feat: StepDetector

move StepDetector from lockstep android to libpasada

pasada-lib/CMakeLists.txt

@@ -5,6 +5,7 @@ SET(PASADA_SRC
         iir_filter.cpp
         ssf_filter.cpp
         pd_signal.cpp
+        step_detector.cpp
 )
 
 if(PASADA_BUILD_TESTS)

pasada-lib/include/ssf_filter.h

@@ -52,6 +52,8 @@ public:
     SsfStepDetector(size_t len_refr);
     double filter(double val);
     double peek_threshold();
+
+    static size_t initial_samples();
 };
 
 /**

pasada-lib/include/step_detector.h

@@ -0,0 +1,52 @@
+//
+// Created by david on 15.03.2026.
+//
+
+#ifndef PASADASUPERPROJECT_STEP_DETECTOR_H
+#define PASADASUPERPROJECT_STEP_DETECTOR_H
+
+#include "iir_filter.h"
+#include "ssf_filter.h"
+#include <vector>
+
+class StepListener {
+public:
+    virtual ~StepListener() {}
+    virtual void playBeat() = 0;
+};
+
+/**
+ * Step detector from accelerometer signal.
+ *
+ * Settling time is 3.0 sec (defined in SsfStepDetector.LEN_INIT),
+ * no steps are detected before.
+ */
+class StepDetector {
+protected:
+    StepListener *listener;
+    IirFilter f_highpass;
+    Filt f_neg;
+    SsfFilter f_ssf;
+    SsfStepDetector f_ssd;
+    RunningQualityFilter f_sqi;
+
+    bool debug;
+    std::vector<double> buf_ssd;
+    std::vector<double> buf_sqi;
+    std::vector<double> buf_out;
+
+public:
+    StepDetector(StepListener *listener, bool debug = false);
+    void filter(std::vector<float> values);
+    std::vector<double> getBufSsd();
+    std::vector<double> getBufSqi();
+    std::vector<double> getBufOut();
+
+    /**
+     * Prime the filters using the given input signal.
+     * Used for debugging (non-realtime processing) to align the signal.
+     */
+    void primeFilters(std::vector<double> sig);
+};
+
+#endif //PASADASUPERPROJECT_STEP_DETECTOR_H

pasada-lib/ssf_filter.cpp

@@ -40,8 +40,10 @@ double SsfFilter::filter(double val) {
     return ssf;
 }
 
+size_t SsfStepDetector::initial_samples() { return (size_t) (3.0 * FPS); }
 
 SsfStepDetector::SsfStepDetector(size_t len_refr) :
+    // note: also change above, in initial_samples()
     LEN_INIT((size_t) (3.0 * FPS)), // initial window length for ssf_threshold
     LEN_TH_WIN((size_t) (3.0 * FPS)), // subsequent window length for ssf_threshold
     num_samples(0),

pasada-lib/step_detector.cpp

@@ -0,0 +1,70 @@
+//
+// Created by david on 15.03.2026.
+//
+
+#include "step_detector.h"
+
+// TODO: we are hardcoding filter coefficients for 60 Hz
+// TODO: this is tolerable for 50 Hz
+
+// TODO: check if we can do with floats instead of doubles
+// (check how much the [already bad] accuracy of filtering suffers)
+
+// TODO: in Java, check if delta timestamps effectively match FPS
+// TODO: FPS constant should be passed as argument to C++ (but we keep an FPS define to validate the coefficients)
+
+// Butterworth filter: order=5, fc=0.5, fs=60, btype='highpass'
+static std::vector<double> hpf_taps_b {0.91875845, -4.59379227,  9.18758454, -9.18758454,  4.59379227, -0.91875845};
+static std::vector<double> hpf_taps_a {1.        , -4.83056552,  9.33652742, -9.02545247,  4.36360803, -0.8441171};
+static size_t upslope_width = 4;
+const size_t len_refr = (size_t) (FPS / (MAX_BPM / 60));
+
+StepDetector::StepDetector(StepListener *listener, bool debug) :
+    listener(listener),
+    f_highpass(hpf_taps_b, hpf_taps_a),
+    f_neg(1, 0, 0, std::vector<double> {-1.0}),
+    f_ssf(upslope_width),
+    f_ssd(len_refr),
+    f_sqi(upslope_width),
+    debug(debug)
+{}
+
+#if (FPS != 60)
+#error "FPS must currently be 60, as highpass taps are pre-computed for that value"
+#endif
+
+void StepDetector::filter(std::vector<float> values) {
+    // TODO: later on, we should use a vector projection towards gravity
+    auto s0 = (double) values[1]; // take y-axis value for now
+    auto s1 = f_highpass.filter(s0);
+    auto s2 = f_neg.filter(s1);
+    auto s3 = f_ssf.filter(s2);
+    auto s4 = f_ssd.filter(s3);
+    auto q5 = f_sqi.filter(s2, s3, s4);
+    if (debug) {
+        buf_ssd.push_back(s4);
+        buf_sqi.push_back(q5);
+        buf_out.push_back(s4 * (q5 > 0.0 ? 1.0 : 0.0));
+    }
+    // is step, step quality is OK, and we have a listener?
+    if(s4 > 0.0 && q5 > 0.0 && listener != nullptr) {
+        listener->playBeat();
+    }
+}
+
+std::vector<double> StepDetector::getBufSsd() { return buf_ssd; }
+std::vector<double> StepDetector::getBufSqi() { return buf_sqi; }
+std::vector<double> StepDetector::getBufOut() { return buf_out; }
+
+void StepDetector::primeFilters(std::vector<double> sig) {
+    const size_t N_INIT = SsfStepDetector::initial_samples();
+    // initialize: feed for priming the filters
+    for (size_t i = 0; i < N_INIT; i++) {
+        const auto a_i = static_cast<float>(sig[i]);
+        filter(std::vector<float> {0.0f, a_i, 0.0f});
+    }
+    // clear debug buffers
+    buf_ssd.clear();
+    buf_sqi.clear();
+    buf_out.clear();
+}