code: private Chunk chunkUUID() {
      final byte [] bs = MemoryManager.malloc1(_len*16,true);
      int j = 0;
      for( int i = 0; i < _len; ++i ) {
        long lo = 0, hi=0;
        if(_id == null || _id.length == 0 || (j < _id.length && _id[j] == i ) ) {
          lo = _ls[j];
          hi = Double.doubleToRawLongBits(_ds[j++]);
          if( _xs != null && _xs[j] == Integer.MAX_VALUE){// NA?
            lo = Long.MIN_VALUE; hi = 0;                  // Canonical NA value
          }
        }
        UDP.set8(bs, 16*i  , lo);
        UDP.set8(bs, 16*i+8, hi);
      }
      assert j == _sparseLen:"j = " + j + ", _sparseLen = " + _sparseLen;
      return new C16Chunk(bs);
    }
docs: Compute a compressed UUID buffer