二进制逆转

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dictScan：http://chenzhenianqing.com/articles/1101.html#comment-46945

摘自 redis 源码

/* Function to reverse bits. Algorithm from:

 * http://graphics.stanford.edu/~seander/bithacks.html#ReverseParallel */

static unsigned long rev(unsigned long v) {

    unsigned long s = 8 * sizeof(v); // bit size; must be power of 2

    unsigned long mask = ~0;

    while ((s >>= 1) > 0) 
    {
        // 将二进制位平均分成 sizeof(v) / s 组

        mask ^= (mask << s);  // mask：每组的低s位置1，高s位置0

        v = ((v >> s) & mask) | ((v << s) & ~mask); // v：每组中的低s位与高s位交换
    }

    return v;

}

验证

#include <iostream>
#include <string>
#include <fstream>
#include <map>
#include <vector>
using namespace std;

void print2(int a[32])
{
    for(int i=31; i>=0; i--)
    {
        cout << a[i];
        if(i<31 && (i)%4 == 0)
            cout << " ";
    }
    cout << endl;
}


void to2(int a[32], unsigned long x)
{
    int i = 0;

    while(x != 0)
    {
        a[i++] = x % 2;
        x = x / 2 ;    
    }  

    print2(a);
}

unsigned long rev(unsigned long v) 
{

    int a[32];

    unsigned long s = 8 * sizeof(v); // bit size; must be power of 2

    unsigned long mask = ~0;

    cout << "sizeof(v)\t\t" << sizeof(v) << endl;
    cout << "s\t\t\t" << s << endl;
    memset(a, 0, sizeof(a));
    cout << "s\t\t\t";
    to2(a, s);

    memset(a, 0, sizeof(a));
    cout << "mask\t\t\t";
    to2(a, mask);

    cout << endl;

    while ((s >>= 1) > 0) 
    {

        memset(a, 0, sizeof(a));
        cout << "s\t\t\t";
        to2(a, s);

        memset(a, 0, sizeof(a));
        cout << "mask\t\t\t";
        to2(a, mask);

        memset(a, 0, sizeof(a));
        cout << "t = mask << s\t\t";
        to2(a, mask << s);

        mask ^= (mask << s);

        memset(a, 0, sizeof(a));
        cout << "mask ^= t\t\t";
        to2(a, mask);

        memset(a, 0, sizeof(a));
        cout << "v1 = v >> s\t\t";
        to2(a, v >> s);

        memset(a, 0, sizeof(a));
        cout << "v11 = v1 & mask\t\t";
        to2(a, (v >> s) & mask);

        memset(a, 0, sizeof(a));
        cout << "v2 = v << s\t\t";
        to2(a, v << s);

        memset(a, 0, sizeof(a));
        cout << "v22=v2&~mask\t\t";
        to2(a, (v << s) & ~mask);

        v = ((v >> s) & mask) | ((v << s) & ~mask);

        cout << "v = v11|v22\t\t";
        memset(a, 0, sizeof(a));
        to2(a, v);

        cout << endl;
    }

    return v;

}

int main() 
{
    int a[32];
    memset(a, 0, sizeof(a));
    int v = 0x32100123;
    cout << "v\t\t\t" << v << endl;
    cout << "v\t\t\t";
    to2(a, v);

    memset(a, 0, sizeof(a));
    unsigned long ans = rev(v);
    cout << "ans\t\t\t";
    to2(a, ans);

    return 0;
}