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SergiiVlasiuk's solution

to Luhn in the Scala Track

Published at Aug 25 2019 · 0 comments
Test suite

Given a number determine whether or not it is valid per the Luhn formula.

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.

The task is to check if a given string is valid.

Validating a Number

Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.

Example 1: valid credit card number

4539 1488 0343 6467

The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling

4_3_ 1_8_ 0_4_ 6_6_

If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:

8569 2478 0383 3437

Then sum all of the digits:

8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

Example 2: invalid credit card number

8273 1232 7352 0569

Double the second digits, starting from the right

7253 2262 5312 0539

Sum the digits

7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57

57 is not evenly divisible by 10, so this number is not valid.

The Scala exercises assume an SBT project scheme. The exercise solution source should be placed within the exercise directory/src/main/scala. The exercise unit tests can be found within the exercise directory/src/test/scala.

To run the tests simply run the command sbt test in the exercise directory.

For more detailed info about the Scala track see the help page.


The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import org.scalatest.{Matchers, FunSuite}

/** @version 1.2.0 */
class LuhnTest extends FunSuite with Matchers {

  test("single digit strings can not be valid") {
    Luhn.valid("1") should be(false)

  test("a single zero is invalid") {
    Luhn.valid("0") should be(false)

  test("a simple valid SIN that remains valid if reversed") {
    Luhn.valid("059") should be(true)

  test("a simple valid SIN that becomes invalid if reversed") {
    Luhn.valid("59") should be(true)

  test("a valid Canadian SIN") {
    Luhn.valid("055 444 285") should be(true)

  test("invalid Canadian SIN") {
    Luhn.valid("055 444 286") should be(false)

  test("invalid credit card") {
    Luhn.valid("8273 1232 7352 0569") should be(false)

  test("valid strings with a non-digit included become invalid") {
    Luhn.valid("055a 444 285") should be(false)

  test("valid strings with punctuation included become invalid") {
    Luhn.valid("055-444-285") should be(false)

  test("valid strings with symbols included become invalid") {
    Luhn.valid("055£ 444$ 285") should be(false)

  test("single zero with space is invalid") {
    Luhn.valid(" 0") should be(false)

  test("more than a single zero is valid") {
    Luhn.valid("0000 0") should be(true)

  test("input digit 9 is correctly converted to output digit 9") {
    Luhn.valid("091") should be(true)

  test("strings with non-digits is invalid") {
    Luhn.valid(":9") should be(false)
object Luhn {
  def valid: String => Boolean = cardNo => Some(cardNo).filter(validFormat).filter(validLuhn).isDefined

  private def validLuhn(cardNo: String): Boolean =
    cardNo.replaceAll("\\D", "").map(_.asDigit).reverse.zipWithIndex.map {
      case (d, idx) if idx % 2 == 0 => d
      case (d, _) if d > 4 => 2 * d - 9
      case (d, _) if d <= 4 => 2 * d
    }.sum % 10 == 0

  private def validFormat(cardNo: String): Boolean = atLeastNDigits(2)(cardNo) && validRegex(cardNo)

  private def validRegex: String => Boolean = s => """[\d ]{2,}""".r.pattern.matcher(s).matches

  private def atLeastNDigits: Int => String => Boolean = n => s => s.filter(Character.isDigit).size >= n

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