Luhn Algorithm (Mod 10)

def luhn_checksum(number: str) -> int:
    """Luhn sum mod 10 of a digit string; 0 means the number is valid."""
    total = 0
    for i, ch in enumerate(reversed(number)):
        d = int(ch)
        if i % 2 == 1:  # every second digit from the right
            d *= 2
            if d > 9:
                d -= 9
        total += d
    return total % 10


def is_luhn_valid(number: str) -> bool:
    digits = number.replace(" ", "").replace("-", "")
    return digits.isascii() and digits.isdigit() and len(digits) > 1 and luhn_checksum(digits) == 0


def luhn_check_digit(payload: str) -> int:
    """Digit to append to payload so that the full number passes Luhn."""
    return (10 - luhn_checksum(payload + "0")) % 10


print(is_luhn_valid("4111 1111 1111 1111"))  # True
print(luhn_check_digit("7992739871"))        # 3

function luhnChecksum(number) {
  let sum = 0;
  for (let i = 0; i < number.length; i++) {
    let d = Number(number[number.length - 1 - i]);
    if (i % 2 === 1) { // every second digit from the right
      d *= 2;
      if (d > 9) d -= 9;
    }
    sum += d;
  }
  return sum % 10; // 0 means valid
}

function isLuhnValid(number) {
  const digits = String(number).replace(/[\s-]/g, '');
  return /^\d{2,}$/.test(digits) && luhnChecksum(digits) === 0;
}

function luhnCheckDigit(payload) {
  return (10 - luhnChecksum(payload + '0')) % 10;
}

console.log(isLuhnValid('4111 1111 1111 1111')); // true
console.log(luhnCheckDigit('7992739871'));       // 3

<?php
function luhn_checksum(string $number): int
{
    $sum = 0;
    $len = strlen($number);
    for ($i = 0; $i < $len; $i++) {
        $d = (int) $number[$len - 1 - $i];
        if ($i % 2 === 1) { // every second digit from the right
            $d *= 2;
            if ($d > 9) {
                $d -= 9;
            }
        }
        $sum += $d;
    }
    return $sum % 10; // 0 means valid
}

function is_luhn_valid(string $number): bool
{
    $digits = preg_replace('/[\s-]/', '', $number);
    return preg_match('/^[0-9]{2,}$/D', $digits) === 1 && luhn_checksum($digits) === 0;
}

function luhn_check_digit(string $payload): int
{
    return (10 - luhn_checksum($payload . '0')) % 10;
}

var_dump(is_luhn_valid('4111 1111 1111 1111')); // bool(true)
echo luhn_check_digit('7992739871'), PHP_EOL;     // 3

public final class Luhn {
    /** Luhn sum mod 10 of a digit string; 0 means the number is valid. */
    static int checksum(String number) {
        int sum = 0;
        for (int i = 0; i < number.length(); i++) {
            int d = number.charAt(number.length() - 1 - i) - '0';
            if (i % 2 == 1) { // every second digit from the right
                d *= 2;
                if (d > 9) d -= 9;
            }
            sum += d;
        }
        return sum % 10;
    }

    static boolean isValid(String number) {
        String digits = number.replaceAll("[\\s-]", "");
        return digits.matches("[0-9]{2,}") && checksum(digits) == 0;
    }

    static int checkDigit(String payload) {
        return (10 - checksum(payload + "0")) % 10;
    }

    public static void main(String[] args) {
        System.out.println(isValid("4111 1111 1111 1111")); // true
        System.out.println(checkDigit("7992739871"));       // 3
    }
}

using System;
using System.Text.RegularExpressions;

public static class Luhn
{
    // Luhn sum mod 10 of a digit string; 0 means the number is valid.
    public static int Checksum(string number)
    {
        int sum = 0;
        for (int i = 0; i < number.Length; i++)
        {
            int d = number[number.Length - 1 - i] - '0';
            if (i % 2 == 1) // every second digit from the right
            {
                d *= 2;
                if (d > 9) d -= 9;
            }
            sum += d;
        }
        return sum % 10;
    }

    public static bool IsValid(string number)
    {
        string digits = Regex.Replace(number, @"[\s-]", "");
        return Regex.IsMatch(digits, @"^[0-9]{2,}$") && Checksum(digits) == 0;
    }

    public static int CheckDigit(string payload) => (10 - Checksum(payload + "0")) % 10;

    public static void Main()
    {
        Console.WriteLine(IsValid("4111 1111 1111 1111")); // True
        Console.WriteLine(CheckDigit("7992739871"));       // 3
    }
}

package main

import (
	"fmt"
	"regexp"
	"strings"
)

// luhnChecksum returns the Luhn sum mod 10 of a digit string; 0 means valid.
func luhnChecksum(number string) int {
	sum := 0
	for i := 0; i < len(number); i++ {
		d := int(number[len(number)-1-i] - '0')
		if i%2 == 1 { // every second digit from the right
			d *= 2
			if d > 9 {
				d -= 9
			}
		}
		sum += d
	}
	return sum % 10
}

var digitsOnly = regexp.MustCompile(`^[0-9]{2,}$`)

func isLuhnValid(number string) bool {
	digits := strings.NewReplacer(" ", "", "-", "").Replace(number)
	return digitsOnly.MatchString(digits) && luhnChecksum(digits) == 0
}

func luhnCheckDigit(payload string) int {
	return (10 - luhnChecksum(payload+"0")) % 10
}

func main() {
	fmt.Println(isLuhnValid("4111 1111 1111 1111")) // true
	fmt.Println(luhnCheckDigit("7992739871"))       // 3
}