// Method for verifying a credit card number:
// 1. Starting with the second digit from the right,
//	  double every other digit
// 2. Break into digits, and add the digits together
// 3. Result must end with 0 (e.g 70)

import java.util.*;

public class Credit {

    public static void main(String[] args) {

        // prompt the user for a credit card number
	Scanner console = new Scanner(System.in);
	System.out.print("Enter credit card number: ");
	String number = console.next();

	// check to see if the credit card number is valid
	System.out.println("Checking credit card number '" + number + "'");

	int sum = 0;
	// Add up the digits formed by doubling every other digit
	for (int i=number.length()-2; i>=0; i = i-2) {
	    char c = number.charAt(i);
	    int n = 2 * Character.getNumericValue(c);
	    sum = sum + n/10 + n%10;	// Break into two digits and add
	    System.out.println("c: " + c + ", n: " + n +
			       ", sum: " + sum);
	}
	// Add the remaining digits
	for (int i=number.length()-1; i>=0; i = i-2) {
	    char c = number.charAt(i);
	    int n = Character.getNumericValue(c);
	    sum = sum + n;
	    System.out.println("c: " + c + ", n: " + n +
			       ", sum: " + sum);
	}

	System.out.println();
	if (sum % 10 == 0) {
	    System.out.println("** Credit card number is valid.");
	} else {
	    System.out.println("** Credit card number is NOT valid!");
	}
    }
}