feat: MixingPlayer - plays event sounds

app/src/main/cpp/MixingPlayer.h

@@ -0,0 +1,117 @@
+//
+// 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