Elementary/Elementary/Audio/AudioMixer.cs

using ManagedBass;
using ManagedBass.Mix;

namespace Elementary.Audio;

public class AudioMixer
{
    private readonly int _mixerStream;
    private MemoryStream?[] _inputStreams;
    private Stream _outStream;
    private object _lock = new();

    public AudioMixer(Stream outStream)
    {
        _outStream = outStream;
        Bass.Init(-1, 44100, DeviceInitFlags.NoSpeakerAssignment, IntPtr.Zero);
        _mixerStream = BassMix.CreateMixerStream(44100, 2, BassFlags.Float | BassFlags.MixerNonStop);
        _inputStreams = new MemoryStream[10];

        Bass.ChannelPlay(_mixerStream);
    }

    /// <summary>
    /// Add a stream to the mixer.
    /// wait for the stream to finish playing 
    /// </summary>
    /// <param name="stream"></param>
    /// <param name="volume"></param>
    public void AddStream(Stream stream, float volume = 1.0f)
    {
        Console.WriteLine("Adding stream to mixer");
        byte[] pcmBytes;
        using (MemoryStream ms = new())
        {
            stream.CopyTo(ms);
            pcmBytes = ms.ToArray();
        }

        lock (_lock)
        {
            for (var i = 0; i < _inputStreams.Length; i++)
            {
                if (_inputStreams[i] == null)
                {
                    _inputStreams[i] = new MemoryStream(pcmBytes);
                    break;
                }
            }
        }
        
        var channel = Bass.CreateStream(pcmBytes, 0, pcmBytes.Length, BassFlags.Float);
        Bass.ChannelSetAttribute(channel, ChannelAttribute.Volume, volume);
        BassMix.MixerAddChannel(_mixerStream, channel, BassFlags.Default);
        
        int length = Bass.ChannelGetData(_mixerStream, new byte[4096], 4096);
        byte[] buffer = new byte[length];
        length = Bass.ChannelGetData(_mixerStream, buffer, length);
        _outStream.Write(buffer, 0, length);
    }

    public void Stop()
    {
        Bass.ChannelStop(_mixerStream);
    }

    public void Dispose()
    {
        lock (_lock)
        {
            foreach (var stream in _inputStreams)
            {
                stream?.Dispose();
            }
        }
        
        _outStream.Flush();
        Bass.StreamFree(_mixerStream);
        Bass.Free();
    }
}