// ID 0 represents a null reference
// From there we count upwards. IDs are not reused when an object is GCed
sclass EphemeralObjectIDs {
  long idCounter;
  new WeakValueMap<Long, O> idToObject;
  Map<O, Long> objectToID = weakIdentityMap();
  
  selfType runnablesReferenceQueue(RunnablesReferenceQueue queue)  {
    idToObject.queue(queue);
    this;
  }
  
  synchronized long remember(O o) {
    if (o == null) ret 0;
    try object objectToID.get(o);
    long id = ++idCounter;
    idToObject.put(id, o);
    objectToID.put(o, id);
    ret id;
  }
  
  synchronized O get(long id) {
    if (id == 0) null;
    ret idToObject.get(id);
  }
  
  synchronized bool wasGarbageCollected(long id) {
    ret id > 0 && id <= idCounter && get(id) == null;
  }
}