Calculate/verify check digit (luhn) of credit cards

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

SPM:

// dependencies: 
.package(url: "[email protected]:paylike/swift-luhn.git", .upToNextMajor(from: "0.2.0")

// target:
.product(name: "PaylikeLuhn", package: "swift-luhn")

Cocoapods: https://cocoapods.org/pods/PaylikeLuhn

pod 'PaylikeLuhn'

// Is the card number valid?
let card = "6123451234567893"
PaylikeLuhn.isValid(card)    // true


// Calculate check digit
// MII + IIN + "Individual account number"
let incomplete = "612345123456789"

PaylikeLuhn.calculateCheckDigit(incomplete);    // "1"

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.