feat: Resampler: Normalizes incoming Android sensor sampling rate

pasada-lib/include/pd_resamp.h

@@ -0,0 +1,51 @@
+//
+// Created by david on 17.05.2026.
+//
+
+#ifndef PASADASUPERPROJECT_PD_RESAMP_H
+#define PASADASUPERPROJECT_PD_RESAMP_H
+
+#include "iir_filter.h"
+
+/** Filter that changes sampling rate between input and output. */
+class ResamplingFilter {
+public:
+    ResamplingFilter() {}
+    virtual ~ResamplingFilter() {}
+    virtual void push(double ts, double val) = 0;
+    virtual bool peek() = 0;
+    virtual double get() = 0;
+};
+
+/** Normalizes incoming Android sensor sampling rate. */
+class Resampler : public ResamplingFilter {
+protected:
+    const size_t INITIAL_SAMPLES = 100;
+    std::vector<double> times;
+    std::vector<double> data;
+    /** circular buffer size */
+    size_t N;
+    /** write index */
+    size_t n;
+    /** read index */
+    size_t m;
+    bool initialized;
+    bool read_valid;
+    /** computed sampling frequency, this will be the output rate */
+    double fs;
+    void compute_fs();
+public:
+    Resampler();
+    /**
+     * Push a value into the buffer.
+     * Caller is responsible for polling via peek() and get() afterward.
+     * @param ts  timestamp in nanoseconds
+     * @param val signal sample
+     */
+    void push(double ts, double val) override;
+    bool peek() override;
+    double get() override;
+    double get_fs() const;
+};
+
+#endif //PASADASUPERPROJECT_PD_RESAMP_H

pasada-lib/include/pd_signal.h

@@ -40,6 +40,10 @@ namespace pd_signal {
     /** two-dimensional mean of a collection of signals */
     void mean(std::vector<double> &out, std::deque<std::vector<double> >& m);
 
+    /** simple mean of 1-d signal */
+    double mean(const std::vector<double>& in);
+    void diff(std::vector<double>& out, const std::vector<double>& in);
+
     /**
      * Convolution of two polynomials given in ASCENDING power order.
      * If <c>p = p_0 + p_1 x + ... + p_{P-1} x^{P-1}</c> and likewise for q,