Luhn Algorithm

 The Luhn Algorithm, also known as the modulus 10 or mod 10 algorithm, is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers. It helps to detect accidental errors in the input data.

Here's a step-by-step explanation of the Luhn Algorithm:

Consider the example of an account number “79927398713“. 

• Starting from the rightmost digit and moving left, double the value of every second digit.
• If the doubling results in a number greater than 9, then add the two digits.

      Second digits from right most are 1, 8, 3, 2, 9

Doubling those digits give 2, 16, 6, 4, 18

      Adding double digits give 2, 7, 6, 4, 9

• Sum all the digits, including both the untouched digits and the doubled digits.

Hence sum is 7 + 9 + 9 + 4 + 7 + 6 + 9 + 7 + 7 + 2 + 3 = 70

• If the total modulo 10 is equal to 0, then the number is valid according to the Luhn Algorithm.

       70 % 10 = 0

package com.example.godel.service;


public class CardNumberValidation {

    /**
     * Validates the card number
     * @param number
     * @return true or false
     */
    public boolean isValid(String number) {
        int cardNumberLength = number.length();
        boolean isSecond = false;
        int i = cardNumberLength-1;
        int sum = 0;
        while(i >= 0) {
            int currentNumber = number.charAt(i) - '0';
            i--;
            if (isSecond) {
                currentNumber *= 2;
            }
            sum += currentNumber / 10;
            sum += currentNumber % 10;
            isSecond = !isSecond;
        }

        return (sum % 10 == 0);
    }

}