# 位操作技巧

1. 取第n位的值（从右边)

int r = (v >> n) & 1;

2.把某一位置1（从右边)

v = v | (1 << n);

3.把某一位取反（从右边)

v = v ^ (1 << n);

4.把最后一个1置0

v = v & (v - 1);

5.把最后一个0置1

v = v | (v + 1);

6.奇偶判断

(v & 1)

7.计算绝对值

int a = v >> 31;
int r = (v ^ a) - a;

8. 取模

//v % (2^n)可以转化为去掉n位之前的值
int mod = v & (2^n - 1); 

9.乘2的n次方

int r = v << n;

10.除2的n次方

int r = v >> n;

11.除2的余

int r = v & 1;

12. 判断语句 v == a ? b : a;

v = a ^ b ^ v;

13.a与b互换

void swap(int& x , int& y)
{
x ^= y;
y ^= x;
x ^= y;
}

14.否为2的幂

bool power2(int v)
{
return ((v & (v - 1)) == 0) && (v != 0);
}

15,求整数的平均值

int average(int x, int y)
{
return (x & y) + ((x ^ y) >> 1);
}

16.取v的符号

int v;
int sign;
//返回-1或0
sign = -(v < 0);
sign = v >> (sizeof(int) * CHAR_BIT - 1);
//返回-1或+1
sign = +1 | (v >> (sizeof(int) * CHAR_BIT - 1);
//返回-1，0，1
sign = (v != 0) | -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));
sign = (v != 0) | (v >> (sizeof(int) * CHAR_BIT - 1));
sign = (v > 0) - (v < 0);

17.判断两数符号是否相反

bool f = ((a ^ b) < 0);

18.没有分支语句的min或者max函数

//min(a,b)
int r = b ^ ((a ^ b) & -(a < b));
//max(a,b)
int r = a ^ ((a ^ b) & -(a < b));

19.是否取反（不带判断语句）

bool negative;
int r = (v ^ -negative) + negative;

unsigned int r = a ^ ((a ^ b) & mask);

21.计算二进制v中1的个数n

unsigned int v;
unsigned int n;
for (n = 0; v; n++)
v &= v - 1

22.二进制位反转

unsigned int
reverse(register unsigned int x)
{
x = (((x & 0xaaaaaaaa) >> 1) | ((x & 0x55555555) << 1));
x = (((x & 0xcccccccc) >> 2) | ((x & 0x33333333) << 2));
x = (((x & 0xf0f0f0f0) >> 4) | ((x & 0x0f0f0f0f) << 4));
x = (((x & 0xff00ff00) >> 8) | ((x & 0x00ff00ff) << 8));
return((x >> 16) | (x << 16));}

22.二进制位反转2

unsigned int
reverse(register unsigned int x)
{
register unsigned int y = 0x55555555;
x = (((x >> 1) & y) | ((x & y) << 1));
y = 0x33333333;
x = (((x >> 2) & y) | ((x & y) << 2));
y = 0x0f0f0f0f;
x = (((x >> 4) & y) | ((x & y) << 4));
y = 0x00ff00ff;
x = (((x >> 8) & y) | ((x & y) << 8));
return((x >> 16) | (x << 16));
}

23.浮点数与0的判断

#define FasI(f)  (*((int *) &(f)))
#define FasUI(f) (*((unsigned int *) &(f)))
#define lt0(f)    (FasUI(f) > 0x80000000U)
#define le0(f)    (FasI(f) <= 0)
#define gt0(f)    (FasI(f) > 0)
#define ge0(f)    (FasUI(f) <= 0x80000000U)

24.下一个2的幂次方

unsigned int nextpowerof2(register unsigned int x)
{
x |= (x >> 1);
x |= (x >> 2);
x |= (x >> 4);
x |= (x >> 8);
x |= (x >> 16);
return(x+1);
}

24.保留最高位的1 （上一个2的幂次方）

unsigned int lastpowerof2(register unsigned int x)
{
x |= (x >> 1);
x |= (x >> 2);
x |= (x >> 4);
x |= (x >> 8);
x |= (x >> 16);
return(x & ~(x >> 1));
}

posted @ 2019-05-31 16:55  游戏程序员刘宇  阅读(470)  评论(0编辑  收藏