _len;
          return;
        }
        if(_id != null)_id[_sparseLen] = _len;
        _ds[_sparseLen++] = d;
      }
      _len++;
      assert _sparseLen <= _len;
    }
docs: Fast-path append double data