Add a way to read jitter in SteamNetConnectionRealTimeStatus_t

Author: zpostfacto

include/steam/steamnetworkingtypes.h

@@ -802,8 +802,17 @@ struct SteamNetConnectionRealTimeStatus_t
 	/// Nagle delay is ignored for the purposes of this calculation.
 	SteamNetworkingMicroseconds m_usecQueueTime;
 
+	/// Highest packet jitter experienced, since the last time this information
+	/// was returned.  (The high water mark is cleared each time you fetch the info.)
+	/// 
+	/// - The units are microseconds, although the measurement precision is usually
+	///   not nearly this precise.
+	/// - A negative value means "no data available".
+	/// - Not all connections are able to measure jitter.
+	int32 m_usecMaxJitter;
+
 	// Internal stuff, room to change API easily
-	uint32 reserved[16];
+	uint32 reserved[15];
 };
 
 /// Quick status of a particular lane

src/steamnetworkingsockets/clientlib/steamnetworkingsockets_connections.cpp

@@ -1953,6 +1953,10 @@ EResult CSteamNetworkConnectionBase::APIGetRealTimeStatus( SteamNetConnectionRea
 		pStatus->m_flOutBytesPerSec = m_statsEndToEnd.m_sent.m_bytes.m_flRate;
 		pStatus->m_flInPacketsPerSec = m_statsEndToEnd.m_recv.m_packets.m_flRate;
 		pStatus->m_flInBytesPerSec = m_statsEndToEnd.m_recv.m_bytes.m_flRate;
+
+		// Max jitter, and clear it
+		pStatus->m_usecMaxJitter = m_statsEndToEnd.m_usecAPIRealtimeStatusMaxJitter;
+		m_statsEndToEnd.m_usecAPIRealtimeStatusMaxJitter = -1;
 	}
 	SNP_PopulateRealTimeStatus( pStatus, nLanes, pLanes, usecNow );
 

src/steamnetworkingsockets/steamnetworking_statsutils.h

@@ -1077,6 +1077,10 @@ struct LinkStatsTrackerEndToEnd : public LinkStatsTrackerBase
 	/// Time when the current interval started
 	SteamNetworkingMicroseconds m_usecSpeedIntervalStart;
 
+	/// Jitter value that is returned in GetConnectionRealTimeStatus and then
+	/// cleared.
+	int m_usecAPIRealtimeStatusMaxJitter;
+
 	///// TX Speed, should match CMsgSteamDatagramLinkLifetimeStats 
 	//int m_nTXSpeed; 
 	//int m_nTXSpeedMax; 
@@ -1169,6 +1173,22 @@ struct LinkStatsTrackerEndToEnd : public LinkStatsTrackerBase
 		}
 	}
 
+	inline void InternalProcessJitterSample( int usecJitter )
+	{
+		LinkStatsTrackerBase::InternalProcessJitterSample( usecJitter );
+
+		// Update user high water mark.
+		// 
+		// Use absolute value here...I think?
+		// If one packet is delayed and then the next packet arrives on time, the second
+		// jitter value will be negative, and both packets will be counted as bad, even
+		// though only the second one was.  (Arriving early usually isn't a problem, it's
+		// arriving late that's bad).  However, for the use cases for this is currently
+		// intended, it's OK.  With any reasonable packet rate, we assume the app
+		// won't be checking this value fast enough for it to matter.
+		m_usecAPIRealtimeStatusMaxJitter = std::max( m_usecAPIRealtimeStatusMaxJitter, abs( usecJitter ) );
+	}
+
 private:
 
 	void UpdateSpeedInterval( SteamNetworkingMicroseconds usecNow );

src/steamnetworkingsockets/steamnetworkingsockets_stats.cpp

@@ -768,6 +768,7 @@ void LinkStatsTrackerEndToEnd::InitInternal( SteamNetworkingMicroseconds usecNow
 
 	m_usecWhenStartedConnectedState = 0;
 	m_usecWhenEndedConnectedState = 0;
+	m_usecAPIRealtimeStatusMaxJitter = -1;
 
 	//m_TXSpeedSample.Clear();
 	//m_nTXSpeed = 0;