sclass EREvent {

}

sclass ERText extends EREvent {

  int startIndex, endIndex;

  S text;

}

sclass ERCaret extends EREvent  {

  int pos;

} 

sclass EditRecorder {

  VF1<EREvent> onEvent;

  *() {}

  *(VF1<EREvent> *onEvent) {}

  // editor state

  S text = "";

  int caret;

  // update the text

  void update(S newText) {

    text = unnull(text);

    newText = unnull(newText);

    S a = text, b = newText;

    int i = lCommonPrefix(a, b);

    a = substring(a, i);

    b = substring(b, i);

    int j = lCommonSuffix(a, b);

    a = substring(a, 0, l(a)-j);

    b = substring(b, 0, l(b)-j);

    // Now differing part remains in a and b;

    // i and j give length of unchanged beginning+end

    if (empty(a) && empty(b)) ret;

    ERText change = nu(ERText, startIndex := i, endIndex := i+l(a), text := b);

    // Verify the change we made

    assertEquals(newText, replayTextEdit(change, text));

    text = newText;

    onEvent(change);

  }

  void updateCaret(int pos) {

    if (pos != caret) {

      onEvent(nu(ERCaret, +pos));

      caret = pos;

    }

  }

  void onEvent(EREvent e) {

    callF(onEvent, e);

  }

  static S replayTextEdit(ERText change, S text) {

    if (change == null) ret text;

    ret substring(text, 0, change.startIndex)

      + change.text

      + substring(text, change.endIndex);

  }

}