code: private void initializeUnknownCategoricalsPerColumn(GenModel model) {
      unknownCategoricalsPerColumn = new ConcurrentHashMap<>();
  
      for (int i = 0; i < model.getNumCols(); i++) {
        String[] domainValues = model.getDomainValues(i);
        if (domainValues != null) {
          unknownCategoricalsPerColumn.put(model.getNames()[i], new AtomicLong());
        }
      }
  
      unknownCategoricalsPerColumn = Collections.unmodifiableMap(unknownCategoricalsPerColumn);
    }
docs: Initializes the map of unknown categoricals per column with an unmodifiable and thread-safe implementation of {@link Map}.
  
  @param model {@link GenModel} the unknown categorical per column map is initialized for