Unusual Speed Difference between Python and C++

Question

I recently wrote a short algorithm to calculate happy numbers in python. The program allows you to pick an upper bound and it will determine all the happy numbers below it.

Answer 1

Here is another way that relies on memorising all the numbers already explored. I obtain a factor x4-5, which is oddly stable against DrAsik's code for 1000 and 1000000, I expected the cache to be more efficient the more numbers we were exploring. Otherwise, the same kind of classic optimizations have been applied. BTW, if the compiler accepts NRVO (/RNVO ? I never remember the exact term) or rvalue references, we wouldn't need to pass the vector as an out parameter.

NB: micro-optimizations are still possible IMHO, and moreover the caching is naive as it allocates much more memory than really needed.

enum Status {
    never_seen,
    being_explored,
    happy,
    unhappy
};

char const* toString[] = { "never_seen", "being_explored", "happy", "unhappy" };


inline size_t sum_squares(size_t i) {
    size_t s = 0;
    while (i) {
        const size_t digit = i%10;
        s += digit * digit;
        i /= 10;
    }
    return s ;
}

struct Cache {
    Cache(size_t dim) : m_cache(dim, never_seen) {}
    void set(size_t n, Status status) {
        if (m_cache.size() <= n) {
            m_cache.resize(n+1, never_seen);
        }
        m_cache[n] = status;
        // std::cout << "(c[" << n << "]<-"< m_cache;
};

void search_happy_lh(size_t upper_bound, std::vector & happy_numbers)
{
    happy_numbers.clear();
    happy_numbers.reserve(upper_bound); // it doesn't improve much the performances

    Cache cache(upper_bound+1);
    std::vector current_stack;

    cache.set(1,happy);
    happy_numbers.push_back(1);
    for (size_t i = 2; i<=upper_bound ; ++i) {
        // std::cout << "\r" << i << std::flush;
        current_stack.clear();
        size_t s= i;
        while ( s != 1 && cache[s]==never_seen)
        {
            current_stack.push_back(s);
            cache.set(s, being_explored);
            s = sum_squares(s);
            // std::cout << " - " << s << std::flush;
        }
        const Status update_with = (cache[s]==being_explored ||cache[s]==unhappy) ? unhappy : happy;
        // std::cout << " => " << s << ":" << toString[update_with] << std::endl;
        for (size_t j=0; j!=current_stack.size(); ++j) {
            cache.set(current_stack[j], update_with);
        }
        if (cache[i] == happy) {
            happy_numbers.push_back(i);
        }
    }
}