is_nan

nan.cpp

// https://en.wikipedia.org/wiki/Half-precision_floating-point_format

#include <cmath>
#include <iostream>

using half = std::uint16_t;

bool is_nan(const half a) {
	return (((a >> 10) & 0x1f) == 0x1f) && (a & 0x3ff);
}

bool is_inf(const half a) {
	return (((a >> 10) & 0x1f) == 0x1f) && !(a & 0x3ff);
}


float half2float(const half a) {
	const auto f = (static_cast<std::uint32_t>(a & 0x8000) << 16) | (static_cast<std::uint32_t>(((a >> 10) & 0x1f) + 127 - 1023) << 23) | (static_cast<std::uint32_t>(a & 0x3ff) << 13);
	return *reinterpret_cast<const float*>(&f);
}

half float2half(const float a) {
	const auto b = *reinterpret_cast<const std::uint32_t*>(&a);
	const auto h = static_cast<std::uint16_t>((b & 0x80000000) >> 16) | static_cast<std::uint16_t>((((b >> 23) & 0xff) + 1023 - 127) << 10) | (static_cast<std::uint16_t>(b >> 13) & 0x3ff);
	return h;
}


int main(int argc, char** argv) {
	const float nan = NAN;
	//const float nan = INFINITY;
	const auto nan_h = float2half(nan);

	std::printf("nan   = %e\n", nan);
	std::printf("han_h = 0x%04hx\n", nan_h);

	if (is_nan(nan_h)) {
		std::cout<< "NaN" <<std::endl;
	}
	if (is_inf(nan_h)) {
		std::cout<< "inf" <<std::endl;
	}
}