/* ----------------------------------------------------------------------
   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
   https://www.lammps.org/, Sandia National Laboratories
   LAMMPS development team: developers@lammps.org

   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 "fix_neigh_history.h"

#include "atom.h"
#include "comm.h"
#include "error.h"
#include "force.h"
#include "memory.h"
#include "modify.h"
#include "my_page.h"
#include "neigh_list.h"
#include "neighbor.h"
#include "pair.h"

#include <cstring>

using namespace LAMMPS_NS;
using namespace FixConst;

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

FixNeighHistory::FixNeighHistory(LAMMPS *lmp, int narg, char **arg) :
    Fix(lmp, narg, arg), pair(nullptr), npartner(nullptr), partner(nullptr), valuepartner(nullptr),
    ipage_atom(nullptr), dpage_atom(nullptr), ipage_neigh(nullptr), dpage_neigh(nullptr)
{
  if (narg != 4) error->all(FLERR, "Illegal fix NEIGH_HISTORY command");

  restart_peratom = 1;
  restart_global = 1;

  create_attribute = 1;
  maxexchange_dynamic = 1;
  use_bit_flag = 1;

  newton_pair = force->newton_pair;

  dnum = utils::inumeric(FLERR, arg[3], false, lmp);
  dnumbytes = dnum * sizeof(double);

  zeroes = new double[dnum];
  for (int i = 0; i < dnum; i++) zeroes[i] = 0.0;

  onesided = 0;
  if (strcmp(id, "LINE_NEIGH_HISTORY") == 0) onesided = 1;
  if (strcmp(id, "TRI_NEIGH_HISTORY") == 0) onesided = 1;

  if (newton_pair)
    comm_reverse = 1;    // just for single npartner value
                         // variable-size history communicated via
                         // reverse_comm_variable()

  // perform initial allocation of atom-based arrays
  // register with atom class

  FixNeighHistory::grow_arrays(atom->nmax);
  atom->add_callback(Atom::GROW);
  atom->add_callback(Atom::RESTART);

  pgsize = oneatom = 0;

  // other per-atom vectors

  firstflag = nullptr;
  firstvalue = nullptr;
  maxatom = 0;

  // per-atom and per-neighbor data structs

  ipage_atom = nullptr;
  dpage_atom = nullptr;
  ipage_neigh = nullptr;
  dpage_neigh = nullptr;

  // initialize npartner to 0 so neighbor list creation is OK the 1st time

  int nlocal = atom->nlocal;
  for (int i = 0; i < nlocal; i++) npartner[i] = 0;
  maxpartner = 0;

  nlocal_neigh = nall_neigh = 0;
  commflag = DEFAULT;
}

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

FixNeighHistory::~FixNeighHistory()
{
  if (copymode) return;

  // unregister this fix so atom class doesn't invoke it any more

  atom->delete_callback(id, Atom::GROW);
  atom->delete_callback(id, Atom::RESTART);

  // delete locally stored arrays

  delete[] zeroes;

  memory->sfree(firstflag);
  memory->sfree(firstvalue);

  memory->destroy(npartner);
  memory->sfree(partner);
  memory->sfree(valuepartner);

  delete[] ipage_atom;
  delete[] dpage_atom;
  delete[] ipage_neigh;
  delete[] dpage_neigh;

  // to better detect use-after-delete errors

  firstflag = nullptr;
  firstvalue = nullptr;

  pair = nullptr;
  npartner = nullptr;
  partner = nullptr;
  valuepartner = nullptr;
}

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

int FixNeighHistory::setmask()
{
  int mask = 0;
  mask |= PRE_EXCHANGE;
  mask |= MIN_PRE_EXCHANGE;
  mask |= POST_NEIGHBOR;
  mask |= MIN_POST_NEIGHBOR;
  return mask;
}

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

void FixNeighHistory::init()
{
  if (atom->tag_enable == 0) error->all(FLERR, "Neighbor history requires that atoms have IDs");

  // this fix must come before any fix which migrates atoms in its pre_exchange()
  // because this fix's pre_exchange() creates per-atom data structure
  // that data must be current for atom migration to carry it along

  for (const auto &ifix : modify->get_fix_list()) {
    if (ifix == this) break;
    if (ifix->pre_exchange_migrate)
      error->all(FLERR,
                 "Pair styles using neighbor history must be defined before "
                 "fix {} {} which migrates atoms in pre_exchange",
                 ifix->id, ifix->style);
  }

