public SortableChunk flush() {
        try (Writer out = newWriter(path)) {
          for (SortableFeature feature : featuresToSort) {
            out.write(feature);
          }
          featuresToSort = null;
          return this;
        } catch (IOException e) {
          throw new UncheckedIOException(e);
        }
      }
A container for all features in a chunk read into memory for sorting.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/ExternalMergeSort.java