/******************************************************************************
 * This program demonstrates the Luhn Algorithm for credit card validation.
 * 
 * Copyright © 2021 Richard Lesh.  All rights reserved.
 *****************************************************************************/

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import org.pureprogrammer.Utils;

public class CreditCardValidator {

    static List<String> GOOD_CASES = Utils.listFromStrings(new String[]{"4321123456789019", "5432123456789020", "65432101234567896"});
    static List<String> BAD_CASES = Utils.listFromStrings(new String[]{"4321123456798019", "4432123456789020", "65431201234567896"});

    static boolean luhnCheck(String s) {
        int checksum = 0;
        final int LENGTH = Utils.cpLength(s);
        final int DOUBLE_DIGIT = LENGTH % 2;
        for (int i = 0; i < LENGTH - 1; ++i) {
            int c = Utils.cpAt(s, i) - "0".codePointAt(0);
            if (i % 2 == DOUBLE_DIGIT) {
                c *= 2;
                if (c > 9) {
                    c -= 9;
                }
            }
            checksum += c;
        }
        checksum *= 9;
        checksum %= 10;
        return Utils.cpAt(s, LENGTH - 1) - "0".codePointAt(0) == checksum;
    }

    public static void main(String[] args) {
        System.out.println("Good Test Cases");
        for (String s : GOOD_CASES) {
            System.out.println(Utils.join("", s, " is ", luhnCheck(s)));
        }
        System.out.println("Bad Test Cases");
        for (String s : BAD_CASES) {
            System.out.println(Utils.join("", s, " is ", luhnCheck(s)));
        }
    }
}