Luhn.scala

/**
 * Created by FScoward on 15/01/18.
 */
object Luhn {

  /**
   * Luhn algorithm
   * */
  def isValid(s: String): Boolean = {
    val list = doubleOddValues(toReverseIntList(s))
    sum(list) % 10 == 0
  }

  /**
   * 偶数桁の値を倍にして返す
   * */
  private def doubleOddValues(list: List[Int]): List[Int] = {
    (0 until list.size).map(i => {
      if (i % 2 == 0) {
        list(i)
      } else {
        list(i) * 2
      }
    }).toList
  }

  /**
   * 文字列をInt型の逆リストに変換
   * */
  private def toReverseIntList(s: String): List[Int] = {
    s.sliding(1).map(_.toInt).toList.reverse
  }

  /**
   * リストの値を合計する
   * 但し、2桁以上の数字は分割して加算する
   * */
  private def sum(list: List[Int]) = {
    list.map(i => {
      i match {
        case i if i > 9 => {
          i.toString.sliding(1).map(_.toInt).sum
        }
        case i => {
          i
        }
      }
    }).toList.sum
  }
}

/* test -> another file [LuhnSpec.scala] */

import org.specs2.Specification

/**
 * Created by FScoward on 15/01/18.
 */
class LuhnSpec extends Specification {
  def is =
    s2"""
         49927398716 and 1234567812345670 is valid $e1
         49927398717 and 1234567812345678 is invalid $e2

      """
  def e1 = {
    /**
     * 49927398716
     * 6 + (2) + 7 + (1+6) + 9 + (6) + 7 + (4) + 9 + (1+8) + 4 = 70
     * */
    Luhn.isValid("49927398716") === true
    Luhn.isValid("1234567812345670") === true

  }

  def e2 = {
    Luhn.isValid("49927398717") === false
    Luhn.isValid("1234567812345678") === false
  }
}