Credit (if statement)

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

// Break credit card number into smaller integers (4x4 for 16-length, 3x5 for 15 length), and performs arithmetic on those smaller pieces to build sum 1 and sum 2.
int segment16_sum1_1(long long a);
int segment16_sum1_2(long long a);
int segment16_sum1_3(long long a);
int segment16_sum1_4(long long a);
int segment16_sum2_1(long long a);
int segment16_sum2_2(long long a);
int segment16_sum2_3(long long a);
int segment16_sum2_4(long long a);

int main(void)
{
    // Acquire credit card string for character and length validation.
    string credit_card1 = get_string("CC#: ");

    // Convert string to long long integer. Establish sum tallies.
    long long credit_card2 = atoll(credit_card1);
    int sum1 = 0;
    int sum2 = 0;

    // Card number validation.
    if (strlen(credit_card1) == 16)
    {
        sum1 = sum1 + segment16_sum1_1(credit_card2);
        printf("SUM 1: %i\n", sum1);
        sum1 = sum1 + segment16_sum1_2(credit_card2);
        printf("SUM 1: %i\n", sum1);
        sum1 = sum1 + segment16_sum1_3(credit_card2);
        printf("SUM 1: %i\n", sum1);
        sum1 = sum1 + segment16_sum1_4(credit_card2);
        printf("SUM 1: %i\n", sum1);

        sum2 = sum2 + segment16_sum2_1(credit_card2);
        printf("SUM 2: %i\n", sum2);
        sum2 = sum2 + segment16_sum2_2(credit_card2);
        printf("SUM 2: %i\n", sum2);
        sum2 = sum2 + segment16_sum2_3(credit_card2);
        printf("SUM 2: %i\n", sum2);
        sum2 = sum2 + segment16_sum2_4(credit_card2);
        printf("SUM 2: %i\n", sum2);
    }

    printf("%i\n", sum1);
    printf("%i\n", sum2);
}

// Doubles every other digit of the first quarter of a 16-length card, starting with the second digit, and adds total to sum 1.
int segment16_sum1_1(long long a)
{
    long first_quarter = a % 10000;
    long divisor1 = 100;
    long divisor2 = 10;
    int sum1 = 0;

    for (int i = 0; i < 2; i++)
    {
        int isolate1 = first_quarter % divisor1;
        int isolate2 = isolate1 % divisor2;
        int isolate3 = (isolate1 - isolate2) / divisor2;
        int doubled = isolate3 * 2;
        printf("whole -- %i\n", doubled);

        // If the sum is a two digit number, set sum to the sum of those two digits.
        if (sum1 > 9)
        {
            int sum1_A = (sum1 - (sum1 % 10)) / 10;
            int sum1_B = (sum1 % 10);
            doubled = sum1_A + sum1_B;
            printf("divided -- %i - %i\n", sum1_A, sum1_B);
        }

        sum1 = sum1 + doubled;
        divisor1 = divisor1 * 100;
        divisor2 = divisor2 * 100;

    }

    return sum1;
}

// Doubles every other digit of the second quarter of a 16-length card, starting with the second digit, and adds total to sum 1.
int segment16_sum1_2(long long a)
{
    long second_quarter = ((a - (a % 10000)) / 10000) % 10000;
    long divisor1 = 100;
    long divisor2 = 10;
    int sum1 = 0;

    for (int i = 0; i < 2; i++)
    {
        int isolate1 = second_quarter % divisor1;
        int isolate2 = isolate1 % divisor2;
        int isolate3 = (isolate1 - isolate2) / divisor2;
        int doubled = isolate3 * 2;
        printf("whole -- %i\n", doubled);

        // If the sum is a two digit number, set sum to the sum of those two digits.
        if (sum1 > 9)
        {
            int sum1_A = (sum1 - (sum1 % 10)) / 10;
            int sum1_B = (sum1 % 10);
            doubled = sum1_A + sum1_B;
            printf("divided -- %i - %i\n", sum1_A, sum1_B);
        }

        sum1 = sum1 + doubled;
        divisor1 = divisor1 * 100;
        divisor2 = divisor2 * 100;

