#include <cstdio>
#include <cassert>
#include <vector>

using prime_vec_t = std::vector<int>;

// https://en.wikipedia.org/wiki/Trial_division
auto prime_factors(int num) -> prime_vec_t {
    prime_vec_t primes{};
    int f = 2;
    while (num > 1) {
        if (num % f == 0) {
            primes.emplace_back(f);
            num = num / f;
        } else {
            f += 1;
        }
    }
    return primes;
}

auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
    auto factors = prime_factors(20);
    for (auto const& factor : factors) {
        std::printf("%d, ", factor);
    }
    std::printf("\n");
    return 0;
}