  // setup data structs

  allocate_pages();
}

/* ----------------------------------------------------------------------
   create pages if first time or if neighbor pgsize/oneatom has changed
   note that latter could cause neighbor history info to be discarded
------------------------------------------------------------------------- */

void FixNeighHistory::allocate_pages()
{
  int create = 0;
  if (ipage_atom == nullptr) create = 1;
  if (pgsize != neighbor->pgsize) create = 1;
  if (oneatom != neighbor->oneatom) create = 1;

  if (create) {
    delete[] ipage_atom;
    delete[] dpage_atom;
    delete[] ipage_neigh;
    delete[] dpage_neigh;

    pgsize = neighbor->pgsize;
    oneatom = neighbor->oneatom;
    int nmypage = comm->nthreads;
    ipage_atom = new MyPage<tagint>[nmypage];
    dpage_atom = new MyPage<double>[nmypage];
    ipage_neigh = new MyPage<int>[nmypage];
    dpage_neigh = new MyPage<double>[nmypage];
    for (int i = 0; i < nmypage; i++) {
      ipage_atom[i].init(oneatom, pgsize);
      dpage_atom[i].init(dnum * oneatom, dnum * pgsize);
      ipage_neigh[i].init(oneatom, pgsize);
      dpage_neigh[i].init(dnum * oneatom, dnum * pgsize);
    }
  }
}

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

void FixNeighHistory::setup_post_neighbor()
{
  post_neighbor();
}

/* ----------------------------------------------------------------------
   copy partner info from neighbor data structs (NDS) to atom arrays
   should be called whenever NDS store current history info
     and need to transfer the info to owned atoms
   e.g. when atoms migrate to new procs, new neigh list built, or between runs
     when atoms may be added or deleted (NDS becomes out-of-date)
   the next post_neighbor() will put this info back into new NDS
   called during run before atom exchanges, including for restart files
   called at end of run via post_run()
   do not call during setup of run (setup_pre_exchange)
     because there is no guarantee of a current NDS (even on continued run)
   if run command does a 2nd run with pre = no, then no neigh list
     will be built, but old neigh list will still have the info
   onesided and newton on and newton off versions
------------------------------------------------------------------------- */

void FixNeighHistory::pre_exchange()
{
  if (onesided)
    pre_exchange_onesided();
  else if (newton_pair)
    pre_exchange_newton();
  else
    pre_exchange_no_newton();
}

/* ----------------------------------------------------------------------
   onesided version for sphere contact with line/tri particles
   neighbor list has I = sphere, J = line/tri
   only store history info with spheres
------------------------------------------------------------------------- */

void FixNeighHistory::pre_exchange_onesided()
{
  int i, j, ii, jj, m, n, inum, jnum;
  int *ilist, *jlist, *numneigh, **firstneigh;
  int *allflags;
  double *allvalues, *onevalues;

  // NOTE: all operations until very end are with nlocal_neigh <= current nlocal
  // because previous neigh list was built with nlocal_neigh
  // nlocal can be larger if other fixes added atoms at this pre_exchange()

  // clear two paged data structures

  ipage_atom->reset();
  dpage_atom->reset();

  // 1st loop over neighbor list, I = sphere, J = tri
  // only calculate npartner for owned spheres

  for (i = 0; i < nlocal_neigh; i++) npartner[i] = 0;

  tagint *tag = atom->tag;
  NeighList *list = pair->list;
  inum = list->inum;
  ilist = list->ilist;
  numneigh = list->numneigh;
  firstneigh = list->firstneigh;

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    jlist = firstneigh[i];
    jnum = numneigh[i];
    allflags = firstflag[i];

    for (jj = 0; jj < jnum; jj++)
      if (allflags[jj]) npartner[i]++;
  }

  // get page chunks to store partner IDs and values for owned atoms

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    n = npartner[i];
    partner[i] = ipage_atom->get(n);
    valuepartner[i] = dpage_atom->get(dnum * n);
    if (partner[i] == nullptr || valuepartner[i] == nullptr)
      error->one(FLERR, "Neighbor history overflow, boost neigh_modify one");
  }

  // 2nd loop over neighbor list, I = sphere, J = tri
  // store partner IDs and values for owned+ghost atoms
  // re-zero npartner to use as counter

  for (i = 0; i < nlocal_neigh; i++) npartner[i] = 0;

