一种快速开平方并取倒数算法

float InvSqrt(float x )
{
	float xhalf = 0.5f * x;
	int i = *( int *)& x;
	i = 0x5f3759df - ( i>>1);
	x = *( float *)& i;
	x = x * (1.5f - xhalf * x * x);

	return x;
}

#include "stdafx.h"
#include <stdio.h>  
#include <stdlib.h>
#include <windows.h>
#include <math.h>

// 开平方取倒数
float InvSqrt(float x )
{
	float xhalf = 0.5f * x;
	int i = *( int *)& x;
	i = 0x5f3759df - ( i>>1);
	x = *( float *)& i;
	x = x * (1.5f - xhalf * x * x);

	return x;
}

int main()
{
	// 比较精度
	float val = 0.0f;
	val = 1.0f;
	printf("计算精度比较: \n");
	printf("输入值:   %f    快速算法: %f    VC函数: %f \n", val, InvSqrt(val), 1.0f / sqrt(val));

	val = 16.0f;
	printf("输入值:  %f    快速算法: %f    VC函数: %f \n", val, InvSqrt(val), 1.0f / sqrt(val));

	val = 25.0f;
	printf("输入值:  %f    快速算法: %f    VC函数: %f \n", val, InvSqrt(val), 1.0f / sqrt(val));

	val = 100.0f;
	printf("输入值: %f    快速算法: %f    VC函数: %f \n", val, InvSqrt(val), 1.0f / sqrt(val));

	printf("\n计算性能比较: \n");
	int count = 1000000;
	DWORD timeStart = 0, timeEnd = 0;
	timeStart = GetTickCount();
	for (int i = 0; i < count; i++)  
	{  
		val = InvSqrt(100.0f);  
	}  
	timeEnd  = GetTickCount();  
	printf("快速算法耗时: %f \n", (timeEnd - timeStart) * 0.001); 

	timeStart = GetTickCount();
	for (int i = 0; i < count; i++)  
	{  
		val = 1.0f / sqrt(100.0f);  
	}  
	timeEnd  = GetTickCount();  
	printf("VC函数耗时:   %f \n", (timeEnd - timeStart) * 0.001); 

	printf("\n");
	system("pause");
	return 0;
}