Playback: append OutputStream playback of recorded buffer

tests/01-record-play-cs/Program.cs

@@ -85,6 +85,62 @@ try
 
     WavWriter.Write(OutWav, buf, SampleRate);
     Console.WriteLine($"Wrote {OutWav} ({buf.Length} frames @ {SampleRate} Hz)");
+
+    Console.WriteLine($"Playing back through device {device}");
+
+    int playOffset = 0;
+    object playGate = new();
+    var doneEvent = new ManualResetEventSlim(false);
+
+    var outParams = new StreamParameters
+    {
+        device = device,
+        channelCount = Channels,
+        sampleFormat = SampleFormat.Int16,
+        suggestedLatency = PortAudio.GetDeviceInfo(device).defaultLowOutputLatency,
+        hostApiSpecificStreamInfo = IntPtr.Zero,
+    };
+
+    Stream.Callback playCb = (IntPtr _, IntPtr output, uint frameCount,
+        ref StreamCallbackTimeInfo _2, StreamCallbackFlags _3, IntPtr _4) =>
+    {
+        lock (playGate)
+        {
+            int remaining = buf.Length - playOffset;
+            int take = (int)Math.Min(frameCount, (uint)remaining);
+            int silence = (int)frameCount - take;
+
+            unsafe
+            {
+                short* dst = (short*)output.ToPointer();
+                if (take > 0)
+                {
+                    fixed (short* src = &buf[playOffset])
+                        Buffer.MemoryCopy(src, dst, take * sizeof(short), take * sizeof(short));
+                    playOffset += take;
+                }
+                if (silence > 0)
+                {
+                    for (int i = take; i < frameCount; i++) dst[i] = 0;
+                }
+            }
+
+            if (playOffset >= buf.Length)
+            {
+                doneEvent.Set();
+                return StreamCallbackResult.Complete;
+            }
+        }
+        return StreamCallbackResult.Continue;
+    };
+
+    using var playStream = new Stream(
+        null, outParams, SampleRate, BlockFrames, StreamFlags.NoFlag, playCb, IntPtr.Zero);
+    playStream.Start();
+    doneEvent.Wait();
+    // Give PortAudio a moment to drain the device buffer before Stop().
+    Thread.Sleep(200);
+    playStream.Stop();
 }
 finally
 {