c implementation of credit card problem from HW1 in CS50

credit.c

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


int countDigits(long long num) { // helper method that counts the number of digits in a number
    int count = 0;
    while (num != 0) {
        num /= 10;
        ++count;
    }
    return count;
}

int getFirst(long long card_number) { // helper method that returns the first digit in a number
    return card_number / pow(10, countDigits(card_number) - 1);
}

int * getDigits(long long card_number, int card_number_length) { // helper method that returns an array of digits from a number
    int *digits = malloc(card_number_length * sizeof(int));
    for (int i = 0; i < card_number_length; i++) {
        
        digits[i] = card_number % 10;
        card_number = card_number / 10;
    }
    return digits;
}

int sumDigits(int *odd, int array_length){ //helper method that adds together the digits of numbers in an array
    int sum = 0;
    for (int i = 0; i < array_length; i++){
        int mod = odd[i];
        if (mod > 9){
            sum += mod % 10;
            sum += 1;
        }
        else{
            sum += mod;
        }
    }
    
    return sum;
}


bool checkCredit(long long card_number, int card_number_length) { //helper method that splits a credit card number into the components needed to verify whether it is a valid code
    
    int *digits = getDigits(card_number,card_number_length);
    int evenSum = 0;
    int odd [(card_number_length + 1)/2];
    int odd_count = 0;
    for (int i = 0; i < card_number_length; i++) { //separate digits
        if (i == 0){
            evenSum += digits[i];
        }
        else if (i % 2 != 0){
            odd[odd_count] = digits[i] * 2;
            odd_count += 1;
        }
        else{
            evenSum += digits[i];
        }
    }
    int oddDigits = sizeof(odd)/sizeof(int);
    int oddSum = sumDigits(odd,oddDigits);
    if ((oddSum + evenSum) % 10 == 0) {
        return true;
    } else {
        return false;
    }
}

int main(void) { // main method!
    long long card_number = 0;
    do { //get card number
        printf("Number:");
        card_number = get_long_long();
    }
    while(countDigits(card_number) < 13 || countDigits(card_number) > 16 ); // check if number is of valid length
    int card_number_length = countDigits(card_number);
    bool cardValid = checkCredit(card_number, card_number_length); //check if card is valid
    
    if (cardValid == false) {
        printf("INVALID\n");
    }
    else if (getFirst(card_number) == 5) {
        printf("MASTERCARD\n");
    }
    else if (getFirst(card_number) == 4) {
        printf("VISA\n");
    }
    else if (getFirst(card_number) == 3) {
        printf("AMEX\n");
    }
}