int count_ones(unsigned a)
{
    int count = 0;

    for (; a; a >>= 1) {
        if (a & 1) count ++;
    }

    return count;
}

int count_ones(unsigned a)
{
    int count = 0;

    for (; a; count ++) {
        a &= a - 1;
    }

    return count;
}

int count_ones(unsigned a)
{
    a = (a & 0x55555555) + ((a >> 1) & 0x55555555);
    a = (a & 0x33333333) + ((a >> 2) & 0x33333333);
    a = (a & 0x0f0f0f0f) + ((a >> 4) & 0x0f0f0f0f);
    a = (a & 0x00ff00ff) + ((a >> 8) & 0x00ff00ff);
    a = (a & 0x0000ffff) + ((a >> 16) & 0x0000ffff);

    return a;
}

const int ones[] = {0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4, 1, 2, 2,\
 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4,\
 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 1, 2, 2, 3, 2,\
 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5,\
 5, 6, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5,\
 6, 4, 5, 5, 6, 5, 6, 6, 7, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,\
 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 2, 3, 3, 4, 3, 4, 4, 5, 3,\
 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 2, 3,\
 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5,\
 6, 5, 6, 6, 7, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 4, 5, 5, 6,\
 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8};

int count_ones(unsigned a)
{
    return (ones[a & 0xff] + ones[(a >> 8) & 0xff] + ones[(a >> 16) & 0xff]\
        + ones[(a >> 24) & 0xff]);
}

sxl@sxl-desktop:~/cpro$ gcc -v

gcc version 4.1.2 20060928 (prerelease) (Ubuntu 4.1.1-13ubuntu5)

sxl@sxl-desktop:~/cpro$ time ./text0 < input >/dev/null

real 1m46.726s

user 1m44.691s

sys 0m1.632s

sxl@sxl-desktop:~/cpro$ time ./text1 < input >/dev/null

real 1m45.945s

user 1m43.698s

sys 0m1.664s

sxl@sxl-desktop:~/cpro$ time ./text2 < input >/dev/null

real 1m42.497s

user 1m40.130s

sys 0m1.700s

sxl@sxl-desktop:~/cpro$ time ./text3 < input >/dev/null

real 1m23.812s

user 1m21.449s

sys 0m1.756s

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    for (int i = 0; i < 100000000; i ++) {
        printf("%d ", rand() % 10000);
    }

    return 0;
}