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| #include <cs50.h> | |
| #include <stdio.h> | |
| #include <math.h> | |
| long exp10(int exponent); | |
| long get_digit(long cc_num, int pos); | |
| bool has_exact_digit(long cc_num, int digit); | |
| string check_credit_card_number(long cc_num); | |
| int main(void) | |
| { | |
| long cc_num = get_long("Number: "); | |
| printf("%s\n", check_credit_card_number(cc_num)); | |
| } | |
| string check_credit_card_number(long cc_num) | |
| { | |
| // validate checksum with Luhn’s Algorithm | |
| long checksum = 0; | |
| int digit_count; | |
| // pos starts at 0, which corresponded to right-most digit | |
| // loop while 10^pos < cc_num | |
| for (int pos = 0; exp10(pos) < cc_num; pos++) | |
| { | |
| long digit = get_digit(cc_num, pos); | |
| // every other digit, starting with the number’s second-to-last digit | |
| // we need to x2 and sum all digit. | |
| // when pos starts with 0 from last. that means said condition are odd pos | |
| bool odd_pos = pos % 2 == 1; | |
| if (odd_pos) | |
| { | |
| long doubled_digit = digit * 2; | |
| if (doubled_digit >= 10) | |
| { | |
| long fst_digit = doubled_digit / 10; | |
| long snd_digit = doubled_digit % 10; | |
| checksum += fst_digit + snd_digit; | |
| } | |
| else | |
| { | |
| checksum += doubled_digit; | |
| } | |
| } | |
| else | |
| { | |
| checksum += digit; | |
| } | |
| // at last round of the loop, pos + 1 will determine digit count | |
| // since pos starts at 0 = 1st digit | |
| digit_count = pos + 1; | |
| } | |
| // valid only last checksum digit is 0 | |
| if (checksum % 10 != 0) | |
| { | |
| return "INVALID"; | |
| } | |
| long first_digit = cc_num / exp10(digit_count - 1); | |
| long first_two_digits = cc_num / exp10(digit_count - 2); | |
| // AMEX starts with 34, 37, 15-digit numbers | |
| if ((first_two_digits == 34 || first_two_digits == 37) && has_exact_digit(cc_num, 15)) | |
| { | |
| return "AMEX"; | |
| } | |
| // MASTERCARD starts with 51, 52, 53, 54, or 55, 16-digit numbers | |
| if (first_two_digits >= 51 && first_two_digits <= 55 && has_exact_digit(cc_num, 16)) | |
| { | |
| return "MASTERCARD"; | |
| } | |
| // VISA starts with 4, 13- and 16-digit numbers. | |
| if (first_digit == 4 && (has_exact_digit(cc_num, 13) || has_exact_digit(cc_num, 16))) | |
| { | |
| return "VISA"; | |
| } | |
| // The rest are invalid | |
| return "INVALID"; | |
| } | |
| long exp10(int exponent) | |
| { | |
| return pow(10, (double)exponent); | |
| } | |
| // getting digit of a certain position | |
| // for example: `1234`, digit at pos=1 -> 3 | |
| // 1234 % 10^(1+1) / 10^1 | |
| // => 1234 % 100 / 10 | |
| // => 34 / 10 | |
| // => 3 (dividing long yields long) | |
| long get_digit(long cc_num, int pos) | |
| { | |
| return (cc_num % exp10(pos + 1)) / exp10(pos); | |
| } | |
| bool has_exact_digit(long cc_num, int digit) | |
| { | |
| return cc_num >= exp10(digit - 1) && cc_num < exp10(digit); | |
| } |
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