Ellipsoid.cpp

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/*
 * Copyright (C) 2010-2026 The ESPResSo project
 * Copyright (C) 2002,2003,2004,2005,2006,2007,2008,2009,2010
 *   Max-Planck-Institute for Polymer Research, Theory Group
 *
 * This file is part of ESPResSo.
 *
 * ESPResSo is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ESPResSo is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#include <shapes/Ellipsoid.hpp>
 
#include <utils/Vector.hpp>
#include <utils/math/sqr.hpp>
 
#include <algorithm>
#include <cmath>
 
namespace Shapes {
void Ellipsoid::calculate_dist(const Utils::Vector3d &pos, double &dist,
                               Utils::Vector3d &vec) const {
 
  /* get particle position in reference frame of ellipsoid */
  Utils::Vector3d const ppos_e = pos - m_center;
 
  /* set appropriate initial point for Newton's method */
  double l = 0.;
  int distance_prefactor = -1;
  if (not inside_ellipsoid(ppos_e)) {
    l = *std::ranges::max_element(m_semiaxes) * ppos_e.norm();
    distance_prefactor = 1;
  }
 
  /* find root via Newton's method */
  double eps = 10.;
  int step = 0;
  while ((eps >= 1e-12) and (step < 100)) {
    auto const l0 = l;
    l -= newton_term(ppos_e, l0);
    eps = std::abs(l - l0);
    step++;
  }
 
  /* calculate dist and vec */
  for (unsigned int i = 0; i < 3; i++) {
    auto const semi_sq = Utils::sqr(m_semiaxes[i]);
    vec[i] = (ppos_e[i] - semi_sq * ppos_e[i] / (l + semi_sq));
  }
 
  dist = distance_prefactor * m_direction * vec.norm();
}
 
bool Ellipsoid::inside_ellipsoid(const Utils::Vector3d &ppos) const {
  return Utils::hadamard_division(ppos, m_semiaxes).norm2() <= 1;
}
 
double Ellipsoid::newton_term(const Utils::Vector3d &ppos,
                              const double &l) const {
  Utils::Vector3d axpos, lax, lax2;
  for (unsigned int i = 0; i < 3; i++) {
    axpos[i] = Utils::sqr(m_semiaxes[i]) * Utils::sqr(ppos[i]);
    lax[i] = l + Utils::sqr(m_semiaxes[i]);
    lax2[i] = Utils::sqr(lax[i]);
  }
 
  return (axpos[0] * lax2[1] * lax2[2] + axpos[1] * lax2[2] * lax2[0] +
          axpos[2] * lax2[0] * lax2[1] - lax2[0] * lax2[1] * lax2[2]) /
         (2 * (axpos[0] * lax[1] * lax2[2] + axpos[1] * lax[2] * lax2[0] +
               axpos[2] * lax[0] * lax2[1] + axpos[0] * lax2[1] * lax[2] +
               axpos[1] * lax2[2] * lax[0] + axpos[2] * lax2[0] * lax[1] -
               lax[0] * lax2[1] * lax2[2] - lax2[0] * lax[1] * lax2[2] -
               lax2[0] * lax2[1] * lax[2]));
}
 
} // namespace Shapes