public long estimate_sum( long mask ) {
      // For short tables, just do the work
      if( _t.length <= 64 ) return sum(mask);
      // For bigger tables, periodically freshen a cached value
      long millis = System.currentTimeMillis();
      if( _fuzzy_time != millis ) { // Time marches on?
        _fuzzy_sum_cache = sum(mask); // Get sum the hard way
        _fuzzy_time = millis;   // Indicate freshness of cached value
      }
      return _fuzzy_sum_cache;  // Return cached sum
    }