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Add Luhn algorithm (Fixes: TheAlgorithms#2537) (TheAlgorithms#2538)
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‎Others/Luhn.java‎

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package Others;
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import java.util.Arrays;
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import java.util.Objects;
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/**
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* The Luhn algorithm or Luhn formula, also known as the "modulus 10" or "mod 10" algorithm,
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* named after its creator, IBM scientist Hans Peter Luhn, is a simple checksum formula
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* used to validate a variety of identification numbers.
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*
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* <p>The algorithm is in the public domain and is in wide use today.
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* It is specified in ISO/IEC 7812-1. It is not intended to be a cryptographically
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* secure hash function; it was designed to protect against accidental errors,
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* not malicious attacks. Most credit cards and many government identification numbers
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* use the algorithm as a simple method of distinguishing valid numbers from mistyped or
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* otherwise incorrect numbers.</p>
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*
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* <p>The Luhn algorithm will detect any single-digit error, as well as almost all
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* transpositions of adjacent digits. It will not, however, detect transposition of the
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* two-digit sequence 09 to 90 (or vice versa). It will detect most of the possible
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* twin errors (it will not detect 22 ↔ 55, 33 ↔ 66 or 44 ↔ 77).</p>
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*
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* <p>The check digit is computed as follows:</p>
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* <ol>
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* <li>Take the original number and starting from the rightmost digit moving left, double the value of every second digit (including the rightmost digit).</li>
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* <li>Replace the resulting value at each position with the sum of the digits of this position's value or just subtract 9 from all numbers more or equal then 10.</li>
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* <li>Sum up the resulting values from all positions (s).</li>
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* <li>The calculated check digit is equal to {@code 10 - s % 10}.</li>
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* </ol>
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*
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* @see <a href="https://en.wikipedia.org/wiki/Luhn_algorithm">Wiki</a>
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*/
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public class Luhn {
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/**
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* Check input digits array by Luhn algorithm.
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* Initial array doesn't change while processing.
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* @param digits array of digits from 0 to 9
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* @return true if check was successful, false otherwise
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*/
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public static boolean luhnCheck(int[] digits) {
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int[] numbers = Arrays.copyOf(digits, digits.length);
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int sum = 0;
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for (int i = numbers.length - 1; i >= 0; i--) {
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if (i % 2 == 0) {
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int temp = numbers[i] * 2;
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if (temp > 9) {
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temp = temp - 9;
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}
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numbers[i] = temp;
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}
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sum += numbers[i];
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}
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return sum % 10 == 0;
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}
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public static void main(String[] args) {
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System.out.println("Luhn algorithm usage examples:");
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int[] validInput = {4, 5, 6, 1, 2, 6, 1, 2, 1, 2, 3, 4, 5, 4, 6, 7};
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int[] invalidInput = {4, 5, 6, 1, 2, 6, 1, 2, 1, 2, 3, 4, 5, 4, 6, 4}; //typo in last symbol
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checkAndPrint(validInput);
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checkAndPrint(invalidInput);
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System.out.println("\nBusiness examples:");
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String validCardNumber = "5265 9251 6151 1412";
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String invalidCardNumber = "4929 3231 3088 1896";
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String illegalCardNumber = "4F15 BC06 3A88 76D5";
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businessExample(validCardNumber);
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businessExample(invalidCardNumber);
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businessExample(illegalCardNumber);
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}
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private static void checkAndPrint(int[] input) {
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String validationResult = Luhn.luhnCheck(input)
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? "valid"
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: "not valid";
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System.out.println("Input " + Arrays.toString(input) + " is " + validationResult);
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}
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/*
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========================
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Business usage example
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========================
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*/
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/**
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* Object representation of credit card.
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*/
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private record CreditCard(int[] digits) {
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private static final int DIGITS_COUNT = 16;
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/**
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* @param cardNumber string representation of credit card number - 16 digits.
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* Can have spaces for digits separation
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* @return credit card object
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* @throws IllegalArgumentException if input string is not 16 digits
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* or if Luhn check was failed
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*/
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public static CreditCard fromString(String cardNumber) {
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Objects.requireNonNull(cardNumber);
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String trimmedCardNumber = cardNumber.replaceAll(" ", "");
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if (trimmedCardNumber.length() != DIGITS_COUNT || !trimmedCardNumber.matches("\\d+")) {
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throw new IllegalArgumentException("{" + cardNumber + "} - is not a card number");
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}
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int[] cardNumbers = toIntArray(trimmedCardNumber);
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boolean isValid = luhnCheck(cardNumbers);
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if (!isValid) {
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throw new IllegalArgumentException("Credit card number {" + cardNumber + "} - have a typo");
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}
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return new CreditCard(cardNumbers);
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}
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/**
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* @return string representation separated by space every 4 digits.
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* Example: "5265 9251 6151 1412"
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*/
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public String number() {
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StringBuilder result = new StringBuilder();
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for (int i = 0; i < DIGITS_COUNT; i++) {
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if (i % 4 == 0 && i != 0) {
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result.append(" ");
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}
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result.append(digits[i]);
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}
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return result.toString();
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}
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@Override
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public String toString() {
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return String.format("%s {%s}", CreditCard.class.getSimpleName(), number());
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}
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private static int[] toIntArray(String string) {
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return string.chars()
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.map(i -> Character.digit(i, 10))
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.toArray();
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}
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}
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private static void businessExample(String cardNumber) {
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try {
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System.out.println("Trying to create CreditCard object from valid card number: " + cardNumber);
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CreditCard creditCard = CreditCard.fromString(cardNumber);
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System.out.println("And business object is successfully created: " + creditCard + "\n");
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} catch (IllegalArgumentException e) {
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System.out.println("And fail with exception message: " + e.getMessage() + "\n");
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}
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}
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}

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