sclass TripleIndex extends VirtualNodeIndex {
  SyncListMultiMap<S, TripleWeb> index = caseInsensitiveSyncListMultiMap();
  new CompactHashSet<TripleWeb> websByID;
  bool activated; // are we the main index (see tripleIndex())

  int numWebs() { ret l(websByID); }
  int numTerms() { ret index.keysSize(); }
  
  Collection<S> indexedTerms() { ret keys(index); }
  
  L<WebNode> get(S query) {
    ret ai_triplesToWebNodes_lazyList(query, index.get(query));
  }
  
  bool hasShortTerm(S s) { ret index.containsKey(s); }

  Web getWeb(GlobalID id) {
    ret webFromTriple(websByID.find(dummyTripleWebWithGlobalID(id)));
  }
  
  void addWeb(Web web) {
    if (web == null) ret;
    if (w == null)
      fail("Skipping non-tripelizable web: " + webToStringShort(web));
    addTriple(ai_webToTripleWeb(web));
  }
  
  void addTriple(TripleWeb w) {
    if (w == null) ret;
    synchronized(index) {
      for (S s : sortedInPlace(ll(w.a, w.b, w.c)))
        index.put(s, w);
      
      // for (S s : lithashset(w.a, w.b, w.c)) index.put(s, w); // only once
        
    }
    websByID.add(w);
  }
  
  void removeWeb(Web web) {
    if (web == null) ret;
    TripleWeb w = ai_webToTripleWeb(web);
    if (w == null) ret;
    GlobalID id = w.globalID();
    websByID.remove(dummyTripleWebWithGlobalID(id));
    indexRemove(w.a, id);
    indexRemove(w.b, id);
    indexRemove(w.c, id);
  }
  
  // internal
  void indexRemove(S term, GlobalID globalID) {
    L<TripleWeb> l = index.get(term);
    for i over l:
      if (eq(l.get(i).globalID(), globalID)) {
        index.remove(term, l.get(i));
        ret;
      }
  }
  
  void clear() {
    index.clear();
    websByID.clear();
  }
  
  void activate {
    if (!activated) {
      activated = true;
      ai_onNewOrRemovedWeb(func(Web web) {
        addWeb(web);
        false;
      }, func(Web web) {
        removeWeb(web);
        false;
      });
    }
  }
}