sclass OptimizedMultiSet<A> {
  new Map<A, Int> map;
  // outer tree map for sorting
  // inner linked hash set for reproducability (HashSet ordering sucks)
  transient MultiSetMap<Int, A> byCount = multiSetMap_innerLinkedHashSet_outerRevTreeMap();
  int size;
  
  int add(A a) { ret add(a, 1); }
  
  int add(A a, int count) {
    if (count <= 0) ret 0;
    size += count;
    Int i = map.get(a);
    if (i != null)
      byCount.remove(i += count, a);
    else
      i = count;
    map.put(a, i);
    byCount.add(i, a);
    ret i;
  }
  
  void remove(A a) {
    Int i = map.get(a);
    if (i != null) {
      --size;
      byCount.remove(i, a);
      if (--i > 0) {
        map.put(a, i);
        byCount.add(i, a);
      } else
        map.remove(a);
    }
  }
  
  bool isEmpty() {
    ret map.isEmpty();
  }
  
  toString {
    ret str(map);
  }
  
  A getMostPopularEntry() {
    ret firstValue(byCount);
  }
  
  void _doneLoading {
    rebuildByCount();
  }
  
  // internal
  void rebuildByCount {
    byCount.clear();
    for (A entry, int count : map)
      byCount.put(count, entry);
  }
  
  int get(A a) {
    ret map.get(a);
  }
  
  int size() {
    ret size;
  }
}