void render(SDL_Renderer *r)
{
    if(!r)
        return;
    // static int step = 1, A = 50, sign = 1;
    int A = 50, B = 35;
    static int sign = 1;
    // int step = 1;
    static int cx = SCREEN_W / 2 - 50,
               cy = SCREEN_H / 2;

    static double phi = 0.0f;
    // static double offset = M_PI/2;

    static SDL_Rect dot = {
        SCREEN_W/2 - 5, SCREEN_H/2 - 5, 10, 10
    };
    dot.x = cx + sign*A*cos(phi*M_PI/180);
    dot.y = cy - sign*B*sin(phi*M_PI/180);


    phi += sign*1;

    if((int)phi % 360 == 0)
    {
        cx += sign*2*A;
        sign = -sign;
    }
    SDL_SetRenderDrawColor(r,0,0,0,255);

    SDL_RenderFillRect(r, &dot);
    SDL_RenderPresent(r);

}