From 74ea1548a929f4936684cc1d9ab59adfc553a85a Mon Sep 17 00:00:00 2001
From: Simon Walton <simon@highfidelity.io>
Date: Thu, 13 Sep 2018 14:24:49 -0700
Subject: [PATCH 1/2] Use chrono::system_clock for usecTimestampNow() on all
 platforms

---
 libraries/shared/src/SharedUtil.cpp | 81 ++---------------------------
 1 file changed, 5 insertions(+), 76 deletions(-)

diff --git a/libraries/shared/src/SharedUtil.cpp b/libraries/shared/src/SharedUtil.cpp
index bb22a1e753..886445824b 100644
--- a/libraries/shared/src/SharedUtil.cpp
+++ b/libraries/shared/src/SharedUtil.cpp
@@ -19,8 +19,8 @@
 #include <time.h>
 #include <mutex>
 #include <thread>
-#include <set>
 #include <unordered_map>
+#include <chrono>
 
 #include <glm/glm.hpp>
 
@@ -122,87 +122,16 @@ void setGlobalInstance(const char* propertyName, const QVariant& variant) {
     }
 }
 
+// Do we still need this?
 static qint64 usecTimestampNowAdjust = 0; // in usec
 void usecTimestampNowForceClockSkew(qint64 clockSkew) {
     ::usecTimestampNowAdjust = clockSkew;
 }
 
-static std::atomic<qint64> TIME_REFERENCE { 0 }; // in usec
-static std::once_flag usecTimestampNowIsInitialized;
-static QElapsedTimer timestampTimer;
-
 quint64 usecTimestampNow(bool wantDebug) {
-    std::call_once(usecTimestampNowIsInitialized, [&] {
-        TIME_REFERENCE = QDateTime::currentMSecsSinceEpoch() * USECS_PER_MSEC; // ms to usec
-        timestampTimer.start();
-    });
-    
-    quint64 now;
-    quint64 nsecsElapsed = timestampTimer.nsecsElapsed();
-    quint64 usecsElapsed = nsecsElapsed / NSECS_PER_USEC;  // nsec to usec
-    
-    // QElapsedTimer may not advance if the CPU has gone to sleep. In which case it
-    // will begin to deviate from real time. We detect that here, and reset if necessary
-    quint64 msecsCurrentTime = QDateTime::currentMSecsSinceEpoch();
-    quint64 msecsEstimate = (TIME_REFERENCE + usecsElapsed) / USECS_PER_MSEC; // usecs to msecs
-    int possibleSkew = msecsEstimate - msecsCurrentTime;
-    const int TOLERANCE = 10 * MSECS_PER_SECOND; // up to 10 seconds of skew is tolerated
-    if (abs(possibleSkew) > TOLERANCE) {
-        // reset our TIME_REFERENCE and timer
-        TIME_REFERENCE = QDateTime::currentMSecsSinceEpoch() * USECS_PER_MSEC; // ms to usec
-        timestampTimer.restart();
-        now = TIME_REFERENCE + ::usecTimestampNowAdjust;
-
-        if (wantDebug) {
-            qCDebug(shared) << "usecTimestampNow() - resetting QElapsedTimer. ";
-            qCDebug(shared) << "    msecsCurrentTime:" << msecsCurrentTime;
-            qCDebug(shared) << "       msecsEstimate:" << msecsEstimate;
-            qCDebug(shared) << "        possibleSkew:" << possibleSkew;
-            qCDebug(shared) << "           TOLERANCE:" << TOLERANCE;
-            
-            qCDebug(shared) << "        nsecsElapsed:" << nsecsElapsed;
-            qCDebug(shared) << "        usecsElapsed:" << usecsElapsed;
-
-            QDateTime currentLocalTime = QDateTime::currentDateTime();
-
-            quint64 msecsNow = now / 1000; // usecs to msecs
-            QDateTime nowAsString;
-            nowAsString.setMSecsSinceEpoch(msecsNow);
-
-            qCDebug(shared) << "                 now:" << now;
-            qCDebug(shared) << "            msecsNow:" << msecsNow;
-
-            qCDebug(shared) << "         nowAsString:" << nowAsString.toString("yyyy-MM-dd hh:mm:ss.zzz");
-            qCDebug(shared) << "    currentLocalTime:" << currentLocalTime.toString("yyyy-MM-dd hh:mm:ss.zzz");
-        }
-    } else {
-        now = TIME_REFERENCE + usecsElapsed + ::usecTimestampNowAdjust;
-    }
-
-    if (wantDebug) {
-        QDateTime currentLocalTime = QDateTime::currentDateTime();
-
-        quint64 msecsNow = now / 1000; // usecs to msecs
-        QDateTime nowAsString;
-        nowAsString.setMSecsSinceEpoch(msecsNow);
-
-        quint64 msecsTimeReference = TIME_REFERENCE / 1000; // usecs to msecs
-        QDateTime timeReferenceAsString;
-        timeReferenceAsString.setMSecsSinceEpoch(msecsTimeReference);
-
-        qCDebug(shared) << "usecTimestampNow() - details... ";
-        qCDebug(shared) << "           TIME_REFERENCE:" << TIME_REFERENCE;
-        qCDebug(shared) << "    timeReferenceAsString:" << timeReferenceAsString.toString("yyyy-MM-dd hh:mm:ss.zzz");
-        qCDebug(shared) << "   usecTimestampNowAdjust:" << usecTimestampNowAdjust;
-        qCDebug(shared) << "             nsecsElapsed:" << nsecsElapsed;
-        qCDebug(shared) << "             usecsElapsed:" << usecsElapsed;
-        qCDebug(shared) << "                      now:" << now;
-        qCDebug(shared) << "                 msecsNow:" << msecsNow;
-        qCDebug(shared) << "              nowAsString:" << nowAsString.toString("yyyy-MM-dd hh:mm:ss.zzz");
-        qCDebug(shared) << "         currentLocalTime:" << currentLocalTime.toString("yyyy-MM-dd hh:mm:ss.zzz");
-    }
-    
-    return now;
+    using namespace std::chrono;
+    static const auto unixEpoch = system_clock::from_time_t(0);
+    return duration_cast<microseconds>(system_clock::now() - unixEpoch).count() + usecTimestampNowAdjust;
 }
 
 float secTimestampNow() {

From 727aea20251af8bbe3c2b77d4ec35b9feda02662 Mon Sep 17 00:00:00 2001
From: Simon Walton <simon@highfidelity.io>
Date: Thu, 13 Sep 2018 18:21:16 -0700
Subject: [PATCH 2/2] Remove comment

---
 libraries/shared/src/SharedUtil.cpp | 1 -
 1 file changed, 1 deletion(-)

diff --git a/libraries/shared/src/SharedUtil.cpp b/libraries/shared/src/SharedUtil.cpp
index 886445824b..012e7aa1f5 100644
--- a/libraries/shared/src/SharedUtil.cpp
+++ b/libraries/shared/src/SharedUtil.cpp
@@ -122,7 +122,6 @@ void setGlobalInstance(const char* propertyName, const QVariant& variant) {
     }
 }
 
-// Do we still need this?
 static qint64 usecTimestampNowAdjust = 0; // in usec
 void usecTimestampNowForceClockSkew(qint64 clockSkew) {
     ::usecTimestampNowAdjust = clockSkew;