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## to Luhn in the Scala Track

Published at Aug 25 2019 · 0 comments
Instructions
Test suite
Solution

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.

## Source

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.

### LuhnTest.scala

``````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") {
pending
Luhn.valid("0") should be(false)
}

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

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

pending
Luhn.valid("055 444 285") should be(true)
}

pending
Luhn.valid("055 444 286") should be(false)
}

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

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

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

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

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

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

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

test("strings with non-digits is invalid") {
pending
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
}``````