lockstep/app/src/main/cpp/MixingPlayer.h

//
// Created by david on 01.02.2026.
//

#ifndef LOCKSTEP_MIXINGPLAYER_H
#define LOCKSTEP_MIXINGPLAYER_H

#include <oboe/Oboe.h>
#include <math.h>
using namespace oboe;

/**
 * Plays event sounds, potentially overlapping in time.
 * Use <c>setStartBeat()</c> to trigger playing sound for an event.
 */
class MixingPlayer: public oboe::AudioStreamDataCallback {
protected:
    std::vector<float> beatSound;
    std::vector<int> beatIdx;
    std::atomic<int> startBeat;
    int numBeatsPlaying;
public:
    explicit MixingPlayer(std::vector<float> beatSound) : beatSound(beatSound), startBeat(0), numBeatsPlaying(0) {}

    virtual ~MixingPlayer() = default;

    /**
     * Play a beat sound by setting the startBeat register.
     * A delay of the audio callback interval will be added.
     */
    void setStartBeat() {
        startBeat.store(1);
    }

    // Call this from Activity onResume()
    int32_t startAudio() {
        std::lock_guard<std::mutex> lock(mLock);
        oboe::AudioStreamBuilder builder;
        // The builder set methods can be chained for convenience.
        Result result = builder.setSharingMode(oboe::SharingMode::Exclusive)
                ->setPerformanceMode(oboe::PerformanceMode::LowLatency)
                ->setChannelCount(kChannelCount)
                ->setSampleRate(kSampleRate)
                ->setSampleRateConversionQuality(oboe::SampleRateConversionQuality::Medium)
                ->setFormat(oboe::AudioFormat::Float)
                ->setDataCallback(this)
                ->openStream(mStream);
        if (result != Result::OK) return (int32_t) result;

        // Typically, start the stream after querying some stream information, as well as some input from the user
        result = mStream->requestStart();
        return (int32_t) result;
    }

    // Call this from Activity onPause()
    void stopAudio() {
        // Stop, close and delete in case not already closed.
        std::lock_guard<std::mutex> lock(mLock);
        if (mStream) {
            mStream->stop();
            mStream->close();
            mStream.reset();
        }
    }

    oboe::DataCallbackResult onAudioReady(oboe::AudioStream *oboeStream, void *audioData, int32_t numFrames) override {
        // fetch startBeat register
        int isStartBeat = startBeat.load();
        if(isStartBeat) {
            if(beatIdx.size() < numBeatsPlaying+1) beatIdx.resize(numBeatsPlaying+1);
            beatIdx[numBeatsPlaying++] = 0;
            startBeat.store(0); // reset startBeat register
        }
        //
        // mix audio events, each event has a separate idx counter in beatIdx
        //
        float *floatData = (float *) audioData;
        int N = (int) beatSound.size();
        for (int i = 0; i < numFrames; i++) {
            float sample = 0.0;
            float norm = (float) numBeatsPlaying;
            for (int k = 0; k < numBeatsPlaying; k++) {
                sample += beatSound[beatIdx[k]];
                beatIdx[k] += 1;
            }
            // reduce beatIdx to the events which have not completed playing
            int l = 0;
            int m = 0;
            for (; m < numBeatsPlaying; l++, m++) {
                while(m < numBeatsPlaying && beatIdx[m] >= N) m++;
                if(m < numBeatsPlaying)
                    beatIdx[l] = beatIdx[m];
                else
                    break; // avoid incrementing l
            }
            //beatIdx.resize(l); // we avoid re-allocating
            numBeatsPlaying = l;
            // set left and right output channels
            for (int j = 0; j < kChannelCount; j++) {
                // normalize sample by numBeatsPlaying (avoids clipping)
                floatData[i * kChannelCount + j] = sample / norm;
            }
        }

        return oboe::DataCallbackResult::Continue;
    }

private:
    std::mutex         mLock;
    std::shared_ptr<oboe::AudioStream> mStream;

    // Stream params
    static int constexpr kChannelCount = 2;
    static int constexpr kSampleRate = 48000;
};

#endif //LOCKSTEP_MIXINGPLAYER_H