Port FluidSim core and scenarios to C++20

include/FluidSim/AudioProcessing.hpp

@@ -0,0 +1,50 @@
+#pragma once
+
+#include <vector>
+
+namespace fluidsim::port {
+
+struct AudioFrame { float left{}, right{}; };
+struct Vec3 { float x{}, y{}, z{}; };
+
+// Flow-derivative sound source used by the C# two-stroke and pulse scenarios.
+class SoundProcessor {
+public:
+    float gain{1.0f};
+    explicit SoundProcessor(int sample_rate = 44100, float listener_distance = 1.0f);
+    float process(float mass_flow_out);
+private:
+    float dt_{}, scale_factor_{}, lowpass_alpha_{}, derivative_alpha_{};
+    float flow_lowpass_{}, previous_flow_{}, smooth_derivative_{};
+};
+
+class RadiationSource {
+public:
+    float flow_tau{0.005f}, smooth_tau{0.020f}, gain{15.0f};
+    Vec3 pipe_end{}, pipe_axis{1.0f, 0.0f, 0.0f};
+    Vec3 listener{0.0f, 0.0f, 1.5f}, listener_right{1.0f, 0.0f, 0.0f};
+    float ear_spacing{0.18f}, head_shadow_cutoff{4000.0f}, speed_of_sound{343.0f};
+    float directivity_balance{0.30f}, air_absorption_exponent{0.002f}, min_air_cutoff{1500.0f};
+
+    explicit RadiationSource(int sample_rate = 44100);
+    AudioFrame process(float mass_flow);
+    float last_mono_pressure() const { return last_mono_pressure_; }
+
+private:
+    class DelayLine;
+    class OnePole;
+    int sample_rate_{};
+    float dt_{}, flow_lp_{}, previous_flow_lp_{}, smooth_derivative_{};
+    float last_mono_pressure_{};
+    std::vector<float> left_delay_, right_delay_;
+    std::size_t write_position_{}, left_offset_{}, right_offset_{};
+    float air_filter_{}, shadow_left_{}, shadow_right_{};
+
+    static float dot(Vec3 a, Vec3 b);
+    static float length(Vec3 value);
+    static Vec3 subtract(Vec3 a, Vec3 b);
+    float one_pole(float input, float cutoff, float& state) const;
+    float delayed(float input, std::vector<float>& buffer, std::size_t offset);
+};
+
+} // namespace fluidsim::port