C++偽GUID

static inline int secondNow() {
    struct timeval v;
    gettimeofday(&v, (struct timezone *) NULL);
    /* Unix Epoch time (time since January 1, 1970 (UTC)) */
    return v.tv_sec + v.tv_usec/1.0e6;
}

static struct tm* secondsSinceEpochToDateTime(struct tm* pTm, uint64_t SecondsSinceEpoch)
{
    uint64_t sec;
    uint quadricentennials, centennials, quadrennials, annuals/*1-ennial?*/;
    uint year, leap;
    uint yday, hour, min;
    uint month, mday, wday;
    static const uint daysSinceJan1st[2][13]=
    {
        {0,31,59,90,120,151,181,212,243,273,304,334,365}, // 365 days, non-leap
        {0,31,60,91,121,152,182,213,244,274,305,335,366}  // 366 days, leap
    };
    /*
     400 years:
     
     1st hundred, starting immediately after a leap year that's a multiple of 400:
     n n n l  \
     n n n l   } 24 times
     ...      /
     n n n l /
     n n n n
     
     2nd hundred:
     n n n l  \
     n n n l   } 24 times
     ...      /
     n n n l /
     n n n n
     
     3rd hundred:
     n n n l  \
     n n n l   } 24 times
     ...      /
     n n n l /
     n n n n
     
     4th hundred:
     n n n l  \
     n n n l   } 24 times
     ...      /
     n n n l /
     n n n L <- 97'th leap year every 400 years
     */
    
    // Re-bias from 1970 to 1601:
    // 1970 - 1601 = 369 = 3*100 + 17*4 + 1 years (incl. 89 leap days) =
    // (3*100*(365+24/100) + 17*4*(365+1/4) + 1*365)*24*3600 seconds
    sec = SecondsSinceEpoch + 11644473600;
    
    wday = (uint)((sec / 86400 + 1) % 7); // day of week
    
    // Remove multiples of 400 years (incl. 97 leap days)
    quadricentennials = (uint)(sec / 12622780800ULL); // 400*365.2425*24*3600
    sec %= 12622780800ULL;
    
    // Remove multiples of 100 years (incl. 24 leap days), can't be more than 3
    // (because multiples of 4*100=400 years (incl. leap days) have been removed)
    centennials = (uint)(sec / 3155673600ULL); // 100*(365+24/100)*24*3600
    if (centennials > 3)
    {
        centennials = 3;
    }
    sec -= centennials * 3155673600ULL;
    
    // Remove multiples of 4 years (incl. 1 leap day), can't be more than 24
    // (because multiples of 25*4=100 years (incl. leap days) have been removed)
    quadrennials = (uint)(sec / 126230400); // 4*(365+1/4)*24*3600
    if (quadrennials > 24)
    {
        quadrennials = 24;
    }
    sec -= quadrennials * 126230400ULL;
    
    // Remove multiples of years (incl. 0 leap days), can't be more than 3
    // (because multiples of 4 years (incl. leap days) have been removed)
    annuals = (uint)(sec / 31536000); // 365*24*3600
    if (annuals > 3)
    {
        annuals = 3;
    }
    sec -= annuals * 31536000ULL;
    
    // Calculate the year and find out if it's leap
    year = 1601 + quadricentennials * 400 + centennials * 100 + quadrennials * 4 + annuals;
    leap = !(year % 4) && (year % 100 || !(year % 400));
    
    // Calculate the day of the year and the time
    yday = sec / 86400;
    sec %= 86400;
    hour = sec / 3600;
    sec %= 3600;
    min = sec / 60;
    sec %= 60;
    
    // Calculate the month
    for (mday = month = 1; month < 13; month++)
    {
        if (yday < daysSinceJan1st[leap][month])
        {
            mday += yday - daysSinceJan1st[leap][month - 1];
            break;
        }
    }
    
    // Fill in C's "struct tm"
    memset(pTm, 0, sizeof(*pTm));
    pTm->tm_sec = sec;          // [0,59]
    pTm->tm_min = min;          // [0,59]
    pTm->tm_hour = hour;        // [0,23]
    pTm->tm_mday = mday;        // [1,31]  (day of month)
    pTm->tm_mon = month - 1;    // [0,11]  (month)
    pTm->tm_year = year - 1900; // 70+     (year since 1900)
    pTm->tm_wday = wday;        // [0,6]   (day since Sunday AKA day of week)
    pTm->tm_yday = yday;        // [0,365] (day since January 1st AKA day of year)
    pTm->tm_isdst = -1;         // daylight saving time flag
    
    return pTm;
}

int main(void) {
    uint64_t time = secondNow();
    struct tm pTm;
    secondsSinceEpochToDateTime(&pTm, time);
    char buf[100] = {0};
    
    int num = rand()%1000;   //  100000     
    printf("rand num:%4d
", num);
    
    sprintf(buf, "%02d%02d%02d%02d%02d%04d", 1+pTm.tm_mon, pTm.tm_mday,
            pTm.tm_hour, pTm.tm_min, pTm.tm_sec, num);  //            ，   buf 
    
    printf("guid:%s
", buf);  //  guid    
}