transient sclass WordIndex<A> {
  // if this is set, it's used for sorting the values
  // which can speed up lookups
  Comparator<A> valueComparator;
  
  S regexp = "\\w+";
  MultiSetMap<S, A> index = ciMultiSetMap(); // sets are better for lookups
  
  *() {}
  *(Comparator<A> *valueComparator) { index = ciMultiSetMap_innerTreeSet(valueComparator); }
  *(Map<A, S> map) { fOr (A a, S text : map) add(a, text); }
  
  void add(A a, S text) {
    Set<S> words = extractWords(text);
    for (S word : words) addWord(a, word);
  }
  
  void addWord(A a, S word) {
    index.add(word, a);
  }
  
  Set<S> extractWords(S text) {
    ret asCISet(extractWords_list(text));
  }
  
  LS extractWords_list(S text) {
    ret regexpExtractAll(regexp, text);
  }
  
  L<IntRange> wordRanges(S text) {
    ret regexpFindRanges(regexp, text);
  }
  
  Set<A> get(S word) {
    ret index.get(word);
  }
  
  void remove(A a, S text) {
    Set<S> words = extractWords(text);
    for (S word : words) index.remove(word, a);
  }
  
  NavigableSet<S> words() { ret (NavigableSet) keys(index); }
  
  int numWords() { ret index.keysSize(); }
  
  // These methods only work when A = S
  
  void add(S s) { add((A) s, s); }
  void remove(S s) { remove((A) s, s); }
}