Added C interface

Makefile

@@ -1,5 +1,11 @@
+example: gravity.o
+	$(CC) -O3 example.c -o example gravity.o ../grapite/libgrapite.a -lcuda -lcudart -lstdc++ -lm
+
+gravity.o:
+	$(CXX) -O3 --std=c++23 gravity.cpp -c
+
 main:
 	$(CXX) -O3 main.cpp ../grapite/libgrapite.a -lcuda -lcudart -o $@
 
 clean:
-	$(RM) main
+	$(RM) main *.o

example.c

@@ -0,0 +1,26 @@
+#include <stdlib.h>
+
+void etics_external_init(const int n_total);
+void etics_external_grav(const double* const A, const int n_act, double x_act[][3], double pot_act[], double a_act[][3]);
+
+int main()
+{
+    const int n_total = 1024;
+    etics_external_init(n_total);
+
+    srand(19640916);
+
+    double x_act[n_total][3], pot_act[n_total], a_act[n_total][3];
+    for (int i = 0; i < n_total; i++) {
+        x_act[i][0] = rand();
+        x_act[i][1] = rand();
+        x_act[i][2] = rand();
+    }
+
+    double A[2048];
+    for (int i = 0; i < 2048; i++) {
+        A[i] = rand();
+    }
+
+    etics_external_grav(A, n_total, x_act, pot_act, a_act);
+}

gravity.cpp

@@ -0,0 +1,30 @@
+#include <span>
+#include "../grapite/etics_interface.h"
+#include "../grapite/particle_manager.h"
+
+static grapite::Active_particles* particles_i;
+static grapite::Etics_interface* etics_interface;
+
+extern "C"
+void etics_external_init(const int n_total)
+{
+    particles_i = new grapite::Active_particles{n_total};
+    etics_interface = new grapite::Etics_interface{n_total};
+}
+
+extern "C"
+void etics_external_grav(const double* const A, const int n_act, double x_act[][3], double pot_act[], double a_act[][3])
+{
+    // Load coefficients
+    std::span A_complex{reinterpret_cast<const Complex*>(A), 1024};
+    std::copy(begin(A_complex), end(A_complex), etics_interface->A_h);
+    etics_interface->GetGpuLock();
+    etics_interface->SendCoeffsToGPU();
+    etics_interface->ReleaseGpuLock();
+
+    // Calculate potentials and forces
+    particles_i->ni_core  = 0;
+    particles_i->ni_total = n_act;
+    particles_i->flush_memory();
+    etics_interface->calculate_gravity(particles_i, grapite::Selector::halo, 0, pot_act, reinterpret_cast<vec3*>(a_act));
+}