transient sclass SingleTextWordIndex {

  S regexp = "\\w+";

  new ElementInstanceMatrix<Int, S> wordMatrix;

  int length;

  *(S text) {

    init(text);

  }

  *(S *regexp, S text) {

    init(text);

  }

  void init(S text) {

    length = l(text);

    wordMatrix.numberToInstance = wordMatrix.instanceToNumber = i -> i;

    for (IntRange r : regexpFindRanges(regexp, text))

      wordMatrix.add(r.start, ll(upper(substring(text, r))));

    wordMatrix.doneAdding();

  }

  LPair<S, Int> wordsAndOffsets(S text) {

    ret map(regexpFindRanges(regexp, text),

      r -> pair(upper(substring(text, r)), r.start));

  }

  // assumes word boundaries left and right of query

  int[] preSearch(S query) {

    ret indicesOfWordCombination(wordsAndOffsets(query));

  }

  int[] indicesOfWordCombination(LPair<S, Int> wordsWithOffsets) {

    int n = l(wordsWithOffsets);

    if (n == 0) null;

    if (n == 1) ret intArray_minus(first(wordsWithOffsets).b, wordMatrix.instancesContainingElement_intArray(first(wordsWithOffsets).a);

    // get entries for words, exit when a word is unknown

    ElementInstanceMatrix.Entry[] entries = new ElementInstanceMatrix.Entry[n];

    for i to n: {

      ElementInstanceMatrix.Entry e = wordMatrix.index.get(wordsWithOffsets.get(i).a);

      if (e == null) null;

      entries[i] = e;

    }

    // go through words again, shift & AND-combine all bit sets

    BitSet bs = leftShiftBitSet(wordsWithOffsets.get(0).b, cloneBitSet(entries[0].bitSet()));

    for (int i = 1; i < n; i++)

      bs.and(leftShiftBitSet(wordsWithOffsets.get(i).b, entries[i].bitSet()));

    ret bitSetToIntArray(bs);

  }

}