gettimeofday

#if defined(_WIN32)

//jrtplib
inline unsigned __int64 CalculateFileTime(unsigned __int64 performancecount, unsigned __int64 performancefrequency)
{
	unsigned __int64 f = performancefrequency;
	unsigned __int64 a = performancecount;
	unsigned __int64 b = a/f;
	unsigned __int64 c = a%f; // a = b*f+c => (a*10'000'000)/f = b*10'000'000+(c*10'000'000)/f

	return b*10'000'000ui64+(c*10'000'000ui64)/f;
}

//https://github.com/Perl/perl5/blob/blead/dist/Time-HiRes/HiRes.xs
union FT_t
{
	unsigned __int64 ft_i64;
	FILETIME ft_val;
};

struct my_ctx
{
	unsigned long run_count;
	unsigned __int64 base_ticks;
	unsigned __int64 tick_frequency;
	FT_t base_systime_as_filetime;
	unsigned __int64 reset_time;
};

/* Number of 100 nanosecond units from 1/1/1601 to 1/1/1970 */
#define EPOCH_BIAS (116444736000000000LL)

/* If the performance counter delta drifts more than 0.5 seconds from the
 * system time then we recalibrate to the system time.  This means we may
 * move *backwards* in time! */
#define MAX_PERF_COUNTER_SKEW (5000000LL) /* 0.5 seconds */

/* Reset reading from the performance counter every five minutes.
 * Many PC clocks just seem to be so bad. */
#define MAX_PERF_COUNTER_TICKS (300000000LL) /* 300 seconds */

/*
 * Windows 8 introduced GetSystemTimePreciseAsFileTime(), but currently we have
 * to support older systems, so for now we provide our own implementation.
 * In the future we will switch to the real deal.
 */
static void _GetSystemTimePreciseAsFileTime(FILETIME *out)
{
	static my_ctx MY_CTX;

	FT_t ft;

	if (MY_CTX.run_count++ == 0 ||
	    MY_CXT.base_systime_as_filetime.ft_i64 > MY_CXT.reset_time)
	{
		::QueryPerformanceFrequency((LARGE_INTEGER*)&MY_CTX.tick_frequency);
		::QueryPerformanceCounter((LARGE_INTEGER*)&MY_CTX.base_ticks);

		::GetSystemTimeAsFileTime(&MY_CTX.base_systime_as_filetime.ft_val);

		ft.ft_i64 = MY_CTX.base_systime_as_filetime.ft_i64;
		MY_CXT.reset_time = ft.ft_i64 + MAX_PERF_COUNTER_TICKS;
	} else
	{
		__int64 diff;
		unsigned __int64 ticks;

		::QueryPerformanceCounter((LARGE_INTEGER*)&ticks);

		ticks -= MY_CTX.base_ticks;

		//ft.ft_i64 = MY_CTX.base_systime_as_filetime.ft_i64 + CalculateFileTime(ticks, MY_CTX.tick_frequency);
		ft.ft_i64 = MY_CTX.base_systime_as_filetime.ft_i64
				+ (ticks / MY_CTX.tick_frequency) * 10'000'000LL
				+ ((ticks % MY_CTX.tick_frequency) * 10'000'000LL) / MY_CTX.tick_frequency;
		diff = ft.ft_i64 - MY_CXT.base_systime_as_filetime.ft_i64;
		if (diff < -MAX_PERF_COUNTER_SKEW || diff > MAX_PERF_COUNTER_SKEW)
		{
			MY_CXT.base_ticks += ticks;
			::GetSystemTimeAsFileTime(&MY_CXT.base_systime_as_filetime.ft_val);
			ft.ft_i64 = MY_CXT.base_systime_as_filetime.ft_i64;
		}
	}
	*out = ft.ft_val;
}

static int gettimeofday(struct timeval *tv, void *tz)
{
	FT_t ft;

	_GetSystemTimePreciseAsFileTime(&ft.ft_val);

	/* seconds since epoch */
	tv->tv_sec = (long)((ft.ft_i64 - EPOCH_BIAS) / 10LL / 1'000'000LL);

	/* microseconds remaining */
	tv->tv_usec = (long)((ft.ft_i64 / 10LL) % 1'000'000LL);

	return 0;
}
#endif