feat: SSF step detector

pasada-lib/include/ssf_filter.h

@@ -0,0 +1,54 @@
+//
+// Created by david on 03.03.2026.
+//
+
+#ifndef PASADASUPERPROJECT_SSF_FILTER_H
+#define PASADASUPERPROJECT_SSF_FILTER_H
+
+#include "iir_filter.h"
+
+#define FPS 60
+#define MAX_BPM 300
+
+/**
+ * Sum-Slope Function filter.
+ *
+ * See Zong et al, 2003: An open-source algorithm to detect onset of arterial blood pressure pulses.
+ */
+class SsfFilter {
+protected:
+    size_t sw;
+    Filt f_delta_u;
+    Filt f_window;
+public:
+    SsfFilter(size_t upslope_width);
+    double filter(double val);
+};
+
+/**
+ * Provides dirac pulses upon steps, based on SSF input.
+ *
+ * Settling time is LEN_INIT (currently 3.0 sec),
+ * no steps are detected before.
+ */
+class SsfStepDetector {
+protected:
+    const size_t LEN_INIT;
+    const size_t LEN_TH_WIN;
+    size_t num_samples;
+    double ssf_threshold;
+    size_t len_refr;
+    size_t n_refr;
+    bool is_refr;
+    double nm1_ssf;
+    Filt f_ssf_mean;
+public:
+    /**
+     * @param len_refr duration of refractory period, in samples
+     */
+    SsfStepDetector(size_t len_refr);
+    double filter(double val);
+    double peek_threshold();
+};
+
+#endif //PASADASUPERPROJECT_SSF_FILTER_H