compute_gravity.cc
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	#include "compute_gravity.hh"
	#include <cmath>
	/* -------------------------------------------------------------------------- */
	void ComputeGravity::compute(System &system) {

	UInt size = system.getNbParticles();
	// first of all reset forces to zero
	for (auto &particle : system)
	particle.getForce() = 0;

	// the force vector
	Vector force;
	// the distance vector
	Vector v_r;
	// the normalized vector
	Vector v_nr;
	// the square of the distance
	Real r2 = 0.;
	// the distance
	Real r = 0.;

	for (UInt p1 = 0; p1 < size; ++p1) {
	Particle &par1 = system.getParticle(p1);
	for (UInt p2 = p1 + 1; p2 < size; ++p2) {
	Particle &par2 = system.getParticle(p2);

	// compute the distance vector and the square of distance

	v_r = par2.getPosition() - par1.getPosition();
	r2 = v_r.squaredNorm();

	if (r2 == 0.)
	continue;
	// compute the distance
	r = sqrt(r2);
	v_nr = v_r / r;

	// compute the pair force
	force = par1.getMass() * par2.getMass() * G / r2 * v_nr;

	// add up the force for both concerned particles
	par2.getForce() -= force;
	par1.getForce() += force;
	}
	}
	}
	/* -------------------------------------------------------------------------- */
	void ComputeGravity::setG(Real G) { this->G = G; }