feat: TempoController: minimal demo

app/src/main/cpp/TempoController.cpp

@@ -37,7 +37,9 @@ void TempoController::seekTo(double oldMarkerPosSec, double newMarkerPosSec) {
  */
 void TempoController::onBeat(double t) {
     std::lock_guard<std::mutex> lock(mMutex);
+    if(lastBeatT != 0.0) dtBeat = t - lastBeatT;
     lastBeatT = t;
+    setStretchFactor();
 }
 
 /**
@@ -46,5 +48,16 @@ void TempoController::onBeat(double t) {
  */
 void TempoController::onStep(double t) {
     std::lock_guard<std::mutex> lock(mMutex);
+    if(lastStepT != 0.0) dtStep = t - lastStepT;
     lastStepT = t;
+    setStretchFactor();
+}
+
+void TempoController::setStretchFactor() {
+    if(dtBeat == 0.0 || dtStep == 0.0) return;
+    double ratio = dtStep / dtBeat;
+    ratio = std::min(3.3, ratio);
+    ratio = std::max(0.3, ratio);
+    this->have_stretch_factor->store(true);
+    this->stretch_factor->store(ratio);
 }

app/src/main/cpp/TempoController.h

@@ -17,6 +17,8 @@ private:
     std::atomic<double> *stretch_factor;
     double lastBeatT;
     double lastStepT;
+    double dtBeat;
+    double dtStep;
 public:
     TempoController(std::atomic<bool> *have_stretch_factor, std::atomic<double> *stretch_factor);
     void reset(std::vector<double> beatTimesSec);
@@ -33,6 +35,8 @@ public:
      * @param t marker position in sec of original music time
      */
     void onStep(double t);
+private:
+    void setStretchFactor();
 };
 
 #endif //LOCKSTEP_TEMPOCONTROLLER_H