Port FluidSim core and scenarios to C++20

include/FluidSim/MechanicalSoundProcessor.hpp

@@ -0,0 +1,49 @@
+#pragma once
+
+#include "FluidSim/Engine.hpp"
+#include <random>
+
+namespace fluidsim::port {
+
+// Synthetic valve, piston, combustion, chain, and gear sounds driven by live
+// engine state; direct port of Core/MechanicalSoundProcessor.cs.
+class MechanicalSoundProcessor {
+public:
+    float master_gain{0.6f}, valve_tick_gain{0.9f}, piston_slap_gain{1.2f};
+    float combustion_gain{0.5f}, gear_whine_gain{0.12f}, cam_chain_gain{0.25f};
+
+    explicit MechanicalSoundProcessor(int sample_rate = 44100);
+    float process(const Crankshaft& crank, const EngineCylinder& cylinder, float load);
+
+private:
+    class Resonator {
+    public:
+        Resonator(float frequency_hz, float q, float sample_rate);
+        float process(float input);
+    private:
+        float b1_{}, b2_{}, gain_{}, y1_{}, y2_{};
+    };
+    class OnePoleLowPass {
+    public:
+        OnePoleLowPass(float cutoff_hz, float sample_rate);
+        float process(float input);
+    private:
+        float alpha_{}, state_{};
+    };
+
+    float dt_{};
+    Resonator intake_tick_, exhaust_tick_;
+    OnePoleLowPass slap_filter_, combustion_filter_, chain_filter_;
+    float slap_envelope_{}, combustion_envelope_{}, chain_envelope_{};
+    float slap_decay_{}, combustion_decay_{}, chain_decay_{};
+    float gear_phase1_{}, gear_phase2_{}, gear_phase3_{};
+    float previous_intake_area_{}, previous_exhaust_area_{}, previous_pressure_{};
+    float previous_cycle_position_{-1.0f}, previous_crank_fraction_{-1.0f};
+    std::mt19937 random_{0x5EEDu};
+    std::uniform_real_distribution<float> unit_{-1.0f, 1.0f};
+
+    float random_sample();
+    static bool crossed_fraction(float previous, float current, float target);
+};
+
+} // namespace fluidsim::port