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); }

}