// 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 Credit3 {

    public static final int NUM_TRIALS = 10000;

    public static void main(String[] args) {

	int numWorked = 0;
	int numFailed = 0;

	while (numWorked < NUM_TRIALS) {
	    String ranCredit = generateRandomCard(10);
	    int sum = luhnSum(ranCredit);

	    if (sum % 10 == 0) {
		numWorked++;
	    } else {
		numFailed++;
	    }
	}
	System.out.println("Num worked: " + numWorked);
	System.out.println("Num failed: " + numFailed);

	double percentWorked;

	percentWorked = ((double) numWorked) /
	    ((double) (numWorked + numFailed));
	percentWorked *= 100.0;
	System.out.println("% worked: " + percentWorked);
    }

    public static int luhnSum(String 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
	}
	// 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;
	}
	return sum;
    }

    public static String generateRandomCard(int num_digits) {
	String card = "";
	Random r = new Random();

	for (int i=0; i<num_digits; i++) {
	    card = card + r.nextInt(10);
	}
	return card;
    }
}