credit_check.c

credit_check.c

#include <stdio.h>
/* 
 * This program uses the Luhn algorithm to validate a card number and
 * print its association (VISA, MASTERCARD or AMERICAN EXPRESS)
 */
int sum_1(long long num);
int sum_2(long long num);
long long get_num();

int main(void) {
	int sum;
	long long digits, num;

	num = get_num();
	sum = sum_1(num) + sum_2(num);
	digits = num;
	while (digits > 100) {
		digits /10;
	}
	if (sum % 10 == 0)
		if (digits / 10 == 4)
			printf("VISA\n");
		else
		switch (digits) {
			case 34: case 37:
				printf("AMEX\n");
				break;
			case 51: case 52: case 53: case 54: case 55:
				printf("MASTERCARD\n");
				break;
			default:
				printf("INVALID\n");
		}
}

long long get_num() {
/* Get a positive number form user*/
	long long num;
	printf("Number: ");
	scanf("%lld\n", &num);
	while (num < 1) {
		printf("Retry: ");
		scanf("%lld\n", &num);
	}
	return num;
}

int sum_1(long long num) {
/* Sum of a product of every second digit form the right multiplied by 2 */
	int sum;
	while (num > 0) {
		num /= 10;
		if (num % 10 * 2 >9) {
			sum += (num % 10 * 2) % 10 + 1;
			num /= 10;
		}
		else {
			num += num % 10 * 2;
			num /= 10;
		}
	}
	return sum;
}

int sum_2(long long num) {
/* Sum of a digits that were not multiplied by 2 */
	int sum;
	while (num > 0) {
		sum += num % 10;
		num /= 100;
	}
	return sum;
}