From facada5d04b0f6aa0ff1b23d2b6f916341b8fb02 Mon Sep 17 00:00:00 2001
From: Ziemas <ziemas@ziemas.se>
Date: Tue, 17 Oct 2023 04:44:35 +0200
Subject: [PATCH] SPU2: Simplify reverb resampling

---
 pcsx2/SPU2/Reverb.cpp | 49 +++++++++++++++++++------------------------
 pcsx2/SPU2/defs.h     |  6 +++---
 2 files changed, 24 insertions(+), 31 deletions(-)

diff --git a/pcsx2/SPU2/Reverb.cpp b/pcsx2/SPU2/Reverb.cpp
index 2f007df04d..5718706711 100644
--- a/pcsx2/SPU2/Reverb.cpp
+++ b/pcsx2/SPU2/Reverb.cpp
@@ -99,47 +99,34 @@ static constexpr std::array<s32, NUM_TAPS> filter_coefs = {
 
 s32 __forceinline V_Core::ReverbDownsample(bool right)
 {
+	int index = (RevbSampleBufPos - NUM_TAPS) & 63;
 	s32 out = 0;
 
-	// Skipping the 0 coefs.
-	for (u32 i = 0; i < NUM_TAPS; i += 2)
+	for (int i = 0; i < NUM_TAPS; i++)
 	{
-		out += RevbDownBuf[right][((RevbSampleBufPos - NUM_TAPS) + i) & 63] * filter_coefs[i];
+		out += RevbDownBuf[right][index + i] * filter_coefs[i];
 	}
 
-	// We also skipped the middle so add that in.
-	out += RevbDownBuf[right][((RevbSampleBufPos - NUM_TAPS) + 19) & 63] * filter_coefs[19];
-
 	out >>= 15;
 	out = std::clamp<s32>(out, INT16_MIN, INT16_MAX);
 
 	return out;
 }
 
-StereoOut32 __forceinline V_Core::ReverbUpsample(bool phase)
+StereoOut32 __forceinline V_Core::ReverbUpsample()
 {
+	int index = (RevbSampleBufPos - NUM_TAPS) & 63;
 	s32 ls = 0, rs = 0;
 
-	if (phase)
+	for (int i = 0; i < NUM_TAPS; i++)
 	{
-		ls += RevbUpBuf[0][(((RevbSampleBufPos - NUM_TAPS) >> 1) + 9) & 63] * filter_coefs[19];
-		rs += RevbUpBuf[1][(((RevbSampleBufPos - NUM_TAPS) >> 1) + 9) & 63] * filter_coefs[19];
-	}
-	else
-	{
-		for (u32 i = 0; i < (NUM_TAPS >> 1) + 1; i++)
-		{
-			ls += RevbUpBuf[0][(((RevbSampleBufPos - NUM_TAPS) >> 1) + i) & 63] * filter_coefs[i * 2];
-		}
-		for (u32 i = 0; i < (NUM_TAPS >> 1) + 1; i++)
-		{
-			rs += RevbUpBuf[1][(((RevbSampleBufPos - NUM_TAPS) >> 1) + i) & 63] * filter_coefs[i * 2];
-		}
+		ls += RevbUpBuf[0][index + i] * (filter_coefs[i] * 2);
+		rs += RevbUpBuf[1][index + i] * (filter_coefs[i] * 2);
 	}
 
-	ls >>= 14;
+	ls >>= 15;
 	ls = std::clamp<s32>(ls, INT16_MIN, INT16_MAX);
-	rs >>= 14;
+	rs >>= 15;
 	rs = std::clamp<s32>(rs, INT16_MIN, INT16_MAX);
 
 	return {ls, rs};
@@ -164,8 +151,10 @@ StereoOut32 V_Core::DoReverb(const StereoOut32& Input)
 		return StereoOut32::Empty;
 	}
 
-	RevbDownBuf[0][RevbSampleBufPos & 63] = Input.Left;
-	RevbDownBuf[1][RevbSampleBufPos & 63] = Input.Right;
+	RevbDownBuf[0][RevbSampleBufPos] = Input.Left;
+	RevbDownBuf[1][RevbSampleBufPos] = Input.Right;
+	RevbDownBuf[0][RevbSampleBufPos + 64] = Input.Left;
+	RevbDownBuf[1][RevbSampleBufPos + 64] = Input.Right;
 
 	bool R = Cycles & 1;
 
@@ -245,9 +234,13 @@ StereoOut32 V_Core::DoReverb(const StereoOut32& Input)
 		_spu2mem[apf2_dst] = clamp_mix(apf2);
 	}
 
-	RevbUpBuf[R][(RevbSampleBufPos >> 1) & 63] = clamp_mix(out);
+	RevbUpBuf[R][RevbSampleBufPos] = clamp_mix(out);
+	RevbUpBuf[!R][RevbSampleBufPos] = 0;
 
-	RevbSampleBufPos++;
+	RevbUpBuf[R][RevbSampleBufPos + 64] = clamp_mix(out);
+	RevbUpBuf[!R][RevbSampleBufPos + 64] = 0;
 
-	return ReverbUpsample(RevbSampleBufPos & 1);
+	RevbSampleBufPos = (RevbSampleBufPos + 1) & 63;
+
+	return ReverbUpsample();
 }
diff --git a/pcsx2/SPU2/defs.h b/pcsx2/SPU2/defs.h
index 0e06a70826..6733ad0ac0 100644
--- a/pcsx2/SPU2/defs.h
+++ b/pcsx2/SPU2/defs.h
@@ -422,8 +422,8 @@ struct V_Core
 
 	V_Reverb Revb; // Reverb Registers
 
-	s32 RevbDownBuf[2][64]; // Downsample buffer for reverb, one for each channel
-	s32 RevbUpBuf[2][64]; // Upsample buffer for reverb, one for each channel
+	s32 RevbDownBuf[2][64 * 2]; // Downsample buffer for reverb, one for each channel
+	s32 RevbUpBuf[2][64 * 2]; // Upsample buffer for reverb, one for each channel
 	u32 RevbSampleBufPos;
 	u32 EffectsStartA;
 	u32 EffectsEndA;
@@ -487,7 +487,7 @@ struct V_Core
 	s32 RevbGetIndexer(s32 offset);
 
 	s32 ReverbDownsample(bool right);
-	StereoOut32 ReverbUpsample(bool phase);
+	StereoOut32 ReverbUpsample();
 
 	StereoOut32 ReadInput();
 	StereoOut32 ReadInput_HiFi();