extern void test_input_data(int n_x, int n_y, double delta_x, 
double delta_y, double kappa, double A, double delta_t, 
int time_steps, double c_zero, double noise_str);

extern void evolve(int n_x, int n_y, double delta_x, 
double delta_y, double kappa, double A, double delta_t, int time_steps, fftw_complex *comp);

extern void generate_snapshot(const char *name, const char *label, int n_x, int n_y, double *c);