Calcuration the Collatz conjecture during compilation time.

collatz.cpp

#include <iostream>
#include <cstdint>

// Calcuration the Collatz conjecture during compilation time.

template<bool EVEN, uint64_t N>
struct InnerCollatz {
    static const uint64_t next = N / 2;
    static const uint64_t value = InnerCollatz<next % 2 == 0, next>::value;
};

template<uint64_t N>
struct InnerCollatz<false, N> {
    static const uint64_t next = (N * 3) + 1;
    static const uint64_t value = InnerCollatz<next % 2 == 0, next>::value;
};

template<>
struct InnerCollatz<false, 1> {
    static const uint64_t value = 1;
};

template<uint64_t N>
struct Collatz {
    static const uint64_t value = InnerCollatz<N % 2 == 0, N>::value;
};

int main() {
    static_assert(Collatz<1>::value == 1);
    static_assert(Collatz<123456789>::value == 1);
    std::cout << Collatz<123456789>::value << std::endl;
}