Luhn credit card verification check in a couple of lines of Scala

LuhnCheck.scala

object LuhnCheck {

  def digitToInt(x:Char)=x.toInt-'0'.toInt

  def digitSeqToInt(ds:Seq[Char])= ds map digitToInt _

  def double ( xs:Seq[Int]) = xs zip (0 to xs.size-1) map { xy => xy._1+xy._1*((xy._2+1) %2) }

  def isValid(cc:String) = (digitSeqToInt (double ( digitSeqToInt (cc)) mkString("") ) sum) % 10 == 0

}

// or from Rosseta Stone
  def luhnTest(number: String): Boolean = {
    val digits = number.reverse.map { _.toString.toInt }
    val s = digits.grouped(2) map { t => t(0) + 
        (if (t.length > 1) (t(1) * 2) % 10 + t(1) / 5 else 0)
    }
    s.sum % 10 == 0
  }