#include <iostream>
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <math.h>

using namespace std;

class cPlanet
{
    public:
        double x, y;    //position x and y components
        double vx, vy;  //velocity x and y components
        double ax, ay;  //acceleration x and y components

        double GM;  //G constant times star mass is a new quantity that is constant
        double xS, yS;  //the star is fixed and its position depends on the screen resolution

        cPlanet(    double x, double y,
                double vx, double vy,
                double GM,
                double xS, double yS)
        {
            this->x = x;
            this->y = y;
            this->vx = vx;
            this->vy = vy;
            this->GM = GM;
            this->xS = xS;
            this->yS = yS;
            ax = 0.0;
            ay = 0.0;
        }

        void verletStep(double dt)
        {
            double xn = x + vx*dt + ax*dt*dt/2;
            double yn = y + vy*dt + ay*dt*dt/2; //calculate future position

            double dx = xn - xS;
            double dy = yn - yS;
            double r3 = pow(dx*dx+dy*dy, 1.5);

            double axn = 0.0;
            double ayn = 0.0;
            if(r3 != 0)
            {
                axn = -GM * dx / r3;
                ayn = -GM * dy / r3;    //calculate future acceleration
            }

            double vxn = vx + (axn + ax)*dt/2;
            double vyn = vy + (ayn + ay)*dt/2;  //calculate future velocity

            x = xn;
            y = yn;
            vx = vxn;
            vy = vyn;
            ax = axn;
            ay = ayn; //apply changes in position, velocity and force
        }
};

int main()
{
    double resX = GetSystemMetrics(SM_CXSCREEN);    //get screen width
    double resY = GetSystemMetrics(SM_CYSCREEN);    //get screen height

    cPlanet planet( resX/2.0, resY/8.0, //x and y
            resY, 0.0,      //vx and vy
            0.75*pow(resY, 3.0),    //GM constant
            resX/2.0, resY/2.0);    //xS and yS

    while(1)
    {
        planet.verletStep(0.02);        //move the planet forward in time using velocity verlet algorithm
        SetCursorPos(planet.x, planet.y);   //move the mouse cursor
        Sleep(20);
    }
}