public DirectoryTaxonomyWriter(Directory directory, OpenMode openMode,

      TaxonomyWriterCache cache) throws IOException {

    dir = directory;

    IndexWriterConfig config = createIndexWriterConfig(openMode);

    indexWriter = openIndexWriter(dir, config);

    // verify (to some extent) that merge policy in effect would preserve category docids

    assert !(indexWriter.getConfig().getMergePolicy() instanceof TieredMergePolicy) :

      "for preserving category docids, merging none-adjacent segments is not allowed";

    // after we opened the writer, and the index is locked, it's safe to check

    // the commit data and read the index epoch

    openMode = config.getOpenMode();

    if (!DirectoryReader.indexExists(directory)) {

      indexEpoch = 1;

    } else {

      String epochStr = null;

      Map<String, String> commitData = readCommitData(directory);

      if (commitData != null) {

        epochStr = commitData.get(INDEX_EPOCH);

      }

      // no commit data, or no epoch in it means an old taxonomy, so set its epoch to 1, for lack

      // of a better value.

      indexEpoch = epochStr == null ? 1 : Long.parseLong(epochStr, 16);

    }

    if (openMode == OpenMode.CREATE) {

      ++indexEpoch;

    }

    FieldType ft = new FieldType(TextField.TYPE_NOT_STORED);

    ft.setOmitNorms(true);

    parentStreamField = new Field(Consts.FIELD_PAYLOADS, parentStream, ft);

    fullPathField = new StringField(Consts.FULL, "", Field.Store.YES);

    nextID = indexWriter.maxDoc();

    if (cache == null) {

      cache = defaultTaxonomyWriterCache();

    }

    this.cache = cache;

    if (nextID == 0) {

      cacheIsComplete = true;

      // Make sure that the taxonomy always contain the root category

      // with category id 0.

      addCategory(new FacetLabel());

    } else {

      // There are some categories on the disk, which we have not yet

      // read into the cache, and therefore the cache is incomplete.

      // We choose not to read all the categories into the cache now,

      // to avoid terrible performance when a taxonomy index is opened

      // to add just a single category. We will do it later, after we

      // notice a few cache misses.

      cacheIsComplete = false;

    }
}