sclass LineComp_PairIndex {
  delegate Node to LineComp_LinkedList_v2.
  
  MultiSetMap<IntPair, Node> nodes = multiSetMap_innerHashSet_outerHashMap();
  MultiSetMap<Int, IntPair> byCount = multiSetMap_innerTreeSet_outerRevTreeMap();
  
  IntPair nodeToPair(Node n) {
    ret n?.pair();
  }
  
  // add node to pair index
  void add(Node n) {
    IntPair p = nodeToPair(n);
    if (p == null) ret;
    int count = nodes.getSize(p);
    nodes.add(p, n);
    byCount.remove(count, p);
    byCount.put(count+1, p);
  }
  
  // remove node from pair index
  void remove(Node n) {
    IntPair p = nodeToPair(n);
    if (p == null) ret;
    int count = nodes.getSize(p);
    if (count == 0) fail("Can't remove pair " + p);
    nodes.remove(p, n);
    byCount.remove(count, p);
    if (count-- > 0) byCount.put(count, p);
  }
  
  IntPair mostPopularDuplicate() {
    ret toInt(firstKey(byCount)) < 2 ? null : firstValue(byCount);
  }
  
  int numberOfDuplicates() {
    ret byCount.size()-l(byCount.get(1));
  }
  
  int getCount(IntPair p) {
    ret nodes.getSize(p);
  }
}