Implement exercise - luhn

[junming403]

No description provided.

[junming403]

Implement new exercise luhn, ready for review.

[arcuru]

The tests look good, but a few things with the formatting and the example need fixing :)

[siebenschlaefer]

Currently all the valid numbers in the tests have an even number of spaces or all the digits are 0. Therefore the following (invalid) attempt passes:

bool luhn::valid(const std::string& str) {
    int sum = 0, digit_count = 0, factor = 1 + str.size() % 2;
    for (auto i = 0u; i < str.size(); ++i) {
        if (str[i] == ' ') continue;
        if (str[i] < '0' || '9' < str[i]) return false;
        ++digit_count;
        factor = 3 - factor;
        int digit = (str[i] - '0') * factor;
        if (digit > 9) digit -= 9;
        sum += digit;
    }
    return digit_count > 1 && sum % 10 == 0;
}

Please add a test where the the number is valid and it has an an odd number of spaces, e.g.

BOOST_AUTO_TEST_CASE(valid_number_with_an_odd_number_of_spaces)
{
    const bool actual = luhn::valid("095 245  88");
    const bool expected {true};
    BOOST_TEST(expected == actual);
}

[arcuru]

Hold on. @junming403 please keep the original test that matches the problem specifications https://github.com/exercism/problem-specifications/blob/master/exercises/luhn/canonical-data.json

@siebenschlaefer that does seem like it might be a problem with the tests, however we do try to keep the test suites here identical the main problem specifications. Please change the canonical test suite in the problem-specifications repo so all the other tracks can benefit too :)

[siebenschlaefer]

@patricksjackson Thanks, I didn't realize the rules are that tight. I submitted a PR to the problem-specifications repo: exercism/problem-specifications#1500

[arcuru]

@siebenschlaefer It's not really that the rules are tight, it's that I don't want to be in a position to maintain a good set of tests, so it's easier to let the central team do it. There are actually some places where we might have an extra test for a C++ specific thing, but those are the only exceptions we generally make.

@junming403 I'll come back and review your updates when I get the chance, it might take until the weekend.

[elyashiv]

digit should be const

[junming403]

Updated according to review

[arcuru]

Thanks!