Collatz Conjecture

collatz.cpp

#include <iostream>
#include <ctime>
#include <tuple>

constexpr const std::tuple<unsigned long, unsigned long> max(const unsigned int a, const unsigned int b)
{
    const auto c = [](auto&& s, const unsigned long n) constexpr -> const unsigned int { 
        if (n == 1) return 1;
                    return 1 + s(s, n % 2 ? n * 3 + 1 : n / 2);
    };
    
    std::tuple<unsigned long, unsigned long> t;
    for (unsigned int i = a ; i <= b ; ++i)
    {
        const auto s = c(c, i);
        if (s > std::get<0>(t)) t = std::make_tuple(s, i);
    }
    return t;
}

int main()
{
    const unsigned int N = 1000000;

    const auto a = std::clock();
    const auto M = max(1, N);
    const auto d = std::clock() - a;
    
    std::cout << "Time: " << d / (double)(CLOCKS_PER_SEC / 1000) << " ms" << std::endl;
    std::cout << "Max: " << std::get<1>(M) << " (" << std::get<0>(M) << ")" << std::endl;
    return 0;
}