    }

    return sum1;
}

// Doubles every other digit of the third quarter of a 16-length card, starting with the second digit, and adds total to sum 1.
int segment16_sum1_3(long long a)
{
    long third_quarter = ((a % 1000000000000) - (a % 100000000)) / 100000000;
    long divisor1 = 100;
    long divisor2 = 10;
    int sum1 = 0;

    for (int i = 0; i < 2; i++)
    {
        int isolate1 = third_quarter % divisor1;
        int isolate2 = isolate1 % divisor2;
        int isolate3 = (isolate1 - isolate2) / divisor2;
        int doubled = isolate3 * 2;
        printf("whole -- %i\n", doubled);

        // If the sum is a two digit number, set sum to the sum of those two digits.
        if (sum1 > 9)
        {
            int sum1_A = (sum1 - (sum1 % 10)) / 10;
            int sum1_B = (sum1 % 10);
            doubled = sum1_A + sum1_B;
            printf("divided -- %i - %i\n", sum1_A, sum1_B);
        }

        sum1 = sum1 + doubled;
        divisor1 = divisor1 * 100;
        divisor2 = divisor2 * 100;

    }

    return sum1;
}

// Doubles every other digit of the fourth quarter of a 16-length card, starting with the second digit, and adds total to sum 1.
int segment16_sum1_4(long long a)
{
    long fourth_quarter = (a - (a % 1000000000000)) / 1000000000000;
    long divisor1 = 100;
    long divisor2 = 10;
    int sum1 = 0;

    for (int i = 0; i < 2; i++)
    {
        int isolate1 = fourth_quarter % divisor1;
        int isolate2 = isolate1 % divisor2;
        int isolate3 = (isolate1 - isolate2) / divisor2;
        int doubled = isolate3 * 2;
        printf("whole -- %i\n", doubled);

        // If the sum is a two digit number, set sum to the sum of those two digits.
        if (sum1 > 9)
        {
            int sum1_A = (sum1 - (sum1 % 10)) / 10;
            int sum1_B = (sum1 % 10);
            doubled = sum1_A + sum1_B;
            printf("divided -- %i - %i\n", sum1_A, sum1_B);
        }

        sum1 = sum1 + doubled;
        divisor1 = divisor1 * 100;
        divisor2 = divisor2 * 100;

    }

    return sum1;
}

// Doubles every other digit of the first quarter of a 16-length card, starting with the first digit, and adds total to sum 1.
int segment16_sum2_1(long long a)
{
    long first_quarter = a % 10000;
    int sum2 = 0;

    sum2 = sum2 + (first_quarter % 10);
    sum2 = sum2 + ((first_quarter % 1000) - (first_quarter % 100)) / 100;
    printf("whole2 -- %i\n", sum2);

    return sum2;
}

// Doubles every other digit of the second quarter of a 16-length card, starting with the first digit, and adds total to sum 1.
int segment16_sum2_2(long long a)
{
    long second_quarter = ((a - (a % 10000)) / 10000) % 10000;
    int sum2 = 0;

    sum2 = sum2 + (second_quarter % 10);
    sum2 = sum2 + ((second_quarter % 1000) - (second_quarter % 100)) / 100;
    printf("whole2 -- %i\n", sum2);

    return sum2;
}

// Doubles every other digit of the third quarter of a 16-length card, starting with the first digit, and adds total to sum 1.
int segment16_sum2_3(long long a)
{
    long third_quarter = ((a % 1000000000000) - (a % 100000000)) / 100000000;
    int sum2 = 0;

    sum2 = sum2 + (third_quarter % 10);
    sum2 = sum2 + ((third_quarter % 1000) - (third_quarter % 100)) / 100;
    printf("whole2 -- %i\n", sum2);

    return sum2;
}

// Doubles every other digit of the fourth quarter of a 16-length card, starting with the first digit, and adds total to sum 1.
int segment16_sum2_4(long long a)
{
    long fourth_quarter = (a - (a % 1000000000000)) / 1000000000000;
    int sum2 = 0;

    sum2 = sum2 + (fourth_quarter % 10);
    sum2 = sum2 + ((fourth_quarter % 1000) - (fourth_quarter % 100)) / 100;
    printf("whole2 -- %i\n", sum2);

    return sum2;
}