optimize StreamingSound allocation and update

src/Audio/StreamingSound.cs

@@ -1,23 +1,22 @@
 using System;
-using System.Collections.Generic;
 using System.Runtime.InteropServices;
 
 namespace MoonWorks.Audio
 {
 	/// <summary>
 	/// For streaming long playback.
-	/// Can be extended to support custom decoders.
+	/// Must be extended with a decoder routine called by FillBuffer.
+	/// See StreamingSoundOgg for an example.
 	/// </summary>
 	public abstract class StreamingSound : SoundInstance
 	{
-		private readonly List<IntPtr> queuedBuffers = new List<IntPtr>();
-		private const int MINIMUM_BUFFER_CHECK = 3;
-
-		private int PendingBufferCount => queuedBuffers.Count;
-
+		private const int BUFFER_COUNT = 3;
+		private readonly IntPtr[] buffers;
+		private int nextBufferIndex = 0;
+		private uint queuedBufferCount = 0;
 		public abstract int BUFFER_SIZE { get; }
 
-		public StreamingSound(
+		public unsafe StreamingSound(
 			AudioDevice device,
 			ushort formatTag,
 			ushort bitsPerSample,
@@ -27,6 +26,12 @@ namespace MoonWorks.Audio
 		) : base(device, formatTag, bitsPerSample, blockAlign, channels, samplesPerSecond)
 		{
 			device.AddDynamicSoundInstance(this);
+
+			buffers = new IntPtr[BUFFER_COUNT];
+			for (int i = 0; i < BUFFER_COUNT; i += 1)
+			{
+				buffers[i] = (IntPtr) NativeMemory.Alloc((nuint) BUFFER_SIZE);
+			}
 		}
 
 		public override void Play()
@@ -78,25 +83,14 @@ namespace MoonWorks.Audio
 				FAudio.FAUDIO_VOICE_NOSAMPLESPLAYED
 			);
 
-			while (PendingBufferCount > state.BuffersQueued)
-			{
-				lock (queuedBuffers)
-				{
-					NativeMemory.Free((void*) queuedBuffers[0]);
-					queuedBuffers.RemoveAt(0);
-				}
-			}
+			queuedBufferCount = state.BuffersQueued;
 
 			QueueBuffers();
 		}
 
 		protected void QueueBuffers()
 		{
-			for (
-				int i = MINIMUM_BUFFER_CHECK - PendingBufferCount;
-				i > 0;
-				i -= 1
-			)
+			for (int i = 0; i < BUFFER_COUNT - queuedBufferCount; i += 1)
 			{
 				AddBuffer();
 			}
@@ -104,32 +98,22 @@ namespace MoonWorks.Audio
 
 		protected unsafe void ClearBuffers()
 		{
-			lock (queuedBuffers)
-			{
-				foreach (IntPtr buf in queuedBuffers)
-				{
-					NativeMemory.Free((void*) buf);
-				}
-				queuedBuffers.Clear();
-			}
+			nextBufferIndex = 0;
+			queuedBufferCount = 0;
 		}
 
 		protected unsafe void AddBuffer()
 		{
-			void* buffer = NativeMemory.Alloc((nuint) BUFFER_SIZE);
+			var buffer = buffers[nextBufferIndex];
+			nextBufferIndex = (nextBufferIndex + 1) % BUFFER_COUNT;
 
 			FillBuffer(
-				buffer,
+				(void*) buffer,
 				BUFFER_SIZE,
 				out int filledLengthInBytes,
 				out bool reachedEnd
 			);
 
-			lock (queuedBuffers)
-			{
-				queuedBuffers.Add((IntPtr) buffer);
-				if (State != SoundState.Stopped)
-				{
 			FAudio.FAudioBuffer buf = new FAudio.FAudioBuffer
 			{
 				AudioBytes = (uint) filledLengthInBytes,
@@ -146,9 +130,8 @@ namespace MoonWorks.Audio
 				ref buf,
 				IntPtr.Zero
 			);
-				}
-			}
 
+			queuedBufferCount += 1;
 
 			/* We have reached the end of the file, what do we do? */
 			if (reachedEnd)
@@ -169,9 +152,14 @@ namespace MoonWorks.Audio
 			out bool reachedEnd
 		);
 
-		protected override void Destroy()
+		protected unsafe override void Destroy()
 		{
 			StopImmediate();
+
+			for (int i = 0; i < BUFFER_COUNT; i += 1)
+			{
+				NativeMemory.Free((void*) buffers[i]);
+			}
 		}
 	}
 }