double currentTime()
{
// Use a combination of ftime and QueryPerformanceCounter.
// ftime returns the information we want, but doesn't have sufficient resolution.
// QueryPerformanceCounter has high resolution, but is only usable to measure time intervals.
// To combine them, we call ftime and QueryPerformanceCounter initially. Later calls will use QueryPerformanceCounter
// by itself, adding the delta to the saved ftime. We periodically re-sync to correct for drift.
static bool started;
static double syncLowResUTCTime;
static double syncHighResUpTime;
static double lastUTCTime;
double lowResTime = lowResUTCTime();
if (!qpcAvailable())
return lowResTime / 1000.0;
double highResTime = highResUpTime();
if (!syncedTime) {
timeBeginPeriod(1); // increase time resolution around low-res time getter
syncLowResUTCTime = lowResTime = lowResUTCTime();
timeEndPeriod(1); // restore time resolution
syncHighResUpTime = highResTime;
syncedTime = true;
}
double highResElapsed = highResTime - syncHighResUpTime;
double utc = syncLowResUTCTime + highResElapsed;
// force a clock re-sync if we've drifted
double lowResElapsed = lowResTime - syncLowResUTCTime;
const double maximumAllowedDriftMsec = 15.625 * 2.0; // 2x the typical low-res accuracy
if (fabs(highResElapsed - lowResElapsed) > maximumAllowedDriftMsec)
syncedTime = false;
// make sure time doesn't run backwards (only correct if difference is < 2 seconds, since DST or clock changes could occur)
const double backwardTimeLimit = 2000.0;
if (utc < lastUTCTime && (lastUTCTime - utc) < backwardTimeLimit)
return lastUTCTime / 1000.0;
lastUTCTime = utc;
return utc / 1000.0;
}
double currentTime()
{
static bool init = false;
static double lastTime;
static DWORD lastTickCount;
if (!init) {
lastTime = lowResUTCTime();
lastTickCount = GetTickCount();
init = true;
return lastTime;
}
DWORD tickCountNow = GetTickCount();
DWORD elapsed = tickCountNow - lastTickCount;
double timeNow = lastTime + (double)elapsed / 1000.;
if (elapsed >= 0x7FFFFFFF) {
lastTime = timeNow;
lastTickCount = tickCountNow;
}
return timeNow;
}