From c256dca7c6ca2c7f8eeffae53c02fe62bd892198 Mon Sep 17 00:00:00 2001
From: Daniel Karbach <daniel.karbach@localhorst.tv>
Date: Mon, 26 Oct 2015 17:39:33 +0100
Subject: [PATCH] measure packet round trip time

---
 src/net/ConnectionHandler.hpp | 12 ++++++++++
 src/net/net.cpp               | 44 ++++++++++++++++++++++++++++++++++-
 2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/src/net/ConnectionHandler.hpp b/src/net/ConnectionHandler.hpp
index 77bcd7b..be01d10 100644
--- a/src/net/ConnectionHandler.hpp
+++ b/src/net/ConnectionHandler.hpp
@@ -13,8 +13,12 @@ class ConnectionHandler {
 public:
 	ConnectionHandler();
 
+	/// packet loss as factor
 	float PacketLoss() const noexcept { return packet_loss; }
+	/// smooth average round trip time in milliseconds
+	float RoundTripTime() const noexcept { return rtt; }
 
+	void PacketSent(std::uint16_t);
 	void PacketLost(std::uint16_t);
 	void PacketReceived(std::uint16_t);
 
@@ -24,6 +28,9 @@ public:
 
 private:
 	void UpdatePacketLoss() noexcept;
+	void UpdateRTT(std::uint16_t) noexcept;
+	bool SamplePacket(std::uint16_t) const noexcept;
+	int HeadDiff(std::uint16_t) const noexcept;
 
 	// called as soon as the remote end ack'd given packet
 	virtual void OnPacketReceived(std::uint16_t) { }
@@ -49,6 +56,11 @@ private:
 	unsigned int packets_received;
 	float packet_loss;
 
+	Uint32 stamps[16];
+	std::size_t stamp_cursor;
+	std::uint16_t stamp_last;
+	float rtt;
+
 };
 
 }
diff --git a/src/net/net.cpp b/src/net/net.cpp
index 9b50479..59176aa 100644
--- a/src/net/net.cpp
+++ b/src/net/net.cpp
@@ -84,6 +84,10 @@ uint16_t Connection::Send(UDPpacket &udp_pack, UDPsocket sock) {
 		throw NetError("SDLNet_UDP_Send");
 	}
 
+	if (HasHandler()) {
+		Handler().PacketSent(seq);
+	}
+
 	FlagSend();
 	return seq;
 }
@@ -151,20 +155,37 @@ uint16_t Connection::SendPing(UDPpacket &udp_pack, UDPsocket sock) {
 ConnectionHandler::ConnectionHandler()
 : packets_lost(0)
 , packets_received(0)
-, packet_loss(0.0f) {
+, packet_loss(0.0f)
+, stamp_cursor(15)
+, stamp_last(0)
+, rtt(64.0f) {
+	Uint32 now = SDL_GetTicks();
+	for (Uint32 &s : stamps) {
+		s = now;
+	}
+}
 
+void ConnectionHandler::PacketSent(uint16_t seq) {
+	if (!SamplePacket(seq)) {
+		return;
+	}
+	stamp_cursor = (stamp_cursor + 1) % 16;
+	stamps[stamp_cursor] = SDL_GetTicks();
+	stamp_last = seq;
 }
 
 void ConnectionHandler::PacketLost(uint16_t seq) {
 	OnPacketLost(seq);
 	++packets_lost;
 	UpdatePacketLoss();
+	UpdateRTT(seq);
 }
 
 void ConnectionHandler::PacketReceived(uint16_t seq) {
 	OnPacketReceived(seq);
 	++packets_received;
 	UpdatePacketLoss();
+	UpdateRTT(seq);
 }
 
 void ConnectionHandler::UpdatePacketLoss() noexcept {
@@ -176,6 +197,27 @@ void ConnectionHandler::UpdatePacketLoss() noexcept {
 	}
 }
 
+void ConnectionHandler::UpdateRTT(std::uint16_t seq) noexcept {
+	if (!SamplePacket(seq)) return;
+	int diff = HeadDiff(seq);
+	if (diff > 0 || diff < -15) {
+		// packet outside observed time frame
+		return;
+	}
+	int cur_rtt = SDL_GetTicks() - stamps[(stamp_cursor + diff + 16) % 16];
+	rtt += (cur_rtt - rtt) * 0.1f;
+}
+
+bool ConnectionHandler::SamplePacket(std::uint16_t seq) const noexcept {
+	// only sample every eighth packet
+	return seq % 8 == 0;
+}
+
+int ConnectionHandler::HeadDiff(std::uint16_t seq) const noexcept {
+	int16_t diff = int16_t(seq) - int16_t(stamp_last);
+	return diff / 8;
+}
+
 
 ostream &operator <<(ostream &out, const IPaddress &addr) {
 	const unsigned char *host = reinterpret_cast<const unsigned char *>(&addr.host);
-- 
2.39.2