cs50-credit-example.c

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

int last_digit(long num);
bool ends_with_zero(int num);
bool is_even(int num);
bool luhn_validate(long num);
int first_n_digits(long num, int len, int n);
int num_len(long num);
bool isAmex(long num);
bool isVisa(long num);
bool isMc(long num);

int main(void)
{
   long num = get_long("Number: ");

    if (luhn_validate(num) == false)
    {
        printf("INVALID\n");
        return 0;
    }

    if (isAmex(num))
        printf("AMEX\n");

    else if (isVisa(num))
        printf("VISA\n");

    else if (isMc(num))
        printf("MASTERCARD\n");

    else
        printf("INVALID\n");
}

int last_digit(long num)
{
  return num % 10;
}

bool ends_with_zero(int num)
{
  return last_digit(num) == 0;
}

bool is_even(int num)
{
  return num % 2 == 0;
}

bool luhn_validate(long num)
{
  int current;
  int sum_of_evens = 0;
  int sum_of_odds = 0;
  int i = 1;

  while(num)
  {
    current = last_digit(num);

    if(is_even(i))
    {
      current = current * 2;

      if(current > 9)
      {
         sum_of_evens += last_digit(current);
         current /= 10;
      }

      sum_of_evens += current;
    }
    else
    {
      sum_of_odds += current;
    }

    num /= 10;
    i += 1;
  }

  return ends_with_zero(sum_of_evens + sum_of_odds);
}

int first_n_digits(long num, int len, int n)
{
    return num / pow(10, len - n);
}

int num_len(long num)
{
    return log10(num) + 1;
}

bool isAmex(long num)
{
    const int AMEX_LEN = 15;
    const int AMEX_HEAD_LARGE = 37;
    const int AMEX_HEAD_SMALL = 34;
    int len = num_len(num);

    if (len == AMEX_LEN)
    {
        int first_2 = first_n_digits(num, len, 2);

        if (first_2 == AMEX_HEAD_SMALL || first_2 == AMEX_HEAD_LARGE)
            return true;
    }
    return false;
}

bool isVisa(long num)
{
    const int VISA_SHORT_LEN = 13;
    const int VISA_LONG_LEN = 16;
    const int VISA_FIRST_DIGIT = 4;
    int len = num_len(num);

    if ((len == VISA_SHORT_LEN) || (len == VISA_LONG_LEN))
    {
        int first = first_n_digits(num, len, 1);

        if (first == VISA_FIRST_DIGIT)
        {
            return true;
        }
    }
    return false;
}

bool isMc(long num)
{
    const int MC_LEN = 16;
    const int MC_MAX = 55;
    const int MC_MIN = 51;
    int len = num_len(num);

    if (len == MC_LEN)
    {
        int first_2 = first_n_digits(num, len, 2);

        if ((first_2 >= MC_MIN) && (first_2 <= MC_MAX))
            return true;
    }
    return false;
}