From 46b59268d384f634078352e93ad6a39b7673e6de Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Thu, 2 Jun 2022 23:26:15 -0700
Subject: [PATCH] GB Audio: Optimize channel 4

---
 src/gb/audio.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/src/gb/audio.c b/src/gb/audio.c
index b1a3f68dd..7a1fe60bf 100644
--- a/src/gb/audio.c
+++ b/src/gb/audio.c
@@ -568,13 +568,24 @@ void GBAudioRun(struct GBAudio* audio, int32_t timestamp, int channels) {
 
 		int32_t last = 0;
 		int32_t diff = timestamp - audio->ch4.lastEvent;
+		int samples = 0;
+		int positiveSamples = 0;
+		int lsb;
+		int coeff = 0x60;
+		if (!audio->ch4.power) {
+			coeff <<= 8;
+		}
 		for (; last + cycles <= diff; last += cycles) {
-			int lsb = audio->ch4.lfsr & 1;
-			audio->ch4.sample = lsb * audio->ch4.envelope.currentVolume;
-			++audio->ch4.nSamples;
-			audio->ch4.samples += audio->ch4.sample;
+			lsb = audio->ch4.lfsr & 1;
 			audio->ch4.lfsr >>= 1;
-			audio->ch4.lfsr ^= (lsb * 0x60) << (audio->ch4.power ? 0 : 8);
+			audio->ch4.lfsr ^= lsb * coeff;
+			++samples;
+			positiveSamples += lsb;
+		}
+		if (samples) {
+			audio->ch4.sample = lsb * audio->ch4.envelope.currentVolume;
+			audio->ch4.nSamples += samples;
+			audio->ch4.samples += positiveSamples * audio->ch4.envelope.currentVolume;
 		}
 
 		audio->ch4.lastEvent += last;