usertype_bitfields.cpp

#define SOL_ALL_SAFETIES_ON 1
#include <sol/sol.hpp>

#include <cstddef>
#include <cstdint>
#include <climits>
#include <type_traits>

namespace itsy_bitsy {

	template <std::size_t sz, typename C = void>
	struct bit_type {
		typedef uint64_t type;
	};

	template <std::size_t sz>
	struct bit_type<sz, std::enable_if_t<(sz <= 1)>> {
		typedef bool type;
	};

	template <std::size_t sz>
	struct bit_type<sz,
	     std::enable_if_t<(sz > 2 && sz <= 16)>> {
		typedef uint16_t type;
	};

	template <std::size_t sz>
	struct bit_type<sz,
	     std::enable_if_t<(sz > 16 && sz <= 32)>> {
		typedef uint32_t type;
	};

	template <std::size_t sz>
	struct bit_type<sz,
	     std::enable_if_t<(sz > 32 && sz <= 64)>> {
		typedef uint64_t type;
	};

	template <std::size_t sz>
	using bit_type_t = typename bit_type<sz>::type;

	template <typename T, typename V>
	bool vcxx_warning_crap(std::true_type, V val) {
		return val != 0;
	}

	template <typename T, typename V>
	T vcxx_warning_crap(std::false_type, V val) {
		return static_cast<T>(val);
	}

	template <typename T, typename V>
	auto vcxx_warning_crap(V val) {
		return vcxx_warning_crap<T>(
		     std::is_same<bool, T>(), val);
	}

	template <typename Base, std::size_t bit_target = 0x0,
	     std::size_t size = 0x1>
	void write(Base& b, bit_type_t<size> bits) {
		typedef bit_type_t<sizeof(Base) * CHAR_BIT>
		     aligned_type;
		static const std::size_t aligned_type_bit_size
		     = sizeof(aligned_type) * CHAR_BIT;
		static_assert(
		     sizeof(Base) * CHAR_BIT >= (bit_target + size),
		     "bit offset and size are too large for the "
		     "desired structure.");
		static_assert((bit_target % aligned_type_bit_size)
		          <= ((bit_target + size)
		               % aligned_type_bit_size),
		     "bit offset and size cross beyond largest "
		     "integral constant boundary.");

		const std::size_t aligned_target
		     = (bit_target + size) / aligned_type_bit_size;
		const aligned_type bits_left
		     = static_cast<aligned_type>(
		          bit_target - aligned_target);
		const aligned_type shifted_mask
		     = ((static_cast<aligned_type>(1) << size) - 1)
		     << bits_left;
		const aligned_type compl_shifted_mask = ~shifted_mask;
		// Jump by native size of a pointer to target
		// then OR the bits
		aligned_type* jumper = static_cast<aligned_type*>(
		     static_cast<void*>(&b));
		jumper += aligned_target;
		aligned_type& aligned = *jumper;
		aligned &= compl_shifted_mask;
		aligned
		     |= (static_cast<aligned_type>(bits) << bits_left);
	}

	template <typename Base, std::size_t bit_target = 0x0,
	     std::size_t size = 0x1>
	bit_type_t<size> read(Base& b) {
		typedef bit_type_t<sizeof(Base) * CHAR_BIT>
		     aligned_type;
		typedef bit_type_t<size> field_type;
		static const std::size_t aligned_type_bit_size
		     = sizeof(aligned_type) * CHAR_BIT;
		static_assert(
		     sizeof(Base) * CHAR_BIT >= (bit_target + size),
		     "bit offset and size are too large for the "
		     "desired structure.");
		static_assert((bit_target % aligned_type_bit_size)
		          <= ((bit_target + size)
		               % aligned_type_bit_size),
		     "bit offset and size cross beyond largest "
		     "integral constant boundary.");

		const std::size_t aligned_target
		     = (bit_target + size) / aligned_type_bit_size;
		const aligned_type bits_left
		     = static_cast<aligned_type>(
		          bit_target - aligned_target);
		const aligned_type mask
		     = (static_cast<aligned_type>(1) << size) - 1;
		// Jump by native size of a pointer to target
		// then OR the bits
		aligned_type* jumper = static_cast<aligned_type*>(
		     static_cast<void*>(&b));
		jumper += aligned_target;
		const aligned_type& aligned = *jumper;
		aligned_type field_bits
		     = (aligned >> bits_left) & mask;
		field_type bits
		     = vcxx_warning_crap<field_type>(field_bits);
		return bits;
	}

} // namespace itsy_bitsy

#include <iostream>

#if defined(_MSC_VER) || defined(__MINGW32__)
#pragma pack(1)
struct alignas(sizeof(uint32_t)) flags_t {
#else
struct __attribute__((packed, aligned(sizeof(uint32_t))))
flags_t {
#endif
	uint8_t C : 1;
	uint8_t N : 1;
	uint8_t PV : 1;
	uint8_t _3 : 1;
	uint8_t H : 1;
	uint8_t _5 : 1;
	uint8_t Z : 1;
	uint8_t S : 1;
	uint16_t D : 14;
} flags { 0, 0, 0, 0, 0, 0, 0, 0, 0 };

int main() {
	std::cout << "=== usertype_bitfields ===" << std::endl;
#ifdef __MINGW32__
	std::cout << "MinGW Detected, packing structs is broken in "
	             "MinGW and this test may fail"
	          << std::endl;
#endif
	sol::state lua;
	lua.open_libraries();

	lua.new_usertype<flags_t>("flags_t",
	     "C",
	     sol::property(itsy_bitsy::read<flags_t, 0>,
	          itsy_bitsy::write<flags_t, 0>),
	     "N",
	     sol::property(itsy_bitsy::read<flags_t, 1>,
	          itsy_bitsy::write<flags_t, 1>),
	     "D",
	     sol::property(itsy_bitsy::read<flags_t, 8, 14>,
	          itsy_bitsy::write<flags_t, 8, 14>));

	lua["f"] = std::ref(flags);

	lua.script(R"(
    print(f.C)
    f.C = true;
    print(f.C)

    print(f.N)
    f.N = true;
    print(f.N)

    print(f.D)
    f.D = 0xDF;
    print(f.D)
)");

	bool C = flags.C;
	bool N = flags.N;
	uint16_t D = flags.D;

	std::cout << std::hex;
	std::cout << "sizeof(flags): " << sizeof(flags)
	          << std::endl;
	std::cout << "C: " << C << std::endl;
	std::cout << "N: " << N << std::endl;
	std::cout << "D: " << D << std::endl;

	SOL_ASSERT(C);
	SOL_ASSERT(N);
	SOL_ASSERT(D == 0xDF);

	std::cout << std::endl;

	return 0;
}