/* ----------------------------------------------------------------------
   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 authors: Leo Silbert (SNL), Gary Grest (SNL)
------------------------------------------------------------------------- */

#include "math.h"
#include "stdio.h"
#include "string.h"
#include "pair_gran_hooke.h"
#include "atom.h"
#include "force.h"
#include "neigh_list.h"

using namespace LAMMPS_NS;

#define MIN(a,b) ((a) < (b) ? (a) : (b))
#define MAX(a,b) ((a) > (b) ? (a) : (b))

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

PairGranHooke::PairGranHooke(LAMMPS *lmp) : PairGranHookeHistory(lmp)
{
  no_virial_compute = 0;
  history = 0;
}

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

void PairGranHooke::compute(int eflag, int vflag)
{
  int i,j,ii,jj,inum,jnum,itype,jtype;
  double xtmp,ytmp,ztmp,delx,dely,delz,fx,fy,fz;
  double radi,radj,radsum,rsq,r,rinv,rsqinv;
  double vr1,vr2,vr3,vnnr,vn1,vn2,vn3,vt1,vt2,vt3;
  double wr1,wr2,wr3;
  double vtr1,vtr2,vtr3,vrel;
  double meff,damp,ccel,tor1,tor2,tor3;
  double fn,fs,ft,fs1,fs2,fs3;
  int *ilist,*jlist,*numneigh,**firstneigh;

  if (eflag || vflag) ev_setup(eflag,vflag);
  else evflag = vflag_fdotr = 0;

  double **x = atom->x;
  double **v = atom->v;
  double **f = atom->f;
  double **omega = atom->omega;
  double **torque = atom->torque;
  double *radius = atom->radius;
  double *rmass = atom->rmass;
  double *mass = atom->mass;
  int *type = atom->type;
  int *mask = atom->mask;
  int nlocal = atom->nlocal;
  int newton_pair = force->newton_pair;

  inum = list->inum;
  ilist = list->ilist;
  numneigh = list->numneigh;
  firstneigh = list->firstneigh;

  // loop over neighbors of my atoms

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    xtmp = x[i][0];
    ytmp = x[i][1];
    ztmp = x[i][2];
    radi = radius[i];
    jlist = firstneigh[i];
    jnum = numneigh[i];

    for (jj = 0; jj < jnum; jj++) {
      j = jlist[jj];
      j &= NEIGHMASK;

      delx = xtmp - x[j][0];
      dely = ytmp - x[j][1];
      delz = ztmp - x[j][2];
      rsq = delx*delx + dely*dely + delz*delz;
      radj = radius[j];
      radsum = radi + radj;

      if (rsq < radsum*radsum) {
	r = sqrt(rsq);
	rinv = 1.0/r;
	rsqinv = 1.0/rsq;

	// relative translational velocity

	vr1 = v[i][0] - v[j][0];
	vr2 = v[i][1] - v[j][1];
	vr3 = v[i][2] - v[j][2];

	// normal component

	vnnr = vr1*delx + vr2*dely + vr3*delz;
	vn1 = delx*vnnr * rsqinv;
	vn2 = dely*vnnr * rsqinv;
	vn3 = delz*vnnr * rsqinv;

	// tangential component

	vt1 = vr1 - vn1;
	vt2 = vr2 - vn2;
	vt3 = vr3 - vn3;

	// relative rotational velocity

	wr1 = (radi*omega[i][0] + radj*omega[j][0]) * rinv;
	wr2 = (radi*omega[i][1] + radj*omega[j][1]) * rinv;
	wr3 = (radi*omega[i][2] + radj*omega[j][2]) * rinv;

	// normal forces = Hookian contact + normal velocity damping

	if (rmass) {
	  meff = rmass[i]*rmass[j] / (rmass[i]+rmass[j]);
	  if (mask[i] & freeze_group_bit) meff = rmass[j];
	  if (mask[j] & freeze_group_bit) meff = rmass[i];
	} else {
	  itype = type[i];
	  jtype = type[j];
	  meff = mass[itype]*mass[jtype] / (mass[itype]+mass[jtype]);
	  if (mask[i] & freeze_group_bit) meff = mass[jtype];
	  if (mask[j] & freeze_group_bit) meff = mass[itype];
	}

	damp = meff*gamman*vnnr*rsqinv;
	ccel = kn*(radsum-r)*rinv - damp;

	// relative velocities

	vtr1 = vt1 - (delz*wr2-dely*wr3);
	vtr2 = vt2 - (delx*wr3-delz*wr1);
	vtr3 = vt3 - (dely*wr1-delx*wr2);
	vrel = vtr1*vtr1 + vtr2*vtr2 + vtr3*vtr3;
	vrel = sqrt(vrel);

	// force normalization

	fn = xmu * fabs(ccel*r);
	fs = meff*gammat*vrel;
	if (vrel != 0.0) ft = MIN(fn,fs) / vrel;
	else ft = 0.0;

	// tangential force due to tangential velocity damping

	fs1 = -ft*vtr1;
	fs2 = -ft*vtr2;
	fs3 = -ft*vtr3;

	// forces & torques

	fx = delx*ccel + fs1;
	fy = dely*ccel + fs2;
	fz = delz*ccel + fs3;
	f[i][0] += fx;
	f[i][1] += fy;
	f[i][2] += fz;

	tor1 = rinv * (dely*fs3 - delz*fs2);
	tor2 = rinv * (delz*fs1 - delx*fs3);
	tor3 = rinv * (delx*fs2 - dely*fs1);
	torque[i][0] -= radi*tor1;
	torque[i][1] -= radi*tor2;
	torque[i][2] -= radi*tor3;

	if (newton_pair || j < nlocal) {
	  f[j][0] -= fx;
	  f[j][1] -= fy;
	  f[j][2] -= fz;
	  torque[j][0] -= radj*tor1;
	  torque[j][1] -= radj*tor2;
	  torque[j][2] -= radj*tor3;
	}

	if (evflag) ev_tally_xyz(i,j,nlocal,newton_pair,
				 0.0,0.0,fx,fy,fz,delx,dely,delz);
      }
    }
  }

  if (vflag_fdotr) virial_compute();
}
