ABStemPlayer/AudioCore/Impl/AudioMixer.cs

namespace AudioCore.Impl;

public sealed class AudioMixer : IAudioMixer
{
    private readonly AudioBufferPool _pool;

    private const int _outputChannels = 2;

    public AudioMixer(AudioBufferPool pool)
    {
        _pool = pool;
    }

    public MixedAudioBlock Mix(
        IReadOnlyList<TimeStretchedAudioBlock> stemBlocks,
        MixerSettings settings)
    {
        if (stemBlocks.Count == 0)
            throw new ArgumentException("No stems provided");

        // All blocks must have same sample rate and frame count
        var first      = stemBlocks[0];
        var frames     = first.Frames;
        var sampleRate = first.SampleRate;
        var position   = first.Position;

        // Rent output buffer
        var outBuf = _pool.Rent(frames * _outputChannels);
        outBuf.Length = frames * _outputChannels;

        Span<float> outSpan = outBuf.Span;
        outSpan.Clear();

        for (var stem = 0; stem < stemBlocks.Count; stem++)
        {
            var s = settings.Stems[stem];
            if (!s.Enabled)
                continue;

            var block = stemBlocks[stem];
            Span<float> inSpan = block.Buffer.Span;

            var inChannels = block.Channels;

            var gain = DbToLinear(s.GainDb);
            var pan = s.Pan;

            var leftGain = gain * (pan <= 0 ? 1f : 1f - pan);
            var rightGain = gain * (pan >= 0 ? 1f : 1f + pan);

            for (var i = 0; i < frames; i++)
            {
                var baseIndex = i * inChannels;

                // If there are no samples for this frame, skip
                if (baseIndex >= inSpan.Length)
                {
                    outSpan[i * 2 + 0] = 0f;
                    outSpan[i * 2 + 1] = 0f;
                    continue;
                }

                if (inChannels == 1)
                {
                    var inp = inSpan[baseIndex];
                    outSpan[i * 2 + 0] += inp * leftGain;
                    outSpan[i * 2 + 1] += inp * rightGain;
                }
                else
                {
                    // ensure both left and right samples exist for multi-channel input
                    if (baseIndex + 1 >= inSpan.Length)
                        continue; // or treat missing right as 0, depending on desired behavior

                    var l = inSpan[baseIndex + 0];
                    var r = inSpan[baseIndex + 1];

                    outSpan[i * 2 + 0] += l * leftGain;
                    outSpan[i * 2 + 1] += r * rightGain;
                }
            }
        }


        return new MixedAudioBlock(outBuf, frames, _outputChannels, sampleRate, position);
    }

    private static float DbToLinear(float db)
        => db <= -80f ? 0f : MathF.Pow(10f, db / 20f);
}