compute_gravity.cc
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	#include <cmath>

	#include "compute_gravity.hh"

	/* -------------------------------------------------------------------------- */

	void ComputeGravity::compute(System& system) {

	// Get the total number of particles
	int nparticles = system.getNbParticles();

	//Initialize types
	Vector v_ij;
	Real r2_ij = 0.;
	Real r_ij = 0;
	Vector force;
	Vector nv_ij;

	// Loop over all the particles and gets each one's parameters
	for (int pi = 0; pi < nparticles; ++pi) {
	Particle & PI = system.getParticle(pi);
	//loop over all next interacting particles
	for (int pj = pi+1; pj < nparticles; ++pj) {
	Particle & PJ = system.getParticle(pj);

	// Gets the vector between the two particles positions
	v_ij = PJ.getPosition() - PI.getPosition();

	// Squared distance (obtained via vector class)
	r2_ij = v_ij.squaredNorm();

	// Obtained distance
	r_ij = sqrt(r2_ij);

	//Obtained corresponding normed vector
	nv_ij=v_ij / r_ij;

	// compute the gravity force vector interaction on i with j
	force = nv_ij* PI.getMass() * PJ.getMass() * G / r2_ij;

	//F_ij=-F_ji Particles have opposite forces
	PJ.getForce() -= force;
	PI.getForce() += force;
	}
	}
	}