feat: play music through librubberband

app/src/main/cpp/MixingPlayer.h

@@ -6,7 +6,10 @@
 #define LOCKSTEP_MIXINGPLAYER_H
 
 #include <oboe/Oboe.h>
-#include <math.h>
+#include <cmath>
+#include <atomic>
+#include "AudioCallback.h"
+
 using namespace oboe;
 
 /**
@@ -20,7 +23,7 @@ protected:
     std::atomic<int> startBeat;
     int numBeatsPlaying;
 public:
-    explicit MixingPlayer(std::vector<float> beatSound) : beatSound(beatSound), startBeat(0), numBeatsPlaying(0) {}
+    explicit MixingPlayer(std::vector<float> beatSound) : beatSound(beatSound), startBeat(0), numBeatsPlaying(0), mHaveMusic(false) {}
 
     virtual ~MixingPlayer() = default;
 
@@ -63,6 +66,12 @@ public:
         }
     }
 
+    void setMusic(std::shared_ptr<AudioCallbackProvider> cb) {
+        std::lock_guard<std::mutex> lock(mLock);
+        mMusic = std::move(cb);
+        mHaveMusic.store((bool) mMusic);
+    }
+
     oboe::DataCallbackResult onAudioReady(oboe::AudioStream *oboeStream, void *audioData, int32_t numFrames) override {
         // fetch startBeat register
         int isStartBeat = startBeat.load();
@@ -78,7 +87,7 @@ public:
         int N = (int) beatSound.size();
         for (int i = 0; i < numFrames; i++) {
             float sample = 0.0;
-            float norm = (float) numBeatsPlaying;
+            float norm = (float) (std::max(numBeatsPlaying, 1));
             for (int k = 0; k < numBeatsPlaying; k++) {
                 sample += beatSound[beatIdx[k]];
                 beatIdx[k] += 1;
@@ -102,12 +111,18 @@ public:
             }
         }
 
+        if(mHaveMusic.load()) {
+            mMusic->onAudioReady(floatData, numFrames);
+        }
+
         return oboe::DataCallbackResult::Continue;
     }
 
 private:
     std::mutex         mLock;
     std::shared_ptr<oboe::AudioStream> mStream;
+    std::shared_ptr<AudioCallbackProvider> mMusic;
+    std::atomic<bool> mHaveMusic;
 
     // Stream params
     static int constexpr kChannelCount = 2;