/* ----------------------------------------------------------------------
   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
   http://lammps.sandia.gov, Sandia National Laboratories
   Steve Plimpton, sjplimp@sandia.gov

   Copyright (2003) Sandia Corporation.  Under the terms of Contract
   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
   certain rights in this software.  This software is distributed under 
   the GNU General Public License.

   See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */

/* ----------------------------------------------------------------------
   Contributing author: Mike Brown (SNL)
------------------------------------------------------------------------- */

#include "math.h"
#include "stdio.h"
#include "string.h"
#include "fix_nve_asphere.h"
#include "math_extra.h"
#include "atom.h"
#include "atom_vec_ellipsoid.h"
#include "force.h"
#include "update.h"
#include "memory.h"
#include "error.h"

using namespace LAMMPS_NS;

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

FixNVEAsphere::FixNVEAsphere(LAMMPS *lmp, int narg, char **arg) : 
  FixNVE(lmp, narg, arg)
{
  avec = (AtomVecEllipsoid *) atom->style_match("ellipsoid");
  if (!avec) 
    error->all("Compute nve/asphere requires atom style ellipsoid");
}

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

void FixNVEAsphere::init()
{
  // check that all particles are finite-size
  // no point particles allowed, spherical is OK

  int *ellipsoid = atom->ellipsoid;
  int *mask = atom->mask;
  int nlocal = atom->nlocal;

  for (int i = 0; i < nlocal; i++)
    if (mask[i] & groupbit)
      if (ellipsoid[i] < 0)
	error->one("Fix nve/asphere requires extended particles");

  FixNVE::init();
}

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

void FixNVEAsphere::initial_integrate(int vflag)
{
  double dtfm;
  double inertia[3];
  double *shape,*quat;

  AtomVecEllipsoid::Bonus *bonus = avec->bonus;
  int *ellipsoid = atom->ellipsoid;
  double **x = atom->x;
  double **v = atom->v;
  double **f = atom->f;
  double **angmom = atom->angmom;
  double **torque = atom->torque;
  double *rmass = atom->rmass;
  int *mask = atom->mask;
  int nlocal = atom->nlocal;
  if (igroup == atom->firstgroup) nlocal = atom->nfirst;

  // set timestep here since dt may have changed or come via rRESPA

  dtq = 0.5 * dtv;

  for (int i = 0; i < nlocal; i++)
    if (mask[i] & groupbit) {
      dtfm = dtf / rmass[i];
      v[i][0] += dtfm * f[i][0];
      v[i][1] += dtfm * f[i][1];
      v[i][2] += dtfm * f[i][2];
      x[i][0] += dtv * v[i][0];
      x[i][1] += dtv * v[i][1];
      x[i][2] += dtv * v[i][2];

      // update angular momentum by 1/2 step
      // update quaternion a full step via Richardson iteration
      // returns new normalized quaternion
      
      angmom[i][0] += dtf * torque[i][0];
      angmom[i][1] += dtf * torque[i][1];
      angmom[i][2] += dtf * torque[i][2];

      // principal moments of inertia

      shape = bonus[ellipsoid[i]].shape;
      quat = bonus[ellipsoid[i]].quat;

      inertia[0] = rmass[i] * (shape[1]*shape[1]+shape[2]*shape[2]) / 5.0;
      inertia[1] = rmass[i] * (shape[0]*shape[0]+shape[2]*shape[2]) / 5.0;
      inertia[2] = rmass[i] * (shape[0]*shape[0]+shape[1]*shape[1]) / 5.0;

      richardson(quat,angmom[i],inertia);
    }
}

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

void FixNVEAsphere::final_integrate()
{
  double dtfm;

  double **v = atom->v;
  double **f = atom->f;
  double **angmom = atom->angmom;
  double **torque = atom->torque;
  double *rmass = atom->rmass;
  int *mask = atom->mask;
  int nlocal = atom->nlocal;
  if (igroup == atom->firstgroup) nlocal = atom->nfirst;

  for (int i = 0; i < nlocal; i++)
    if (mask[i] & groupbit) {
      dtfm = dtf / rmass[i];
      v[i][0] += dtfm * f[i][0];
      v[i][1] += dtfm * f[i][1];
      v[i][2] += dtfm * f[i][2];
  
      angmom[i][0] += dtf * torque[i][0];
      angmom[i][1] += dtf * torque[i][1];
      angmom[i][2] += dtf * torque[i][2];
    }
}

/* ----------------------------------------------------------------------
   Richardson iteration to update quaternion accurately
------------------------------------------------------------------------- */

void FixNVEAsphere::richardson(double *q, double *m, double *moments)
{
  // compute omega at 1/2 step from m at 1/2 step and q at 0

  double w[3];
  omega_from_mq(q,m,moments,w);

  // full update from dq/dt = 1/2 w q

  double wq[4];
  MathExtra::multiply_vec_quat(w,q,wq);

  double qfull[4];
  qfull[0] = q[0] + dtq * wq[0];
  qfull[1] = q[1] + dtq * wq[1];
  qfull[2] = q[2] + dtq * wq[2];
  qfull[3] = q[3] + dtq * wq[3];
  MathExtra::normalize4(qfull);

  // 1st half of update from dq/dt = 1/2 w q

  double qhalf[4];
  qhalf[0] = q[0] + 0.5*dtq * wq[0];
  qhalf[1] = q[1] + 0.5*dtq * wq[1];
  qhalf[2] = q[2] + 0.5*dtq * wq[2];
  qhalf[3] = q[3] + 0.5*dtq * wq[3];
  MathExtra::normalize4(qhalf);

  // re-compute omega at 1/2 step from m at 1/2 step and q at 1/2 step
  // recompute wq

  omega_from_mq(qhalf,m,moments,w);
  MathExtra::multiply_vec_quat(w,qhalf,wq);

  // 2nd half of update from dq/dt = 1/2 w q

  qhalf[0] += 0.5*dtq * wq[0];
  qhalf[1] += 0.5*dtq * wq[1];
  qhalf[2] += 0.5*dtq * wq[2];
  qhalf[3] += 0.5*dtq * wq[3];
  MathExtra::normalize4(qhalf);

  // corrected Richardson update

  q[0] = 2.0*qhalf[0] - qfull[0];
  q[1] = 2.0*qhalf[1] - qfull[1];
  q[2] = 2.0*qhalf[2] - qfull[2];
  q[3] = 2.0*qhalf[3] - qfull[3];
  MathExtra::normalize4(q);
}

/* ----------------------------------------------------------------------
   compute omega from angular momentum
   w = omega = angular velocity in space frame
   wbody = angular velocity in body frame
   project space-frame angular momentum onto body axes
     and divide by principal moments
------------------------------------------------------------------------- */

void FixNVEAsphere::omega_from_mq(double *q, double *m, double *moments,
				  double *w)
{
  double rot[3][3];
  MathExtra::quat_to_mat(q,rot);
  
  double wbody[3];
  MathExtra::transpose_times_column3(rot,m,wbody);
  wbody[0] /= moments[0];
  wbody[1] /= moments[1];
  wbody[2] /= moments[2];
  MathExtra::times_column3(rot,wbody,w);
}