用位数组计算整数中1的个数

//

//  main.c

//  bitcounts

//

//  Created by Cheney Shen on 11-4-16.

//  Copyright 2011年 __MyCompanyName__. All rights reserved.

//



#include <stdio.h>



static int bitcounts[] = 

{0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4};



int bitcount(unsigned int u) {

    int n = 0;

    for(; u != 0; u >>= 4)

        n += bitcounts[u & 0x0f];

    return n;

}



int main (int argc, const char * argv[])

{

    return printf("%d", bitcount(1111111111111111111));

}