Implementation of Cows and Bulls game

cows_bulls.cpp

#include <cstdint>
#include <algorithm>
#include <tuple>

template <class... Ts>
auto constexpr persist(Ts volatile... ts)
{ std::tuple{ ts... }; }

auto constexpr hamming_weight(std::uint16_t const mask,
	std::uint16_t const value)
{
	auto ret = value & mask;
	ret = ((ret & 0xAAAA) >> 1) + (ret & 0x5555);
	ret = ((ret & 0xCCCC) >> 2) + (ret & 0x3333);
	ret = ((ret & 0xF0F0) >> 4) + (ret & 0x0F0F);
	ret = ((ret & 0xFF00) >> 8) + (ret & 0x00FF);
	return ret;
}

auto constexpr generate(std::size_t const n,
	std::uint16_t const key, std::uint16_t const guess)
{
	auto const mask = 0xFFFF >> (16 - n);
	auto const diff = key ^ guess;

	auto const key_misplaced   = hamming_weight(mask, key   & diff);
	auto const guess_misplaced = hamming_weight(mask, guess & diff);
	
	auto const bulls = n - key_misplaced - guess_misplaced;
	auto const cows  = std::min(key_misplaced, guess_misplaced) * 2;
	
	return std::tuple{ bulls, cows };
}

int main(int argc, char **argv)
{
	#if 1
	auto const key   = static_cast<std::uint16_t>(argv[0][0]);
	auto const guess = static_cast<std::uint16_t>(argv[0][1]);
	#else
	auto constexpr key   = std::uint16_t{ 0b1011'1111 };
	auto constexpr guess = std::uint16_t{ 0b1101'1110 };
	#endif

	auto const [bulls, cows] = generate(8, key, guess);
	persist(bulls, cows);
}