Compute Luhn checksum for credit card or check digit for ABA Routing Transit Number RTN

Checksum.scala

trait Checksum {
  val seed: Seq[Int]
  def op(c: Int, x: Int): Int

  def coefficients: Stream[Int] = Stream.continually(seed).flatten

  /** Computes the checksum for a sequence of Ints beginning with the least significant. */
  def compute(xs: Seq[Int]): Int = (10 - xs.zip(coefficients).map(x => op(x._1, x._2)).sum % 10) % 10

  /** Computes the check digit for a string of digits. */
  def compute(x: String): Char = compute(Checksum.digits(x)).toString.head

  /** Validates that the first Int in a squence is the correct checksum for the remaining valus. */
  def validate(xs: Seq[Int]): Boolean = xs.head == compute(xs.tail)

  /** Validates that the string of digits has the correct checksum. */
  def validate(x: String): Boolean = validate(Checksum.digits(x))
}
object Checksum {
  def digits(s: String): Seq[Int] = s.filter('0' to '9' contains _).map(_.asDigit).reverse
}
object RTN extends Checksum {
  val seed = Seq(7, 3, 1)
  def op(c: Int, x: Int): Int = c * x
}
object Luhn extends Checksum{
  val seed = Seq(2, 1)
  def op(c: Int, x: Int): Int = {
    val p = c * x
    if (p < 10) p else p - 9
  }
}