class Ls [T](hd : Option[T], tl : Ls[T]){
    def head : Option[T] = hd
    def tail : Ls[T] = tl;
    def foldr[ACC](acc: ACC,fun:(ACC,Option[T])=>ACC):ACC = {
        hd match {
            case None => acc
            case Some(x) => tl.foldr(fun(acc,Some(x)),fun)
        }
    }
    def concat(x: T):Ls[T] = {
        new Ls[T](Some(x),this)
    }
    override def clone() = {
        this.foldr[Ls[T]](new Ls[T](None,null),(acc: Ls[T],x:Option[T]) => x match{
            case None => acc
            case Some(y) => acc.concat(y)
        })
    }
    def len():Int = {
        foldr[Int](0,(acc: Int,x:Option[T]) =>
            x match{
                case None => acc
                case Some(y) => acc+1
            }
        )
    }
};
object LsOp{
    def subl[T](src : Ls[T], start : Int, end : Int):Option[Ls[T]] = {
        if((start<end) && (end < src.len()-1))
            __subl[T](start,end,0,new Ls[T](None,null),src)
        else
            None
    }
    private[this] def __subl[T](start : Int, end : Int, count : Int, acc : Ls[T], ls : Ls[T]):Option[Ls[T]] = {
        ls.head match {
            case None => None
            case Some(x) =>
                if(count < start){
                    __subl[T](start,end,count+1,acc,ls.tail)
                }else if((count<=end)&&(count >=start)){
                    __subl[T](start,end,count+1,acc.concat(x),ls.tail)
                }else{
                    Some(acc)
                }
        }
    }
    def optionListToStr[T](ls : Option[Ls[T]]):String = {
        ls match{
            case None => "an error occured while performing a list operation"
            case Some(x) => listToStr(x)
        }
    }
    def listToStr[T](ls: Ls[T]):String = {
        val str = __listToStr(ls,"")
        str.substring(0,str.length-1)
    }
    private[this] def __listToStr[T](ls : Ls[T],acc: String):String = {
        ls.head match{
            case None => acc
            case Some(x) => __listToStr[T](ls.tail,x+","+acc)
        }
    }
};


object Main extends App{
    println("test")
    val ls = new Ls[Int](Some(1),new Ls[Int](Some(2),new Ls[Int](Some(26),new Ls[Int](Some(32),new Ls[Int](None,null)))))
    println(ls.concat(1).clone().len())
    println(LsOp.optionListToStr(LsOp.subl(ls,0,2)));
};