feat: Resampler: Normalizes incoming Android sensor sampling rate

pasada-lib/pd_signal.cpp

@@ -6,6 +6,7 @@
 #include <stdexcept>
 #include <algorithm>
 #include <iostream>
+#include <numeric>
 
 namespace pd_signal {
 
@@ -141,6 +142,25 @@ void mean(std::vector<double> &out, std::deque<std::vector<double> >& m) {
     mean_tpl(out, m);
 }
 
+double mean(const std::vector<double> &in) {
+    if (in.empty()) {
+        throw std::invalid_argument("mean: input vector is empty");
+    }
+    double sum = std::accumulate(in.begin(), in.end(), 0.0);
+    return sum / static_cast<double>(in.size());
+}
+
+void diff(std::vector<double>& out, const std::vector<double>& in) {
+    if (in.size() < 2) {
+        out.clear();
+        return;
+    }
+    out.resize(in.size() - 1);
+    for (std::size_t i = 1; i < in.size(); ++i) {
+        out[i - 1] = in[i] - in[i - 1];
+    }
+}
+
 // Convolution of two polynomials in ascending power order.
 void polymul(std::vector<cplx>& out,
              const std::vector<cplx>& p, const std::vector<cplx>& q) {