Paylike's luhn implementation in kotlin

Calculate/verify check digit (luhn) of credit cards

This library is a clone of the original javascript implementation from Paylike.

/**
 * Is the card number valid?
 */
val card = '6123451234567893'
val result = PaylikeLuhn.isValid(card) // true

/**
 * Calculate check digit
 * MII + IIN + "Individual account number"
 */
val checksum = PaylikeLuhn.calculateCheckDigit(card) // "3"

Implemented according to ISO/IEC 7812-1:2015(E) which goes:

1. Double the value of alternate digits beginning with the first right-hand digit (low order).

2. Add the individual digits comprising the products obtained in Step 1 to each of the unaffected digits in the original number.

3. Subtract the total obtained in Step 2 from the next higher number ending in 0 [this is the equivalent of calculating the “tens complement” of the low-order digit (unit digit) of the total]. If the total obtained in Step 2 is a number ending in zero (30, 40, etc.), the check digit is 0.umber ending in 0 [this is the equivalent of calculating the “tens complement” of the low-order digit (unit digit) of the total]. If the total obtained in Step 2 is a number ending in zero (30, 40, etc.), the check digit is 0.