【收集向】位操作技巧 bitwise operation trick

来源：互联网发布：centos 7 gcc 安装包编辑：程序博客网时间：2024/06/05 21:10

参考:

http://www.codeproject.com/Articles/28681/Bitwise-Operation-Explained
http://blog.csdn.net/haoni123321/article/details/7415498

//=============Let`s begin================//

bitwise operation trick

交换两数

void bitwise_swap(int& a, int& b) {    a ^= b;    b ^= a;    a ^= b;}

取反

int bitwise_sign_reverse(int& a) {    return ~a + 1;}

取绝对值

// version 1int bitwise_abs(int& a) {    return (a >> 31) ? ( ~a + 1 ) : a;}// version 2int bitwise_abs(int& a) {    int highest = a >> 31;    return (highest & ( ~a + 1 )) | (~highest & a);}// version 3int bitwise_abs(int& a) {    int i = a >> 31;    return ((a^i) - i);}

是否为2的幂

// 对 x 为正整数有效// version1bool bitwise_is_power_of_two(int x) {    return ( x & ( x - 1 ) ) == 0;}// version2bool bitwise_is_power_of_two(int x) {    return ( (x & (-x) ) == x );}

非科学测试

本机环境 win7 64bit, 8GB ram, amd 1.90GHz, 4 core

bitwise_swap vs swap (C++11, utility)

多次测试，bitwise_swap 耗时大约为 swap 的 2/3 多一点。

#include <iostream>#include <cstdio>#include <ctime>#include <climits>#include <utility>using namespace std;#define rep(i, s, t) for(int (i)=(s);(i)<=(t);++(i))void bitwise_swap(int& a, int& b) {    a ^= b; b ^= a; a ^= b;}int main() {    float t, delta;    t = clock();    rep(i, 1, 10000000) {        int x, y;        swap(x, y);    }    delta = clock() - t;    printf("%.8f seconds\n", delta / CLOCKS_PER_SEC);    t = clock();    rep(i, 1, 10000000) {        int x, y;        bitwise_swap(x, y);    }    delta = clock() - t;    printf("%.8f seconds\n", delta / CLOCKS_PER_SEC);    return 0;}

其它

其他的手写bitwise操作基本无法打败C++库函数，甚至会慢很多。
C++ 的效率果然是吊吊的。。。

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