Some constexpr stuff

constexpr.cpp

namespace detail {
constexpr auto factorial(size_t n) -> size_t {
    return n <= 1 ? 1 : (n * factorial(n - 1));
}

constexpr auto binomial(size_t n, size_t k) -> size_t {
    if (n == k || k == 0) {
        return 1;
    } else {
        if (n == 0) {
            return 0;
        } else {
            return binomial(n - 1, k - 1) + binomial(n - 1, k);
        }
    }
}

constexpr auto multiset_coefficient(size_t n, size_t k) -> size_t {
    return binomial(n + k - 1, n);
}
} // namespace detail