asclass ConceptFieldIndexBase<A extends Concept, Val> implements IConceptIndex, IFieldIndex<A, Val>, IConceptCounter, AutoCloseable {

  Concepts concepts;

  Class<A> cc;

  S field;

  Map<A, Val> objectToValue = syncHashMap();

  MultiSetMap<Val, A> valueToObject; // initialized in subclass

  // bool indexNulls = true; // set this only when there are few null values in this field in the db. or when the point is to index a non-existing field (for singletons)

  *() { init(); }

  *(Class<A> cc, S field) { this(db_mainConcepts(), cc, field); }

  *(Concepts *concepts, Class<A> *cc, S *field) {

    this();

    concepts.addConceptIndex(this);

    updateAll();

    register();

    updateAll();

  }

  void updateAll() {

    for (A c : setToIndex())

      updateImpl(c);

  }

  Cl<A> setToIndex() {

    ret list(concepts, cc);

  }

  abstract void init();

  abstract void register();

  public void update(Concept c) {

    if (!isInstance(cc, c)) ret;

    updateImpl((A) c);

  }

  synchronized void updateImpl(A c) {

    Val newValue = cast cget(c, field);

    Val oldValue = objectToValue.get(c);

    if (newValue == oldValue && (oldValue != null || objectToValue.containsKey(c))) ret;

    valueToObject.remove(oldValue, c);

    valueToObject.put(newValue, c);

    put(objectToValue, c, newValue);

  }

  public synchronized void remove(Concept c) {

    if (!isInstance(cc, c)) ret;

    if (!objectToValue.containsKey(c)) ret;

    Val value = objectToValue.get(c);

    objectToValue.remove(c);

    valueToObject.remove(value, (A) c);

  }

  synchronized A get(Val value) {

    ret valueToObject.getFirst(value);

  }

  // older version - may return empty list as null

  // should be phased out everywhere!

  public synchronized Cl<A> getAll(Val value) {

    ret valueToObject.get(value);

  }

  public synchronized L<Val> allValues() { ret cloneKeys_noSync(valueToObject.data); }

  public ItIt<A> objectIterator() {

    ret navigableMultiSetMapValuesIterator_concurrent(valueToObject, this);

  }

  public synchronized MultiSet<Val> allValues_multiSet() {

    ret multiSetMapToMultiSet(valueToObject);

  }

  public Class<? extends Concept> conceptClass() { ret cc; }

  public int countConcepts() { ret l(objectToValue); }

  public Cl<Concept> allConcepts() { ret (Cl) keys(objectToValue); }

  O mutex() { this; }

  close {

    concepts.removeConceptIndex(this);

  }

}