persistable sclass UpDownSequence extends RandomAccessAbstractList<UpDown> {

  new BitBuffer moves; // up is true

  *(S s) {

    for (c : characters(upper(s)))

      if (c == 'U') add(UpDown.up);

      else if (c == 'D') add(UpDown.down);

  }

  public UpDown get(int i) {

    ret UpDown.fromBool(moves.get(i));

  }

  public bool add(UpDown move) {

    moves.add(move.bit);

    true;

  }

  toString { ret join(countIterator moveChar(size())); }

  S moveChar(int i) { ret moves.get(i) ? "U" : "D"; }

  public int size() { ret moves.size(); }

  void addUp { add(UpDown.up); }

  void addDown { add(UpDown.down); }

  static UpDownSequence fromCellNumbers aka fromInts(L<Int> cellNumbers) {

    new UpDownSequence seq;

    if (nempty(cellNumbers)) {

      int cn = first(cellNumbers);

      for (int i = 1; i < l(cellNumbers); i++) {

        int cn2 = cellNumbers.get(i);

        int cn3 = clamp(cn2, cn-10, cn+10);

        while (cn3 > cn) {

          ping();

          ++cn;

          seq.addUp();

        }

        while (cn3 < cn) {

          ping();

          --cn;

          seq.addDown();

        }

        cn = cn2;

      }

    }

    ret seq;

  }

}