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);
  }
}