基本位运算

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
/* a + b */
int sum(int a, int b)
{
    int t;
    while(b)
    {
        t = (a & b) << 1;
        a = a ^ b;
        b = t;
    }
    return a;
}
/* -a */
int neg(int a)
{
    return sum(~a, 1);
}
/* a - b */
int sub(int a, int b)
{
    return sum(a, neg(b));
}
/* 判断a是不是负数 */
int ispos(int a)
{
    if(a & (1 << 31) )
        return 0;
    return 1;    
}
/* a * b */
int mul(int a, int b)
{
    int ans = 0;
    int flag = 0;
    if(!ispos(a))
    {
        a = neg(a);
        flag ^= 1;
    }
    if(!ispos(b))
    {
        b = neg(b);
        flag ^= 1;
    }
    while(b)
    {
        if(b & 1) ans = sum(ans, a);
        a = a << 1;
        b = b >> 1;
    }
    if(flag)
        ans = neg(ans);
    return ans;
}
/* a / b */
int Div(int a, int b)
{
    int ans = 0;
    int flag = 0;
    if(!ispos(a))
    {
        a = neg(a);
        flag ^= 1;
    }
    if(!ispos(b))
    {
        b = neg(b);
        flag ^= 1;
    }
    int i;
    for(i = 31; i >= 0; i = sub(i, 1))
    {
        if( (a >> i) >= b)
        {
            ans = sum(ans, 1 << i);
            a = sub(a, b << i);
        }
    }
    if(flag)
        ans = neg(ans);
    return ans;
}

int main()
{
    struct timeval stime, etime;
    int a, b;
    long long i, j, u, v;
    i = j = u = v = 0;
    gettimeofday(&stime, NULL);
    for(a = -1000; a <= 1000; ++a)
        for(b = -1000; b <= 1000; ++b)
    {
        i += sum(a, b);
        j += sub(a, b);
        u += mul(a, b);
        if(b != 0)
            v += Div(a, b);
    }
    gettimeofday(&etime, NULL);
    printf("%ld\n", etime.tv_sec * 1000 + etime.tv_usec / 1000 - stime.tv_sec * 1000 - stime.tv_usec / 1000);
    return 0;
}