meeting-assistant/MeetingAssistant.Tests/AudioMixingTests.cs

using MeetingAssistant.Recording;
using Microsoft.Extensions.Logging.Abstractions;

namespace MeetingAssistant.Tests;

public sealed class AudioMixingTests
{
    [Fact]
    public async Task CompositeAudioSourceMixesMicrophoneAndSystemAudioIntoOnePcmStream()
    {
        var microphone = new FixedAudioSource(Pcm16(10_000));
        var system = new FixedAudioSource(Pcm16(20_000));
        var source = new CompositeMeetingAudioSource(
            microphone,
            system,
            NullLogger<CompositeMeetingAudioSource>.Instance);

        var chunks = await ReadChunks(source);

        Assert.Single(chunks);
        Assert.Equal(30_000, BitConverter.ToInt16(chunks[0].Pcm));
    }

    [Fact]
    public async Task CompositeAudioSourceClampsMixedSamples()
    {
        var microphone = new FixedAudioSource(Pcm16(30_000));
        var system = new FixedAudioSource(Pcm16(10_000));
        var source = new CompositeMeetingAudioSource(
            microphone,
            system,
            NullLogger<CompositeMeetingAudioSource>.Instance);

        var chunks = await ReadChunks(source);

        Assert.Equal(short.MaxValue, BitConverter.ToInt16(chunks[0].Pcm));
    }

    private static async Task<IReadOnlyList<AudioChunk>> ReadChunks(IMeetingAudioSource source)
    {
        var chunks = new List<AudioChunk>();
        await foreach (var chunk in source.CaptureAsync(CancellationToken.None))
        {
            chunks.Add(chunk);
        }

        return chunks;
    }

    private static byte[] Pcm16(short sample)
    {
        return BitConverter.GetBytes(sample);
    }

    private sealed class FixedAudioSource : IMeetingAudioSource
    {
        private readonly AudioChunk chunk;

        public FixedAudioSource(byte[] pcm)
        {
            chunk = new AudioChunk(pcm, 16000, 1);
        }

        public async IAsyncEnumerable<AudioChunk> CaptureAsync(
            [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken)
        {
            await Task.Yield();
            yield return chunk;
        }
    }
}