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    jlist = firstneigh[i];
    jnum = numneigh[i];
    allflags = firstflag[i];
    allvalues = firstvalue[i];

    for (jj = 0; jj < jnum; jj++) {
      if (allflags[jj]) {
        onevalues = &allvalues[dnum * jj];
        j = jlist[jj];
        j &= NEIGHMASK;
        m = npartner[i]++;
        partner[i][m] = tag[j];
        memcpy(&valuepartner[i][dnum * m], onevalues, dnumbytes);
      }
    }
  }

  // set maxpartner = max # of partners of any owned atom
  // maxexchange = max # of values for any Comm::exchange() atom

  maxpartner = 0;
  for (i = 0; i < nlocal_neigh; i++) maxpartner = MAX(maxpartner, npartner[i]);
  maxexchange = (dnum + 1) * maxpartner + 1;

  // zero npartner values from previous nlocal_neigh to current nlocal

  int nlocal = atom->nlocal;
  for (i = nlocal_neigh; i < nlocal; i++) npartner[i] = 0;
}

/* ----------------------------------------------------------------------
   newton ON version
   performs reverse comm to acquire partner values from ghost atoms
------------------------------------------------------------------------- */

void FixNeighHistory::pre_exchange_newton()
{
  int i, j, ii, jj, m, n, inum, jnum;
  int *ilist, *jlist, *numneigh, **firstneigh;
  int *allflags;
  double *allvalues, *onevalues, *jvalues;
  int *type = atom->type;

  // NOTE: all operations until very end are with
  //   nlocal_neigh  <= current nlocal and nall_neigh
  // because previous neigh list was built with nlocal_neigh & nghost_neigh
  // nlocal can be larger if other fixes added atoms at this pre_exchange()

  // clear two paged data structures

  ipage_atom->reset();
  dpage_atom->reset();

  // 1st loop over neighbor list
  // calculate npartner for owned+ghost atoms

  // Ensure npartner is zeroed across all atoms, nall_neigh can be less than nall
  // when writing restarts when comm calls are made but modify->post_neighbor() isn't
  int nall = atom->nlocal + atom->nghost;
  for (i = 0; i < MAX(nall_neigh, nall); i++) npartner[i] = 0;

  tagint *tag = atom->tag;
  NeighList *list = pair->list;
  inum = list->inum;
  ilist = list->ilist;
  numneigh = list->numneigh;
  firstneigh = list->firstneigh;

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    jlist = firstneigh[i];
    jnum = numneigh[i];
    allflags = firstflag[i];

    for (jj = 0; jj < jnum; jj++) {
      if (allflags[jj]) {
        npartner[i]++;
        j = jlist[jj];
        j &= NEIGHMASK;
        npartner[j]++;
      }
    }
  }

  // perform reverse comm to augment owned npartner counts with ghost counts

  commflag = NPARTNER;
  comm->reverse_comm(this);

  // get page chunks to store partner IDs and values for owned+ghost atoms

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    n = npartner[i];
    partner[i] = ipage_atom->get(n);
    valuepartner[i] = dpage_atom->get(dnum * n);
    if (partner[i] == nullptr || valuepartner[i] == nullptr) {
      error->one(FLERR, "Neighbor history overflow, boost neigh_modify one");
    }
  }

  for (i = nlocal_neigh; i < nall_neigh; i++) {
    n = npartner[i];
    partner[i] = ipage_atom->get(n);
    valuepartner[i] = dpage_atom->get(dnum * n);
    if (partner[i] == nullptr || valuepartner[i] == nullptr) {
      error->one(FLERR, "Neighbor history overflow, boost neigh_modify one");
    }
  }

  // 2nd loop over neighbor list
  // store partner IDs and values for owned+ghost atoms
  // re-zero npartner to use as counter

  for (i = 0; i < MAX(nall_neigh, nall); i++) npartner[i] = 0;

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    jlist = firstneigh[i];
    jnum = numneigh[i];
    allflags = firstflag[i];
    allvalues = firstvalue[i];

    for (jj = 0; jj < jnum; jj++) {
      if (allflags[jj]) {
        onevalues = &allvalues[dnum * jj];
        j = jlist[jj];
        j &= NEIGHMASK;
        m = npartner[i]++;
        partner[i][m] = tag[j];
        memcpy(&valuepartner[i][dnum * m], onevalues, dnumbytes);
        m = npartner[j]++;
        partner[j][m] = tag[i];
        jvalues = &valuepartner[j][dnum * m];
        if (pair->nondefault_history_transfer)
          pair->transfer_history(onevalues, jvalues, type[i], type[j]);
        else
          for (n = 0; n < dnum; n++) jvalues[n] = -onevalues[n];
      }
    }
  }

