sclass LineComp_PairCounts {

  new MultiSet<IntPair> counts;

  MultiSetMap<Int, IntPair> byCount = multiSetMap_innerTreeSet_outerRevTreeMap();

  void addAll(Iterable<IntPair> l) { fOr (IntPair p : l) add(p); }

  void removeAll(Iterable<IntPair> l) { fOr (IntPair p : l) remove(p); }

  void add(int a, int b) { add(IntPair(a, b)); }

  void remove(int a, int b) { remove(IntPair(a, b)); }

  // increment pair's count

  void add(IntPair p) {

    int count = counts.get(p);

    byCount.remove(count, p);

    counts.add(p);

    byCount.put(count+1, p);

  }

  // decrement pair's count

  void remove(IntPair p) {

    int count = counts.get(p);

    if (count == 0) fail("Can't remove pair " + p);

    byCount.remove(count, p);

    counts.remove(p);

    if (count-- > 0) byCount.put(count, p);

  }

  IntPair mostPopularDuplicate() {

    ret toInt(firstKey(byCount)) < 2 ? null : firstValue(byCount);

  }

  int numberOfDuplicates() {

    ret counts.size()-l(byCount.get(1));

  }

  int getCount(IntPair p) {

    ret counts.get(p);

  }

}