From b0db9272211960f52afb7cf80472045a0b69694d Mon Sep 17 00:00:00 2001
From: Dale Whinham <daleyo@gmail.com>
Date: Thu, 1 Dec 2022 13:51:41 +0000
Subject: [PATCH] Add ability to reverse global stereo output

Closes #307.
---
 CHANGELOG.md       |  4 ++++
 include/config.def |  1 +
 sdcard/mt32-pi.cfg |  7 +++++++
 src/mt32pi.cpp     | 23 +++++++++++++++++++----
 4 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1153c5d..cb81023 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Ability to reverse global left and right audio channels (new configuration file option) to work around broken hardware (issue #307).
+
 ### Changed
 
 - Update to libmt32emu v2.7.0.
diff --git a/include/config.def b/include/config.def
index 01cc73c..aed5624 100644
--- a/include/config.def
+++ b/include/config.def
@@ -53,6 +53,7 @@ BEGIN_SECTION(audio)
 CFG(output_device,		TAudioOutputDevice,		AudioOutputDevice,			TAudioOutputDevice::PWM				)
 CFG(sample_rate,		int,				AudioSampleRate,			48000						)
 CFG(chunk_size,			int,				AudioChunkSize,				256						)
+CFG(reversed_stereo,		bool,				AudioReversedStereo,			false						)
 END_SECTION
 
 BEGIN_SECTION(control)
diff --git a/sdcard/mt32-pi.cfg b/sdcard/mt32-pi.cfg
index a94f311..78cecf5 100644
--- a/sdcard/mt32-pi.cfg
+++ b/sdcard/mt32-pi.cfg
@@ -147,6 +147,13 @@ sample_rate = 48000
 # Values: 2-2048 (256*)
 chunk_size = 256
 
+# Set whether the stereo channels should be swapped or not.
+#
+# Use this option to work around wrongly-wired audio hardware.
+#
+# Values: on, off*
+reversed_stereo = off
+
 # -----------------------------------------------------------------------------
 # Control options
 # -----------------------------------------------------------------------------
diff --git a/src/mt32pi.cpp b/src/mt32pi.cpp
index 6ace8e8..d39aa5b 100644
--- a/src/mt32pi.cpp
+++ b/src/mt32pi.cpp
@@ -562,6 +562,7 @@ void CMT32Pi::AudioTask()
 
 	// Circle's "fast path" for I2S 24-bit really expects 32-bit samples
 	const bool bI2S = m_pConfig->AudioOutputDevice == CConfig::TAudioOutputDevice::I2S;
+	const bool bReversedStereo = m_pConfig->AudioReversedStereo;
 	const u8 nBytesPerSample = bI2S ? sizeof(s32) : (sizeof(s8) * 3);
 	const u8 nBytesPerFrame = 2 * nBytesPerSample;
 
@@ -578,11 +579,25 @@ void CMT32Pi::AudioTask()
 
 		m_pCurrentSynth->Render(FloatBuffer, nFrames);
 
-		// Convert to signed 24-bit integers
-		for (size_t i = 0; i < nFrames * nChannels; ++i)
+		if (bReversedStereo)
 		{
-			s32* const pSample = reinterpret_cast<s32*>(IntBuffer + i * nBytesPerSample);
-			*pSample = FloatBuffer[i] * Sample24BitMax;
+			// Convert to signed 24-bit integers with channel swap
+			for (size_t i = 0; i < nFrames * nChannels; i += nChannels)
+			{
+				s32* const pLeftSample = reinterpret_cast<s32*>(IntBuffer + i * nBytesPerSample);
+				s32* const pRightSample = reinterpret_cast<s32*>(IntBuffer + (i + 1) * nBytesPerSample);
+				*pLeftSample = FloatBuffer[i + 1] * Sample24BitMax;
+				*pRightSample = FloatBuffer[i] * Sample24BitMax;
+			}
+		}
+		else
+		{
+			// Convert to signed 24-bit integers
+			for (size_t i = 0; i < nFrames * nChannels; ++i)
+			{
+				s32* const pSample = reinterpret_cast<s32*>(IntBuffer + i * nBytesPerSample);
+				*pSample = FloatBuffer[i] * Sample24BitMax;
+			}
 		}
 
 		const int nResult = m_pSound->Write(IntBuffer, nWriteBytes);