  // perform reverse comm to augment
  // owned atom partner/valuepartner with ghost info
  // use variable variant because size of packed data can be arbitrarily large
  //   if many touching neighbors for large particle

  commflag = PERPARTNER;
  comm->reverse_comm_variable(this);

  // set maxpartner = max # of partners of any owned atom
  // maxexchange = max # of values for any Comm::exchange() atom

  maxpartner = 0;
  for (i = 0; i < nlocal_neigh; i++) maxpartner = MAX(maxpartner, npartner[i]);
  maxexchange = (dnum + 1) * maxpartner + 1;

  // zero npartner values from previous nlocal_neigh to current nlocal

  int nlocal = atom->nlocal;
  for (i = nlocal_neigh; i < nlocal; i++) npartner[i] = 0;
}

/* ----------------------------------------------------------------------
   newton OFF version
   do not need partner values from ghost atoms
   assume J values are negative of I values
------------------------------------------------------------------------- */

void FixNeighHistory::pre_exchange_no_newton()
{
  int i, j, ii, jj, m, n, inum, jnum;
  int *ilist, *jlist, *numneigh, **firstneigh;
  int *allflags;
  double *allvalues, *onevalues, *jvalues;
  int *type = atom->type;

  // NOTE: all operations until very end are with nlocal_neigh <= current nlocal
  // because previous neigh list was built with nlocal_neigh
  // nlocal can be larger if other fixes added atoms at this pre_exchange()

  // clear two paged data structures

  ipage_atom->reset();
  dpage_atom->reset();

  // 1st loop over neighbor list
  // calculate npartner for owned atoms

  for (i = 0; i < nlocal_neigh; i++) npartner[i] = 0;

  tagint *tag = atom->tag;
  NeighList *list = pair->list;
  inum = list->inum;
  ilist = list->ilist;
  numneigh = list->numneigh;
  firstneigh = list->firstneigh;

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    jlist = firstneigh[i];
    jnum = numneigh[i];
    allflags = firstflag[i];

    for (jj = 0; jj < jnum; jj++) {
      if (allflags[jj]) {
        npartner[i]++;
        j = jlist[jj];
        j &= NEIGHMASK;
        if (j < nlocal_neigh) npartner[j]++;
      }
    }
  }

  // get page chunks to store partner IDs and values for owned atoms

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    n = npartner[i];
    partner[i] = ipage_atom->get(n);
    valuepartner[i] = dpage_atom->get(dnum * n);
    if (partner[i] == nullptr || valuepartner[i] == nullptr)
      error->one(FLERR, "Neighbor history overflow, boost neigh_modify one");
  }

  // 2nd loop over neighbor list
  // store partner IDs and values for owned+ghost atoms
  // re-zero npartner to use as counter

  for (i = 0; i < nlocal_neigh; i++) npartner[i] = 0;

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    jlist = firstneigh[i];
    jnum = numneigh[i];
    allflags = firstflag[i];
    allvalues = firstvalue[i];

    for (jj = 0; jj < jnum; jj++) {
      if (allflags[jj]) {
        onevalues = &allvalues[dnum * jj];
        j = jlist[jj];
        j &= NEIGHMASK;
        m = npartner[i]++;
        partner[i][m] = tag[j];
        memcpy(&valuepartner[i][dnum * m], onevalues, dnumbytes);
        if (j < nlocal_neigh) {
          m = npartner[j]++;
          partner[j][m] = tag[i];
          jvalues = &valuepartner[j][dnum * m];
          if (pair->nondefault_history_transfer)
            pair->transfer_history(onevalues, jvalues, type[i], type[j]);
          else
            for (n = 0; n < dnum; n++) jvalues[n] = -onevalues[n];
        }
      }
    }
  }

  // set maxpartner = max # of partners of any owned atom
  // maxexchange = max # of values for any Comm::exchange() atom

  maxpartner = 0;
  for (i = 0; i < nlocal_neigh; i++) maxpartner = MAX(maxpartner, npartner[i]);
  maxexchange = (dnum + 1) * maxpartner + 1;

