sclass LineComp_PairIndex {
  bool verbose;
  new Map<IntPair, PairInfo> pairInfos;
  new LinkedHashMap<S, Ch> chains;
  
  // buckets
  Bucket oneBucket = new Bucket(1), highest = oneBucket; // 1-bucket is lowest
  int bucketCount = 1;

  class PairInfo {
    IntPair pair;
    Bucket bucket;
    new LinkedHashSet<Node> nodes;
    
    *(IntPair *pair) {}
    
    public bool equals(O o) { if (o cast PairInfo) ret pair.equals(o.pair); false; }
    public int hashCode() { ret pair.hashCode(); }
  }
  
  class Bucket {
    Bucket higher, lower;
    int level; // number of duplicates of each IntPair in this bucket
    new LinkedHashSet<PairInfo> pairs;
    
    *(int *level) {}
    
    Bucket oneHigher() {
      if (higher != null && higher.level == level+1) ret higher;
      Bucket b = new(level+1);
      ++bucketCount;
      b.lower = this;
      b.higher = higher;
      higher = b;
      if (highest == this) highest = b;
      ret b;
    }
    
    void add(PairInfo pair) {
      pairs.add(pair);
      pair.bucket = this;
    }
    
    void movePairOneHigher(PairInfo pair) {
      oneHigher().add(pair);
      remove(pair);
    }
    
    void remove(PairInfo pair) {
      pairs.remove(pair);
      if (empty(pairs) && this != oneBucket)
        dropMe();
    }
    
    void dropMe {
      if (higher != null) higher.lower = lower; else highest = lower;
      lower.higher = higher;
      level = -1; // mark invalid for safety
    }
    
    void movePairOneLower(PairInfo pair) {
      if (this == oneBucket) forgetPair(pair);
      else oneLower().add(pair);
      remove(pair);
    }
  }
  
  Bucket oneBucket() { ret oneBucket; }
  
  // a "chain" of nodes (one input file)
  class Ch {
    Node tail;
    
    *(L<Int> l) {
      fOr (int i : l) add(i);
    }
    
    L<Int> toList() {
      new L<Int> l;
      Node node = tail;
      while (node != null) {
        l.add(node.value);
        node = node.prev;
      }
      ret reverseInPlace(l);
    }
    
    void add(int i) {
      new Node n;
      n.ch = this;
      n.value = i;
      n.prev = tail;
      if (tail != null) tail.next = n;
      tail = n;
      addToIndex(n.prev);
    }
  }
  
  class Node {
    Ch ch;
    int value;
    Node prev, next;
    
    int a() { ret value; }
    int b() { ret next == null ? -1 : next.value; }
    IntPair pair() { ret next == null ? null : IntPair(a(), b()); }
  }
  
  IntPair nodeToPair(Node n) {
    ret n?.pair();
  }
  
  // add node to pair index (add to lowest or move to higher bucket)
  void addToIndex(Node n) {
    IntPair p = nodeToPair(n), ret if null;
    PairInfo pi = getPairInfo(p);
    if (pi == null)
      oneBucket.add(pi);
    else
      pi.bucket.movePairOneHigher(pi);
  }
  
  // remove node from pair index (move to lower bucket or drop)
  void removeFromIndex(Node n) {
    IntPair p = nodeToPair(n), ret if null;
    PairInfo pi = getPairInfo(p);
    if (pi == null) fail("Can't remove pair " + p);
    pi.bucket.movePairOneLower(pi);
  }
  
  IntPair mostPopularDuplicate() {
    ret toInt(firstKey(byCount)) < 2 ? null : firstValue(byCount);
  }
  
  // returns number of buckets this time
  int numberOfDuplicates() {
    ret bucketCount-1; // byCount.size()-l(byCount.get(1));
  }
  
  int getCount(IntPair p) {
    ret nodes.getSize(p);
  }
  
  void replacePair(int pa, int pb, int replaceWith) {
    IntPair p = IntPair(pa, pb);
    Set<Node> set = nodes.get(p);
    for (Node n : cloneList(set)) {
      continue if n.a() != pa || n.b() != pb; // nodes might have been deleted or changed
      replacePair(n, replaceWith);
    }
  }
  
  void replacePair(Node node, int replaceWith) {
    removeFromIndex(node.prev);
    removeFromIndex(node);
    removeFromIndex(node.next);
    node.value = replaceWith;
    deleteNode(node.next);
    addToIndex(node.prev);
    addToIndex(node);
  }
  
  void deleteNode(Node node) {
    if (node.next != null) node.next.prev = node.prev; else node.ch.tail = node.prev;
    if (node.prev != null) node.prev.next = node.next;
    node.value = -1; // mark invalid
  }
  
  void newChain(S version, L<Int> encoding) {
    chains.put(version, new Ch(encoding));
  }
  
  PairInfo getPairInfo(IntPair pair) {
    ret pairInfos.get(pair);
    /*ret pair == null ? null : getOrCreate(pairInfos, pair, () -> {
      PairInfo pi = new(pair);
      oneBucket.add(pi);
      ret pi;
    });*/
  }
  
  void forgetPair(PairInfo pair) {
    pairInfos.remove(pair.pair);
    pair.bucket = null;
  }
}