package planeta;

public class Planeta {
	private double r; 	//distancia al centro de fuerzas
	private double a, b, c, d;
	private double l1, l2, l3, l4;
	private double k1, k2, k3, k4;
	private double m1, m2, m3, m4;
	private double q1, q2, q3, q4;
	private double dt; 		//intervalo

	public double t;
	public double x;
	public double y;
	public double Vx;
	public double Vy;

Planeta(double x, double y, double Vx, double Vy, double t, double dt){
	this.x=x;
	this.y=y;
	this.Vx=Vx;
	this.Vy=Vy;
	this.dt=dt;
	this.t=t;
}

public void resolver(){
//resolución del sistema de ecuaciones diferenciales de segundo orden
//por el procedimiento de Runge-Kutta
	a=x; b=y; c=Vx; d=Vy;
	l1=c*dt; q1=d*dt;
	r=Math.sqrt(a*a+b*b);
	k1=-a*dt/r/r/r; m1=-b*dt/r/r/r;
	a=x+l1/2; b=y+q1/2; c=Vx+k1/2; d=Vy+m1/2;
	l2=c*dt; q2=d*dt;
	r=Math.sqrt(a*a+b*b);
	k2=-a*dt/r/r/r; m2=-b*dt/r/r/r;
	a=x+l2/2; b=y+q2/2; c=Vx+k2/2; d=Vy+m2/2;
	l3=c*dt; q3=d*dt;
	r=Math.sqrt(a*a+b*b);
	k3=-a*dt/r/r/r; m3=-b*dt/r/r/r;
	a=x+l3; b=y+q3; c=Vx+k3; d=Vy+m3;
	l4=c*dt; q4=d*dt;
	r=Math.sqrt(a*a+b*b);
	k4=-a*dt/r/r/r; m4=-b*dt/r/r/r;
//valores de las variables en el instante t+dt
	t+=dt;
	x+=(l1+2*l2+2*l3+l4)/6;
	y+=(q1+2*q2+2*q3+q4)/6;
	Vx+=(k1+2*k2+2*k3+k4)/6;
	Vy+=(m1+2*m2+2*m3+m4)/6;
  }
}