Port FluidSim core and scenarios to C++20

include/FluidSim/TwoStrokeCylinder.hpp

@@ -0,0 +1,33 @@
+#pragma once
+
+#include "FluidSim/Crankcase.hpp"
+
+namespace fluidsim::port {
+
+// Two-stroke variant of the reciprocating cylinder model. The shared four-
+// stroke implementation supplies geometry, state balance, and combustion
+// parameters; cycle events and pumping torque are specialized here.
+class TwoStrokeCylinder final : public EngineCylinder {
+public:
+    TwoStrokeCylinder(float bore, float stroke, float connecting_rod_length,
+                      float compression_ratio, float transfer_duration_degrees,
+                      float exhaust_duration_degrees);
+
+    float transfer_open() const { return 180.0f - transfer_duration_ * 0.5f; }
+    float transfer_close() const { return 180.0f + transfer_duration_ * 0.5f; }
+    float exhaust_open() const { return 180.0f - exhaust_duration_ * 0.5f; }
+    float exhaust_close() const { return 180.0f + exhaust_duration_ * 0.5f; }
+    void set_crankcase(Crankcase& crankcase) { crankcase_ = &crankcase; }
+
+    float valve_area(float cycle_degrees, bool inlet) const override;
+    void pre_step(Crankshaft& crank, float dt, float throttle) override;
+
+private:
+    float transfer_duration_{}, exhaust_duration_{};
+    Crankcase* crankcase_{};
+
+    static float lift(float angle, float opens, float closes, float peak_lift);
+    void handle_cycle_events(float previous_degrees, float current_degrees);
+};
+
+} // namespace fluidsim::port