Original code: // https://en.wikipedia.org/wiki/Fast_inverse_square_root#Overview_of_the_code

_original_inverse_square_root.c

float Q_rsqrt_original(float number)
{
	long i;
	float x2, y;
	const float threehalfs = 1.5F;

	x2 = number * 0.5F;
	y = number;
	i = *(long*)&y;                       // evil floating point bit level hacking
	i = 0x5f3759df - (i >> 1);               // what the fuck? 
	y = *(float*)&i;
	y = y * (threehalfs - (x2 * y * y));   // 1st iteration
//	y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

	return y;
}

_refactored_inverse_square_root.c

inline float InverseSquare_intial_fast_guess(float number)
{
	long i = *(long*)&number;                       // evil floating point bit level hacking
	i = 0x5f3759df - (i >> 1);               // what the fuck? 
	return *(float*)&i;
}


inline float InverseSquare_improve_estimate_with_newtons_method(float number, float estimate)
{
	const float x2 = number * 0.5F;
	return estimate * (1.5F - (x2 * estimate * estimate));
}


float Q_get_inverse_square_root(float number)
{
	float estimate = InverseSquare_intial_fast_guess(number);
	estimate = InverseSquare_improve_estimate_with_newtons_method(number, estimate); // 1st iteration
	// estimate = improve_estimate_with_newtons_method(number, estimate); // 2nd iteration, this can be removed
	return estimate;
}

everything_with-tests.cpp

#include "pch.h"
#include "CppUnitTest.h"

using namespace Microsoft::VisualStudio::CppUnitTestFramework;

// https://en.wikipedia.org/wiki/Fast_inverse_square_root#Overview_of_the_code

inline float InverseSquare_intial_fast_guess(float number)
{
	long i = *(long*)&number;                       // evil floating point bit level hacking
	i = 0x5f3759df - (i >> 1);               // what the fuck? 
	return *(float*)&i;
}


inline float InverseSquare_improve_estimate_with_newtons_method(float number, float estimate)
{
	const float x2 = number * 0.5F;
	return estimate * (1.5F - (x2 * estimate * estimate));
}


float Q_get_inverse_square_root(float number)
{
	float estimate = InverseSquare_intial_fast_guess(number);
	estimate = InverseSquare_improve_estimate_with_newtons_method(number, estimate); // 1st iteration
	// estimate = improve_estimate_with_newtons_method(number, estimate); // 2nd iteration, this can be removed
	return estimate;
}

float Q_rsqrt_original(float number)
{
	long i;
	float x2, y;
	const float threehalfs = 1.5F;

	x2 = number * 0.5F;
	y = number;
	i = *(long*)&y;                       // evil floating point bit level hacking
	i = 0x5f3759df - (i >> 1);               // what the fuck? 
	y = *(float*)&i;
	y = y * (threehalfs - (x2 * y * y));   // 1st iteration
//	y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

	return y;
}

namespace UnitTest1
{
	TEST_CLASS(UnitTest1)
	{
	public:
		TEST_METHOD(TestMethod1)
		{
			for (float i = 0; i < 10000; i += 3.14f)
			{
				Assert::AreEqual(Q_rsqrt_original(i), Q_get_inverse_square_root(i));
			}
		}
	};
}