docs: @Override
      public LocationIndex prepareIndex() {
          initBuffer();
          initAlgo(latSize, lonSize);
          StopWatch sw = new StopWatch().start();
          GHBitSet filledIndices = fillQuadtree(latSize * lonSize);
          int fillQT = filledIndices.getCardinality();
          float res1 = sw.stop().getSeconds();
          sw = new StopWatch().start();
          int counter = fillEmptyIndices(filledIndices);
          float fillEmpty = sw.stop().getSeconds();
          logger.info("filled quadtree index array in " + res1 + "s. size is " + getCapacity()
                  + " (" + fillQT + "). filled empty " + counter + " in " + fillEmpty + "s");
          flush();
          return this;
      }