сс.c

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>

string INVALID = "INVALID";
string AMEX = "AMEX";
string MASTERCARD = "MASTERCARD";
string VISA = "VISA";

typedef struct {
  unsigned int d[16];
  unsigned int len;
} CC;

CC getDigits(long cc) {
    CC digits;
    int i = 0;
    
    while (cc > 0) {
        digits.d[i] = cc % 10;
        cc /= 10;
        i++;
    }
    
    digits.len = i;
    
    return digits;
}

bool checkLength(CC cc) {
    if (cc.len < 13 || cc.len > 16 || cc.len == 14) {
        return false;
    }
    
    return true;
}

bool isMC(CC cc) {
    int last = cc.len - 1;
    return cc.d[last] == 5 && cc.d[last - 1] > 0 && cc.d[last - 1] < 6;
}

bool isVisa(CC cc) {
    int last = cc.len - 1;
    return cc.d[last] == 4;
}

bool isAmex(CC cc) {
    int last = cc.len - 1;
    return cc.d[last] == 3 && (cc.d[last - 1] == 4 || cc.d[last - 1] == 7);
}

bool checkStartsWith(CC cc) {
    if (isVisa(cc)) {
        return cc.len == 13 || cc.len == 16;
    }
    
    if (isMC(cc)) {
        return cc.len == 16;
    }
    
    if (isAmex(cc)) {
        return cc.len == 15;
    }
    
    return false;
}

bool checkSum(CC cc) {
    int sum = 0; 

    for (int i = cc.len - 1; i >= 0; i--) {
        int toadd = cc.d[i] * (i%2 ? 2 : 1);
        
        if (toadd >= 10) {
            sum += toadd % 10 + toadd / 10;
        } else {
            sum += toadd;
        }
    }

    return sum % 10 == 0;
}

int main(void) {
    CC cc = getDigits(get_long("Gimme credit card: "));
    if (
        checkLength(cc) &&
        checkStartsWith(cc) &&
        checkSum(cc)
    ) {
        if (isAmex(cc)) {
            printf("%s\n", AMEX);
        } else if (isVisa(cc)) {
            printf("%s\n", VISA);
        } else if (isMC(cc)) {
            printf("%s\n", MASTERCARD);
        }
    } else {
        printf("%s\n", INVALID);
    }
}