Port FluidSim core and scenarios to C++20

include/FluidSim/OutdoorExhaustReverb.hpp

@@ -0,0 +1,55 @@
+#pragma once
+
+#include "FluidSim/AudioProcessing.hpp"
+#include <vector>
+
+namespace fluidsim::port {
+
+// Eight-line frequency-dependent feedback delay network with early reflections
+// and transient bloom, ported from Core/OutdoorExhaustReverb.cs.
+class OutdoorExhaustReverb {
+public:
+    float dry_mix{1.00f}, early_mix{0.10f}, tail_mix{0.28f};
+    float pre_delay_ms{5.0f}, ground_delay_ms{9.0f}, wall_delay_ms{45.0f};
+    float low_decay_seconds{1.50f}, mid_decay_seconds{0.70f}, high_decay_seconds{0.20f};
+    float band_low_hz{300.0f}, band_high_hz{3000.0f};
+    float transient_feedback_boost{0.35f}, transient_hf_boost{3.0f};
+    float transient_decay_ms{110.0f}, saturation_drive{1.2f};
+
+    explicit OutdoorExhaustReverb(int sample_rate = 44100);
+    AudioFrame process_stereo(float input);
+    float process(float input);
+
+private:
+    class DelayLine {
+    public:
+        explicit DelayLine(std::size_t size = 1);
+        float read() const;
+        float read_write(float value);
+        void write(float value);
+    private:
+        std::vector<float> buffer_;
+        std::size_t position_{};
+    };
+    class OnePoleLowPass {
+    public:
+        OnePoleLowPass(float cutoff_hz, int sample_rate);
+        float process(float value);
+    private:
+        float alpha_{}, state_{};
+    };
+
+    int sample_rate_{};
+    float dt_{}, gain_low_{}, gain_mid_{}, gain_high_{};
+    std::vector<DelayLine> fdn_;
+    std::vector<OnePoleLowPass> low_filters_, mid_filters_;
+    std::vector<float> read_buffer_, mix_buffer_;
+    DelayLine pre_delay_, ground_left_, ground_right_, wall_left_, wall_right_;
+    OnePoleLowPass ground_filter_left_, ground_filter_right_, wall_filter_left_, wall_filter_right_;
+    float fast_envelope_{}, slow_envelope_{}, bloom_{};
+    float fast_attack_{}, fast_release_{}, slow_attack_{}, slow_release_{}, bloom_decay_{};
+
+    static std::size_t delay_samples(int sample_rate, float milliseconds);
+};
+
+} // namespace fluidsim::port