/* ----------------------------------------------------------------------
   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.
------------------------------------------------------------------------- */

#include "math.h"
#include "pair_dpd_tstat.h"
#include "atom.h"
#include "update.h"
#include "force.h"
#include "neigh_list.h"
#include "comm.h"
#include "random_mars.h"
#include "error.h"

using namespace LAMMPS_NS;

#define EPSILON 1.0e-10

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

PairDPDTstat::PairDPDTstat(LAMMPS *lmp) : PairDPD(lmp)
{
  single_enable = 0;
}

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

void PairDPDTstat::compute(int eflag, int vflag)
{
  int i,j,ii,jj,inum,jnum,itype,jtype;
  double xtmp,ytmp,ztmp,delx,dely,delz,fpair;
  double vxtmp,vytmp,vztmp,delvx,delvy,delvz;
  double rsq,r,rinv,dot,wd,randnum,factor_dpd;
  int *ilist,*jlist,*numneigh,**firstneigh;

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

  // adjust sigma if target T is changing

  if (t_start != t_stop) {
    double delta = update->ntimestep - update->beginstep;
    if (delta != 0.0) delta /= update->endstep - update->beginstep;
    temperature = t_start + delta * (t_stop-t_start);
    double boltz = force->boltz;
    for (i = 1; i <= atom->ntypes; i++)
      for (j = i; j <= atom->ntypes; j++)
        sigma[i][j] = sigma[j][i] = sqrt(2.0*boltz*temperature*gamma[i][j]);
  }

  double **x = atom->x;
  double **v = atom->v;
  double **f = atom->f;
  int *type = atom->type;
  int nlocal = atom->nlocal;
  double *special_lj = force->special_lj;
  int newton_pair = force->newton_pair;
  double dtinvsqrt = 1.0/sqrt(update->dt);

  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];
    vxtmp = v[i][0];
    vytmp = v[i][1];
    vztmp = v[i][2];
    itype = type[i];
    jlist = firstneigh[i];
    jnum = numneigh[i];

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

      delx = xtmp - x[j][0];
      dely = ytmp - x[j][1];
      delz = ztmp - x[j][2];
      rsq = delx*delx + dely*dely + delz*delz;
      jtype = type[j];

      if (rsq < cutsq[itype][jtype]) {
        r = sqrt(rsq);
        if (r < EPSILON) continue;     // r can be 0.0 in DPD systems
        rinv = 1.0/r;
        delvx = vxtmp - v[j][0];
        delvy = vytmp - v[j][1];
        delvz = vztmp - v[j][2];
        dot = delx*delvx + dely*delvy + delz*delvz;
        wd = 1.0 - r/cut[itype][jtype];
        randnum = random->gaussian();

        // drag force = -gamma * wd^2 * (delx dot delv) / r
        // random force = sigma * wd * rnd * dtinvsqrt;

        fpair = -gamma[itype][jtype]*wd*wd*dot*rinv;
        fpair += sigma[itype][jtype]*wd*randnum*dtinvsqrt;
        fpair *= factor_dpd*rinv;

        f[i][0] += delx*fpair;
        f[i][1] += dely*fpair;
        f[i][2] += delz*fpair;
        if (newton_pair || j < nlocal) {
          f[j][0] -= delx*fpair;
          f[j][1] -= dely*fpair;
          f[j][2] -= delz*fpair;
        }

        if (evflag) ev_tally(i,j,nlocal,newton_pair,
                             0.0,0.0,fpair,delx,dely,delz);
      }
    }
  }

  if (vflag_fdotr) virial_fdotr_compute();
}

/* ----------------------------------------------------------------------
   global settings
------------------------------------------------------------------------- */

void PairDPDTstat::settings(int narg, char **arg)
{
  if (narg != 4) error->all(FLERR,"Illegal pair_style command");

  t_start = force->numeric(arg[0]);
  t_stop = force->numeric(arg[1]);
  cut_global = force->numeric(arg[2]);
  seed = force->inumeric(arg[3]);

  temperature = t_start;

  // initialize Marsaglia RNG with processor-unique seed

  if (seed <= 0) error->all(FLERR,"Illegal pair_style command");
  delete random;
  random = new RanMars(lmp,seed + comm->me);

  // reset cutoffs that have been explicitly set

  if (allocated) {
    int i,j;
    for (i = 1; i <= atom->ntypes; i++)
      for (j = i+1; j <= atom->ntypes; j++)
        if (setflag[i][j]) cut[i][j] = cut_global;
  }
}

/* ----------------------------------------------------------------------
   set coeffs for one or more type pairs
------------------------------------------------------------------------- */

void PairDPDTstat::coeff(int narg, char **arg)
{
  if (narg < 3 || narg > 4) 
    error->all(FLERR,"Incorrect args for pair coefficients");
  if (!allocated) allocate();

  int ilo,ihi,jlo,jhi;
  force->bounds(arg[0],atom->ntypes,ilo,ihi);
  force->bounds(arg[1],atom->ntypes,jlo,jhi);

  double a0_one = 0.0;
  double gamma_one = force->numeric(arg[2]);

  double cut_one = cut_global;
  if (narg == 4) cut_one = force->numeric(arg[3]);

  int count = 0;
  for (int i = ilo; i <= ihi; i++) {
    for (int j = MAX(jlo,i); j <= jhi; j++) {
      a0[i][j] = a0_one;
      gamma[i][j] = gamma_one;
      cut[i][j] = cut_one;
      setflag[i][j] = 1;
      count++;
    }
  }

  if (count == 0) error->all(FLERR,"Incorrect args for pair coefficients");
}

/* ----------------------------------------------------------------------
   proc 0 writes to restart file
------------------------------------------------------------------------- */

void PairDPDTstat::write_restart_settings(FILE *fp)
{
  fwrite(&t_start,sizeof(double),1,fp);
  fwrite(&t_stop,sizeof(double),1,fp);
  fwrite(&cut_global,sizeof(double),1,fp);
  fwrite(&seed,sizeof(int),1,fp);
  fwrite(&mix_flag,sizeof(int),1,fp);
}

/* ----------------------------------------------------------------------
   proc 0 reads from restart file, bcasts
------------------------------------------------------------------------- */

void PairDPDTstat::read_restart_settings(FILE *fp)
{
  if (comm->me == 0) {
    fread(&t_start,sizeof(double),1,fp);
    fread(&t_stop,sizeof(double),1,fp);
    fread(&cut_global,sizeof(double),1,fp);
    fread(&seed,sizeof(int),1,fp);
    fread(&mix_flag,sizeof(int),1,fp);
  }
  MPI_Bcast(&t_start,1,MPI_DOUBLE,0,world);
  MPI_Bcast(&t_stop,1,MPI_DOUBLE,0,world);
  MPI_Bcast(&cut_global,1,MPI_DOUBLE,0,world);
  MPI_Bcast(&seed,1,MPI_INT,0,world);
  MPI_Bcast(&mix_flag,1,MPI_INT,0,world);

  // initialize Marsaglia RNG with processor-unique seed
  // same seed that pair_style command initially specified

  if (random) delete random;
  random = new RanMars(lmp,seed + comm->me);
}
