Add Vanderpol example (#41)

Author: markaren

export/examples/CMakeLists.txt

@@ -2,3 +2,4 @@
 add_subdirectory(BouncingBall)
 add_subdirectory(SimplePendulum)
 add_subdirectory(Resource)
+add_subdirectory(VanDerPol)

export/examples/VanDerPol/CMakeLists.txt

@@ -0,0 +1,6 @@
+
+add_library(van_der_pol SHARED
+        "van_der_pol.cpp"
+)
+
+generateFMU(van_der_pol "fmi3")

export/examples/VanDerPol/van_der_pol.cpp

@@ -0,0 +1,87 @@
+
+#include <fmu4cpp/fmu_base.hpp>
+
+#include <cmath>
+
+using namespace fmu4cpp;
+
+class VanDerPol : public fmu_base {
+public:
+    FMU4CPP_CTOR(VanDerPol) {
+
+        register_variable(real("x0", &x0_).setCausality(causality_t::OUTPUT).setInitial(initial_t::EXACT));
+        register_variable(real("x1", &x1_).setCausality(causality_t::OUTPUT).setInitial(initial_t::EXACT));
+        register_variable(real("mu", &mu_).setCausality(causality_t::PARAMETER).setVariability(variability_t::FIXED).setInitial(initial_t::EXACT));
+
+        register_variable(real("x0Der", &x0Der_).setCausality(causality_t::LOCAL).setInitial(initial_t::CALCULATED));//.setDerivative("x0", {"x0"}));
+        register_variable(real("x1Der", &x1Der_).setCausality(causality_t::LOCAL).setInitial(initial_t::CALCULATED));//.setDerivative("x1", {"x0", "x1"}));
+
+        VanDerPol::reset();
+    }
+
+    bool do_step(double dt) override {
+        if (dt <= 0.0) return false;
+
+        // derivatives:
+        // x' = y
+        // y' = mu * (1 - x^2) * y - x
+        auto dx = [&](double x, double y) { return y; };
+        auto dy = [&](double x, double y) { return mu_ * (1.0 - x * x) * y - x; };
+
+        // RK4
+        double k1x = dx(x0_, x1_);
+        double k1y = dy(x0_, x1_);
+
+        double x2 = x0_ + 0.5 * dt * k1x;
+        double y2 = x1_ + 0.5 * dt * k1y;
+        double k2x = dx(x2, y2);
+        double k2y = dy(x2, y2);
+
+        double x3 = x0_ + 0.5 * dt * k2x;
+        double y3 = x1_ + 0.5 * dt * k2y;
+        double k3x = dx(x3, y3);
+        double k3y = dy(x3, y3);
+
+        double x4 = x0_ + dt * k3x;
+        double y4 = x1_ + dt * k3y;
+        double k4x = dx(x4, y4);
+        double k4y = dy(x4, y4);
+
+        x0Der_ = k1x;
+        x1Der_ = k1y;
+
+        x0_ += (dt / 6.0) * (k1x + 2.0 * k2x + 2.0 * k3x + k4x);
+        x1_ += (dt / 6.0) * (k1y + 2.0 * k2y + 2.0 * k3y + k4y);
+
+        return true;
+    }
+
+
+    void reset() override {
+        // default initial conditions
+        x0_ = 2.0;
+        x1_ = 0.0;
+        mu_ = 1.0;// stiffness parameter; adjust if needed
+
+        x0Der_ = 0.0;
+        x1Der_ = 0.0;
+    }
+
+private:
+    double x0_{0.0};
+    double x1_{0.0};
+    double mu_{1.0};
+
+    double x0Der_{};
+    double x1Der_{};
+};
+
+model_info fmu4cpp::get_model_info() {
+    model_info info;
+    info.modelName = "VanDerPol";
+    info.description = "VanDerPol oscillator";
+    info.modelIdentifier = FMU4CPP_MODEL_IDENTIFIER;
+    return info;
+}
+
+FMU4CPP_INSTANTIATE(VanDerPol);