FIX(server): Cap receiver ranges at a vol. diff of 5dB

The factor difference cap works for larger volume adjustments, but for
low volumes, the difference can easily exceed 5dB (from -24dB and lower,
a difference of 0.05 in the factor is a difference of more than 5dB).

src/murmur/AudioReceiverBuffer.h

@@ -62,6 +62,17 @@ class AudioReceiverBuffer {
 	std::vector< AudioReceiver > &getReceivers(bool receivePositionalData);
 
 
+	/**
+	 * If two receivers differ in their volume adjustment factors by this much or more, they will never end up in the
+	 * same receiver range
+	 */
+	constexpr static const float maxFactorDiff = 0.05f;
+	/**
+	 * If two receivers differ in their volume adjustment decibels by this much or more, they will never end up in the
+	 * same receiver range
+	 */
+	constexpr static const int maxDecibelDiff = 5;
+
 	template< typename Iterator > static ReceiverRange< Iterator > getReceiverRange(Iterator begin, Iterator end) {
 		ZoneScoped;
 
@@ -82,8 +93,14 @@ class AudioReceiverBuffer {
 			return lhs.getContext() == rhs.getContext()
 				   && Mumble::Protocol::protocolVersionsAreCompatible(lhs.getReceiver().m_version,
 																	  rhs.getReceiver().m_version)
-				   // Allow a little variance between volume adjustments
-				   && std::abs(lhs.getVolumeAdjustment().factor - rhs.getVolumeAdjustment().factor) < 0.05f;
+				   // The factor difference caps audible differences for high volume adjustments (where 1dB is already a
+				   // big difference). Thus, this is a cap on the absolute loudness difference.
+				   && std::abs(lhs.getVolumeAdjustment().factor - rhs.getVolumeAdjustment().factor) < maxFactorDiff
+				   // The dB difference caps audible difference for low volume adjustments. E.g. a factor difference of
+				   // 0.05 for an adjustment of -24dB is a difference of more than 5dB and therefore audible. Thus, this
+				   // is a cap on the relative loudness difference.
+				   && std::abs(lhs.getVolumeAdjustment().dbAdjustment - rhs.getVolumeAdjustment().dbAdjustment)
+						  < maxDecibelDiff;
 		});
 
 		return range;

src/tests/TestAudioReceiverBuffer/TestAudioReceiverBuffer.cpp

@@ -39,14 +39,14 @@ Version::full_t vOld2 = Version::fromComponents(1, 3, 1);
 Version::full_t vOld3 = Version::fromComponents(1, 4, 0);
 Version::full_t vNew  = Mumble::Protocol::PROTOBUF_INTRODUCTION_VERSION;
 
-std::array< ServerUser, 5 > users = { ServerUser(0, vOld1), ServerUser(1, vOld2), ServerUser(2, vOld3),
-									  ServerUser(3, vNew), ServerUser(4, vNew) };
+std::array< ServerUser, 6 > users = { ServerUser(0, vOld1), ServerUser(1, vOld2), ServerUser(2, vOld3),
+									  ServerUser(3, vNew),  ServerUser(4, vNew),  ServerUser(5, vNew) };
 
-ServerUser deafUser(5, vOld1, true);
-ServerUser selfDeafUser(6, vNew, false, true);
-ServerUser contextUser1(7, vNew, false, false, "context1");
-ServerUser contextUser2(8, vNew, false, false, "context2");
-ServerUser contextUser3(9, vNew, false, false, "context1");
+ServerUser deafUser(6, vOld1, true);
+ServerUser selfDeafUser(7, vNew, false, true);
+ServerUser contextUser1(8, vNew, false, false, "context1");
+ServerUser contextUser2(9, vNew, false, false, "context2");
+ServerUser contextUser3(10, vNew, false, false, "context1");
 
 
 struct Range {
@@ -203,6 +203,41 @@ private slots:
 		QCOMPARE(volumeReceiver->getVolumeAdjustment().factor, 1.4f);
 	}
 
+	void test_getReceiverRange() {
+		AudioReceiverBuffer buffer;
+
+		ServerUser &sender = contextUser2;
+
+		buffer.addReceiver(sender, users[0], Mumble::Protocol::AudioContext::NORMAL, false,
+						   VolumeAdjustment::fromFactor(1.22));
+		buffer.addReceiver(sender, users[1], Mumble::Protocol::AudioContext::NORMAL, false,
+						   VolumeAdjustment::fromFactor(1.22 + 0.6 * AudioReceiverBuffer::maxFactorDiff));
+		buffer.addReceiver(sender, users[2], Mumble::Protocol::AudioContext::NORMAL, false,
+						   VolumeAdjustment::fromFactor(1.22 + 1.1 * AudioReceiverBuffer::maxFactorDiff));
+		buffer.addReceiver(sender, users[3], Mumble::Protocol::AudioContext::NORMAL, false,
+						   VolumeAdjustment::fromDBAdjustment(-60));
+		buffer.addReceiver(sender, users[4], Mumble::Protocol::AudioContext::NORMAL, false,
+						   VolumeAdjustment::fromDBAdjustment(-60 + AudioReceiverBuffer::maxDecibelDiff - 1));
+		buffer.addReceiver(sender, users[5], Mumble::Protocol::AudioContext::NORMAL, false,
+						   VolumeAdjustment::fromDBAdjustment(-60 + AudioReceiverBuffer::maxDecibelDiff));
+
+		buffer.preprocessBuffer();
+
+		std::vector< AudioReceiver > receivers = buffer.getReceivers(false);
+		auto receiverRange = AudioReceiverBuffer::getReceiverRange(receivers.begin(), receivers.end());
+
+		std::array< int, 4 > expectedGroupSizes = { 2, 1, 2, 1 };
+
+		for (std::size_t i = 0; i < expectedGroupSizes.size(); ++i) {
+			QVERIFY(receiverRange.begin != receiverRange.end);
+			QCOMPARE(std::distance(receiverRange.begin, receiverRange.end), expectedGroupSizes.at(i));
+			receiverRange = AudioReceiverBuffer::getReceiverRange(receiverRange.end, receivers.end());
+		}
+
+		QVERIFY(receiverRange.begin == receiverRange.end);
+	}
+
+
 	void test_encoding() {
 		AudioReceiverBuffer buffer;