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; }