class notepad
{
  var hist = List[Tuple3[Int, Char, Boolean]]() // actions. int - pos, char - sym (added or deleted), boolean - true, if action = adding
  var posInHist = 0 // actions 0, 1, ..., posInHist-1 from hist applied to "" will give text
  var text = "" // current text

  def addSym(pos: Int, sym: Char)
  {
    if (posInHist < hist.length )
      hist = hist.slice(0,posInHist)

    text = text.substring(0, pos) + sym + text.substring(pos,text.length)
    hist = hist ::: (pos, sym, true) :: Nil
    posInHist += 1
  }

  def delSym(pos: Int)
  {
    if (posInHist < hist.length )
      hist = hist.slice(0,posInHist)

    var sym: Char = text(pos)

    text = text.substring(0, pos) + text.substring(pos+1,text.length)
    hist = hist ::: (pos, sym, false) :: Nil
    posInHist += 1
  }

  def undo()
  {
    if (posInHist > 0)
    {
      if (hist(posInHist-1)._3)// add
      {
        text = text.substring(0,hist(posInHist-1)._1) + text.substring(hist(posInHist-1)._1+1, text.length)
      }
      else
      {
        text = text.substring(0,hist(posInHist-1)._1) + hist(posInHist-1)._2 + text.substring(hist(posInHist-1)._1, text.length)
      }
      posInHist -= 1
    }
  }

  def redo()
  {
    if (posInHist < hist.length )
    {
      if (hist(posInHist)._3)// add
      {
        text = text.substring(0,hist(posInHist)._1) + hist(posInHist)._2 + text.substring(hist(posInHist)._1, text.length)
      }
      else
      {
        text = text.substring(0,hist(posInHist)._1) + text.substring(hist(posInHist)._1+1, text.length)
      }

      posInHist += 1
    }
  }

}

object test {
  def main(args: Array[String])
  {
    var n = new notepad
    n.addSym(0,'a')
    n.addSym(1,'b')
    n.addSym(2,'c')
    println(n.text)
    n.undo()
    println(n.text)
    n.redo()
    println(n.text)
    n.undo()
    n.undo()
    println(n.text)
    n.redo()
    println(n.text)
    n.redo()
    println(n.text)
    println("---------------")

    n.delSym(1)
    println(n.text)
    n.undo()
    println(n.text)
    n.redo()
    println(n.text)
    n.addSym(2,'d')
    println(n.text)
    n.undo()
    println(n.text)
    n.undo()
    println(n.text)
  }
}