Validate a credit card using the Luhn formula.

credit.c

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

long get_number();
int get_number_length(long number);
int get_sum_of_digits(long number);
long get_first_n_digits(long number, int n);

bool validate_number(long number, long number_length);

int main(void) {
  long number = get_number();
  long number_length = get_number_length(number);
  
  bool valid = validate_number(number, number_length);

  if (!valid) {
    printf("INVALID\n");
  } else {
    long first_digit = get_first_n_digits(number, 1);
    long first_two_digits = get_first_n_digits(number, 2);
    switch(number_length) {
      case 15:
        if (first_two_digits == 34 || first_two_digits == 37) {
          printf("AMEX\n");
        } else {
          printf("INVALID\n");
        }
        break;
      case 16:
        if (first_digit == 4) {
          printf("VISA\n");
        } else {
          switch(first_two_digits) {
          case 51:
          case 52:
          case 53:
          case 54:
          case 55:
            printf("MASTERCARD\n");
            break;
          default:
            printf("INVALID\n");
          }   
        }
        break;
      case 13:
        if (first_digit == 4) {
          printf("VISA\n");
        } else {
          printf("INVALID\n");
        }
        break;
      default:
        printf("INVALID\n");
        break;
    }
  }
}

long get_number() {
  printf("Number: ");
  long number = get_long_long();
  
  if (number > 0 && 13 <= get_number_length(number) && get_number_length(number) <= 16) {
    return number;
  } else {
    printf("INVALID\n");
    exit(0);
  }
}

int get_number_length(long number) {
  int i;
  for (i = 0; number != 0; i++, number = number / 10){};
  return i;
}

int get_sum_of_digits(long number) {
  int sum = 0;
  int number_length = get_number_length(number);
  for (int i = 0; i < number_length; i++) {
    long modulo_divider = pow(10, i);
    long current_digit = (number / modulo_divider) % 10;
    sum += current_digit;
  }
  return sum;
}

long get_first_n_digits(long number, int n) {
  long divider = pow(10, n);
  if (number < divider) {
    return number;
  }
  return get_first_n_digits(number / 10, n);
}

bool validate_number(long number, long number_length) {
  long even_numbers_sum = 0;
  long odd_numbers_sum = 0;
  for (int i = 0; i < number_length; i++) {
    int counter = i + 1;
    long modulo_divider = pow(10, i);
    
    long current_number = (number / modulo_divider) % 10;
    
    if (counter % 2 == 0) {
      int temporary_number = current_number * 2;
      even_numbers_sum += get_sum_of_digits(temporary_number);
    } else {
      odd_numbers_sum += current_number;
    }
  }
  
  long final_sum = even_numbers_sum + odd_numbers_sum;
  return final_sum % 10 == 0;
}