LUHN

Intro

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.

The task is to check if a given string is valid.

Validating a Number

Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.

Example 1: valid credit card number

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4539 1488 0343 6467

The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling

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4_3_ 1_8_ 0_4_ 6_6_

If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:

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8569 2478 0383 3437

Then sum all of the digits:

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8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

Example 2: invalid credit card number

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8273 1232 7352 0569

Double the second digits, starting from the right

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7253 2262 5312 0539

Sum the digits

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7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57

57 is not evenly divisible by 10, so this number is not valid.

Task

Given a number determine whether or not it is valid per the Luhn formula.

The Code

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#include "luhn.h"
#include <bits/stdint-uintn.h>
#include <cctype>
#include <cstdio>
#include <string>
#include <vector>

namespace luhn {
bool valid(std::string test) {
std::vector<int8_t> var{};
for (auto c : test) {
    if (std::isspace(c))
    continue;
    if (std::isdigit(c)) {
    var.push_back(c - '0');
    continue;
    }
    return false;
}

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#include <string>
#if !defined(LUHN_H)
#define LUHN_H
#include "string"

namespace luhn {
    bool valid(std::string);
}  // namespace luhn

#endif // LUHN_H

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