From 3ca82b64afa0ffd66fba1f0facb3b82176fa4381 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Mon, 15 Mar 2021 20:45:21 -0700
Subject: [PATCH] GB Audio: Fix some channel 4 timing edge cases

---
 CHANGES        |  1 +
 src/gb/audio.c | 11 +++++++----
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/CHANGES b/CHANGES
index 593b8278c..f583abfc0 100644
--- a/CHANGES
+++ b/CHANGES
@@ -29,6 +29,7 @@ Emulation fixes:
  - GB: Downgrade DMG-only ROMs from CGB mode even without boot ROM
  - GB: Fix marking BIOS as unmapped when skipping BIOS (fixes mgba.io/i/2061)
  - GB Audio: Fix serializing sweep time
+ - GB Audio: Fix some channel 4 timing edge cases
  - GB MBC: Fix MBC1 mode changing behavior
  - GB MBC: Fix some MBC3 bit masking
  - GB Video: Fix state after skipping BIOS (fixes mgba.io/i/1715 and mgba.io/i/1716)
diff --git a/src/gb/audio.c b/src/gb/audio.c
index f20fa3fac..121db67d2 100644
--- a/src/gb/audio.c
+++ b/src/gb/audio.c
@@ -390,6 +390,7 @@ void GBAudioWriteNR44(struct GBAudio* audio, uint8_t value) {
 			}
 		}
 		if (audio->playingCh4 && audio->ch4.envelope.dead != 2) {
+			audio->ch4.lastEvent = mTimingCurrentTime(audio->timing);
 			mTimingDeschedule(audio->timing, &audio->ch4Event);
 			mTimingSchedule(audio->timing, &audio->ch4Event, 0);
 		}
@@ -581,14 +582,16 @@ void GBAudioUpdateFrame(struct GBAudio* audio, struct mTiming* timing) {
 		if (audio->playingCh4 && !audio->ch4.envelope.dead) {
 			--audio->ch4.envelope.nextStep;
 			if (audio->ch4.envelope.nextStep == 0) {
-				int8_t sample = audio->ch4.sample > 0;
-				audio->ch4.samples -= audio->ch4.sample;
+				int8_t sample = audio->ch4.sample;
 				_updateEnvelope(&audio->ch4.envelope);
 				if (audio->ch4.envelope.dead == 2) {
 					mTimingDeschedule(timing, &audio->ch4Event);
 				}
-				audio->ch4.sample = sample * audio->ch4.envelope.currentVolume;
-				audio->ch4.samples += audio->ch4.sample;
+				audio->ch4.sample = (sample > 0) * audio->ch4.envelope.currentVolume;
+				if (audio->ch4.nSamples) {
+					audio->ch4.samples -= sample;
+					audio->ch4.samples += audio->ch4.sample;
+				}
 			}
 		}
 		break;