From 07586af6d1f0ac6e6c84e4d0ccba2a61d573d7ec Mon Sep 17 00:00:00 2001
From: Hisymak <kozten@seznam.cz>
Date: Sun, 29 Apr 2018 14:56:35 +0200
Subject: [PATCH] Fix OPL Synth note cutoffs and issues after removal of MusLib

This should fix bug reported in https://forum.zdoom.org/viewtopic.php?
f=104&t=56682.
There were two problems:
1. Algorithm to select a free voice was not appropriate. Changed to a
simple "least-recently-freed voice" algorithm.
2. Sustained voices were treated as used voices and never got replaced.
Now sustained voices are preferentially replaced when there are no free
voices.
---
 src/sound/oplsynth/musicblock.cpp | 35 ++++++++++++++++++++++++-------
 src/sound/oplsynth/musicblock.h   |  2 ++
 2 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/src/sound/oplsynth/musicblock.cpp b/src/sound/oplsynth/musicblock.cpp
index 32254f6707..bbbf6f0750 100644
--- a/src/sound/oplsynth/musicblock.cpp
+++ b/src/sound/oplsynth/musicblock.cpp
@@ -53,6 +53,8 @@ int musicBlock::releaseVoice(uint32_t slot, uint32_t killed)
 	struct OPLVoice *ch = &voices[slot];
 	io->WriteFrequency(slot, ch->note, ch->pitch, 0);
 	ch->index = ~0u;
+	ch->sustained = false;
+	if (!killed) ch->timestamp = ++timeCounter;
 	if (killed) io->MuteChannel(slot);
 	return slot;
 }
@@ -65,15 +67,25 @@ int musicBlock::releaseVoice(uint32_t slot, uint32_t killed)
 
 int musicBlock::findFreeVoice()
 {
+	// We want to prefer the least recently freed voice, as more recently
+	// freed voices can still play a tone from their release state.
+	// Sustained voices are replaced when there are no free voices.
+	uint32_t min_value = ~0u;
+	int result = -1;
 	for (uint32_t i = 0; i < io->NumChannels; ++i)
 	{
-		if (voices[i].index == ~0u)
+		uint32_t voice_value = voices[i].timestamp + (voices[i].sustained ? (1 << 31) : 0);
+		if ((voices[i].index == ~0u || voices[i].sustained) && (voice_value < min_value))
 		{
-			releaseVoice(i, 1);
-			return i;
+			min_value = voice_value;
+			result = i;
 		}
 	}
-	return -1;
+	if (result >= 0)
+	{
+		releaseVoice(result, 1);
+	}
+	return result;
 }
 
 //----------------------------------------------------------------------------
@@ -94,7 +106,6 @@ int musicBlock::replaceExistingVoice()
 	// Lower numbered MIDI channels implicitly have a higher priority
 	// than higher-numbered channels, eg. MIDI channel 1 is never
 	// discarded for MIDI channel 2.
-
 	int result = 0;
 
 	for (uint32_t i = 0; i < io->NumChannels; ++i)
@@ -236,7 +247,11 @@ void musicBlock::noteOff(uint32_t id, uint8_t note)
 	{
 		if (voices[i].index == id && voices[i].key == note)
 		{
-			if (sustain >= MIN_SUSTAIN) voices[i].sustained = true;
+			if (sustain >= MIN_SUSTAIN)
+			{
+				voices[i].sustained = true;
+				voices[i].timestamp = ++timeCounter;
+			}
 			else releaseVoice(i, 0);
 		}
 	}
@@ -357,7 +372,11 @@ void musicBlock::notesOff(uint32_t id, int value)
 	{
 		if (voices[i].index == id)
 		{
-			if (oplchannels[id].Sustain >= MIN_SUSTAIN) voices[i].sustained = true;
+			if (oplchannels[id].Sustain >= MIN_SUSTAIN) 
+			{
+				voices[i].sustained = true;
+				voices[i].timestamp = ++timeCounter;
+			}
 			else releaseVoice(i, 0);
 		}
 	}
@@ -475,5 +494,7 @@ void musicBlock::stopAllVoices()
 	for (uint32_t i = 0; i < io->NumChannels; i++)
 	{
 		if (voices[i].index != ~0u) releaseVoice(i, 1);
+		voices[i].timestamp = 0;
 	}
+	timeCounter = 0;
 }
diff --git a/src/sound/oplsynth/musicblock.h b/src/sound/oplsynth/musicblock.h
index 815e3d2679..f6cf2150c1 100644
--- a/src/sound/oplsynth/musicblock.h
+++ b/src/sound/oplsynth/musicblock.h
@@ -15,6 +15,7 @@ struct OPLVoice
 	bool sustained;
 	int8_t	fine_tuning;
 	int	pitch;
+	uint32_t timestamp;
 };
 
 struct musicBlock {
@@ -23,6 +24,7 @@ struct musicBlock {
 
 	OPLChannel oplchannels[NUM_CHANNELS];
 	OPLio *io;
+	uint32_t timeCounter;
 
 	struct GenMidiInstrument OPLinstruments[GENMIDI_NUM_TOTAL];