1 files changed, 125 insertions, 0 deletions
diff --git a/src/Ryujinx.Audio/Backends/CompatLayer/Downmixing.cs b/src/Ryujinx.Audio/Backends/CompatLayer/Downmixing.cs
new file mode 100644
index 00000000..6959c158
--- /dev/null
+++ b/src/Ryujinx.Audio/Backends/CompatLayer/Downmixing.cs
@@ -0,0 +1,125 @@
+using System;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+namespace Ryujinx.Audio.Backends.CompatLayer
+{
+    public static class Downmixing
+    {
+        [StructLayout(LayoutKind.Sequential, Pack = 1)]
+        private struct Channel51FormatPCM16
+        {
+            public short FrontLeft;
+            public short FrontRight;
+            public short FrontCenter;
+            public short LowFrequency;
+            public short BackLeft;
+            public short BackRight;
+        }
+
+        [StructLayout(LayoutKind.Sequential, Pack = 1)]
+        private struct ChannelStereoFormatPCM16
+        {
+            public short Left;
+            public short Right;
+        }
+
+        private const int Q15Bits = 16;
+        private const int RawQ15One = 1 << Q15Bits;
+        private const int RawQ15HalfOne = (int)(0.5f * RawQ15One);
+        private const int Minus3dBInQ15 = (int)(0.707f * RawQ15One);
+        private const int Minus6dBInQ15 = (int)(0.501f * RawQ15One);
+        private const int Minus12dBInQ15 = (int)(0.251f * RawQ15One);
+
+        private static readonly int[] DefaultSurroundToStereoCoefficients = new int[4]
+        {
+            RawQ15One,
+            Minus3dBInQ15,
+            Minus12dBInQ15,
+            Minus3dBInQ15
+        };
+
+        private static readonly int[] DefaultStereoToMonoCoefficients = new int[2]
+        {
+            Minus6dBInQ15,
+            Minus6dBInQ15
+        };
+
+        private const int SurroundChannelCount = 6;
+        private const int StereoChannelCount = 2;
+        private const int MonoChannelCount = 1;
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static ReadOnlySpan<Channel51FormatPCM16> GetSurroundBuffer(ReadOnlySpan<short> data)
+        {
+            return MemoryMarshal.Cast<short, Channel51FormatPCM16>(data);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static ReadOnlySpan<ChannelStereoFormatPCM16> GetStereoBuffer(ReadOnlySpan<short> data)
+        {
+            return MemoryMarshal.Cast<short, ChannelStereoFormatPCM16>(data);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static short DownMixStereoToMono(ReadOnlySpan<int> coefficients, short left, short right)
+        {
+            return (short)((left * coefficients[0] + right * coefficients[1]) >> Q15Bits);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static short DownMixSurroundToStereo(ReadOnlySpan<int> coefficients, short back, short lfe, short center, short front)
+        {
+            return (short)((coefficients[3] * back + coefficients[2] * lfe + coefficients[1] * center + coefficients[0] * front + RawQ15HalfOne) >> Q15Bits);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static short[] DownMixSurroundToStereo(ReadOnlySpan<int> coefficients, ReadOnlySpan<short> data)
+        {
+            int samplePerChannelCount = data.Length / SurroundChannelCount;
+
+            short[] downmixedBuffer = new short[samplePerChannelCount * StereoChannelCount];
+
+            ReadOnlySpan<Channel51FormatPCM16> channels = GetSurroundBuffer(data);
+
+            for (int i = 0; i < samplePerChannelCount; i++)
+            {
+                Channel51FormatPCM16 channel = channels[i];
+
+                downmixedBuffer[i * 2] = DownMixSurroundToStereo(coefficients, channel.BackLeft, channel.LowFrequency, channel.FrontCenter, channel.FrontLeft);
+                downmixedBuffer[i * 2 + 1] = DownMixSurroundToStereo(coefficients, channel.BackRight, channel.LowFrequency, channel.FrontCenter, channel.FrontRight);
+            }
+
+            return downmixedBuffer;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static short[] DownMixStereoToMono(ReadOnlySpan<int> coefficients, ReadOnlySpan<short> data)
+        {
+            int samplePerChannelCount = data.Length / StereoChannelCount;
+
+            short[] downmixedBuffer = new short[samplePerChannelCount * MonoChannelCount];
+
+            ReadOnlySpan<ChannelStereoFormatPCM16> channels = GetStereoBuffer(data);
+
+            for (int i = 0; i < samplePerChannelCount; i++)
+            {
+                ChannelStereoFormatPCM16 channel = channels[i];
+
+                downmixedBuffer[i] = DownMixStereoToMono(coefficients, channel.Left, channel.Right);
+            }
+
+            return downmixedBuffer;
+        }
+
+        public static short[] DownMixStereoToMono(ReadOnlySpan<short> data)
+        {
+            return DownMixStereoToMono(DefaultStereoToMonoCoefficients, data);
+        }
+
+        public static short[] DownMixSurroundToStereo(ReadOnlySpan<short> data)
+        {
+            return DownMixSurroundToStereo(DefaultSurroundToStereoCoefficients, data);
+        }
+    }
+}
+\ No newline at end of file