From 6158a4fb8d6bc8e7df42504bad7e18d9b5c667a3 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Wed, 5 Dec 2018 19:47:57 -0800
Subject: [PATCH] GB Audio: Skip frame if enabled when clock is high

---
 include/mgba/internal/gb/audio.h     |  1 +
 include/mgba/internal/gb/serialize.h |  1 +
 src/gb/audio.c                       | 17 +++++++++++++++++
 src/gb/timer.c                       |  3 ++-
 4 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/mgba/internal/gb/audio.h b/include/mgba/internal/gb/audio.h
index 837e0a1b3..ed3a76325 100644
--- a/include/mgba/internal/gb/audio.h
+++ b/include/mgba/internal/gb/audio.h
@@ -187,6 +187,7 @@ struct GBAudio {
 	uint8_t* nr52;
 
 	int frame;
+	bool skipFrame;
 
 	int32_t sampleInterval;
 	enum GBAudioStyle style;
diff --git a/include/mgba/internal/gb/serialize.h b/include/mgba/internal/gb/serialize.h
index 14cfd12d4..7b1feb93e 100644
--- a/include/mgba/internal/gb/serialize.h
+++ b/include/mgba/internal/gb/serialize.h
@@ -195,6 +195,7 @@ DECL_BITS(GBSerializedAudioFlags, Frame, 22, 3);
 DECL_BIT(GBSerializedAudioFlags, Ch1SweepEnabled, 25);
 DECL_BIT(GBSerializedAudioFlags, Ch1SweepOccurred, 26);
 DECL_BIT(GBSerializedAudioFlags, Ch3Readable, 27);
+DECL_BIT(GBSerializedAudioFlags, SkipFrame, 28);
 
 DECL_BITFIELD(GBSerializedAudioEnvelope, uint32_t);
 DECL_BITS(GBSerializedAudioEnvelope, Length, 0, 7);
diff --git a/src/gb/audio.c b/src/gb/audio.c
index ee5ec9ed8..08ba1fa9e 100644
--- a/src/gb/audio.c
+++ b/src/gb/audio.c
@@ -470,7 +470,15 @@ void GBAudioWriteNR52(struct GBAudio* audio, uint8_t value) {
 		}
 		*audio->nr52 &= ~0x000F;
 	} else if (!wasEnable) {
+		audio->skipFrame = false;
 		audio->frame = 7;
+
+		if (audio->p) {
+			unsigned timingFactor = 0x400 >> !audio->p->doubleSpeed;
+			if (audio->p->timer.internalDiv & timingFactor) {
+				audio->skipFrame = true;
+			}
+		}
 	}
 }
 
@@ -483,6 +491,13 @@ void _updateFrame(struct mTiming* timing, void* user, uint32_t cyclesLate) {
 }
 
 void GBAudioUpdateFrame(struct GBAudio* audio, struct mTiming* timing) {
+	if (!audio->enable) {
+		return;
+	}
+	if (audio->skipFrame) {
+		audio->skipFrame = false;
+		return;
+	}
 	int frame = (audio->frame + 1) & 7;
 	audio->frame = frame;
 
@@ -929,6 +944,7 @@ void GBAudioPSGSerialize(const struct GBAudio* audio, struct GBSerializedPSGStat
 	uint32_t ch4Flags = 0;
 
 	flags = GBSerializedAudioFlagsSetFrame(flags, audio->frame);
+	flags = GBSerializedAudioFlagsSetSkipFrame(flags, audio->skipFrame);
 	STORE_32LE(audio->frameEvent.when - mTimingCurrentTime(audio->timing), 0, &state->ch1.nextFrame);
 
 	flags = GBSerializedAudioFlagsSetCh1Volume(flags, audio->ch1.envelope.currentVolume);
@@ -987,6 +1003,7 @@ void GBAudioPSGDeserialize(struct GBAudio* audio, const struct GBSerializedPSGSt
 
 	LOAD_32LE(flags, 0, flagsIn);
 	audio->frame = GBSerializedAudioFlagsGetFrame(flags);
+	audio->skipFrame = GBSerializedAudioFlagsGetSkipFrame(flags);
 
 	LOAD_32LE(ch1Flags, 0, &state->ch1.envelope);
 	audio->ch1.envelope.currentVolume = GBSerializedAudioFlagsGetCh1Volume(flags);
diff --git a/src/gb/timer.c b/src/gb/timer.c
index 32c9efdc5..e990b3534 100644
--- a/src/gb/timer.c
+++ b/src/gb/timer.c
@@ -80,7 +80,8 @@ void GBTimerDivReset(struct GBTimer* timer) {
 			mTimingSchedule(&timer->p->timing, &timer->irq, 7 - (timer->p->cpu->executionState & 3));
 		}
 	}
-	if (timer->internalDiv & 0x200) {
+	unsigned timingFactor = 0x400 >> !timer->p->doubleSpeed;
+	if (timer->internalDiv & timingFactor) {
 		GBAudioUpdateFrame(&timer->p->audio, &timer->p->timing);
 	}
 	timer->p->memory.io[REG_DIV] = 0;