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