  // zero npartner values from previous nlocal_neigh to current nlocal

  int nlocal = atom->nlocal;
  for (i = nlocal_neigh; i < nlocal; i++) npartner[i] = 0;
}

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

void FixNeighHistory::min_pre_exchange()
{
  pre_exchange();
}

/* ----------------------------------------------------------------------
   called after neighbor list is build
   recover history info stored temporarily in per-atom partner lists
     and store afresh in per-neighbor firstflag and firstvalue lists
------------------------------------------------------------------------- */

void FixNeighHistory::post_neighbor()
{
  int i, j, m, ii, jj, nn, np, inum, jnum, rflag;
  tagint jtag;
  int *ilist, *jlist, *numneigh, **firstneigh;
  int *allflags;
  double *allvalues;

  // store atom counts used for new neighbor list which was just built

  int nlocal = atom->nlocal;
  int nall = nlocal + atom->nghost;
  nlocal_neigh = nlocal;
  nall_neigh = nall;

  // realloc firstflag and firstvalue if needed

  if (maxatom < nlocal) {
    memory->sfree(firstflag);
    memory->sfree(firstvalue);
    maxatom = nall;
    firstflag = (int **) memory->smalloc(maxatom * sizeof(int *), "neighbor_history:firstflag");
    firstvalue =
        (double **) memory->smalloc(maxatom * sizeof(double *), "neighbor_history:firstvalue");
  }

  // loop over newly built neighbor list
  // repopulate entire per-neighbor data structs
  //   whether with old-neigh partner info or zeroes

  ipage_neigh->reset();
  dpage_neigh->reset();

  tagint *tag = atom->tag;
  NeighList *list = pair->list;
  inum = list->inum;
  ilist = list->ilist;
  numneigh = list->numneigh;
  firstneigh = list->firstneigh;

  for (ii = 0; ii < inum; ii++) {
    i = ilist[ii];
    jlist = firstneigh[i];
    jnum = numneigh[i];
    firstflag[i] = allflags = ipage_neigh->get(jnum);
    firstvalue[i] = allvalues = dpage_neigh->get(jnum * dnum);
    np = npartner[i];
    nn = 0;

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

      if (use_bit_flag) {
        rflag = histmask(j) | pair->beyond_contact;
        j &= HISTMASK;
        jlist[jj] = j;
      } else {
        rflag = 1;
      }

      // Remove special bond bits
      j &= NEIGHMASK;

      // rflag = 1 if r < radsum in npair_size() method or if pair interactions extend further
      // preserve neigh history info if tag[j] is in old-neigh partner list
      // this test could be more geometrically precise for two sphere/line/tri
      // if use_bit_flag is turned off, always record data since not all npair classes
      // apply a mask for history (and they could use the bits for special bonds)

      if (rflag) {
        jtag = tag[j];
        for (m = 0; m < np; m++)
          if (partner[i][m] == jtag) break;
        if (m < np) {
          allflags[jj] = 1;
          memcpy(&allvalues[nn], &valuepartner[i][dnum * m], dnumbytes);
        } else {
          allflags[jj] = 0;
          memcpy(&allvalues[nn], zeroes, dnumbytes);
        }
      } else {
        allflags[jj] = 0;
        memcpy(&allvalues[nn], zeroes, dnumbytes);
      }
      nn += dnum;
    }
  }
}

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

void FixNeighHistory::min_post_neighbor()
{
  post_neighbor();
}

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

void FixNeighHistory::post_run()
{
  pre_exchange();
}

/* ----------------------------------------------------------------------
   memory usage of local atom-based arrays
------------------------------------------------------------------------- */

double FixNeighHistory::memory_usage()
{
  int nmax = atom->nmax;
  double bytes = (double) nmax * sizeof(int);      // npartner
  bytes += (double) nmax * sizeof(tagint *);       // partner
  bytes += (double) nmax * sizeof(double *);       // valuepartner
  bytes += (double) maxatom * sizeof(int *);       // firstflag
  bytes += (double) maxatom * sizeof(double *);    // firstvalue

  int nmypage = comm->nthreads;
  for (int i = 0; i < nmypage; i++) {
    bytes += ipage_atom[i].size();
    bytes += dpage_atom[i].size();
    bytes += ipage_neigh[i].size();
    bytes += dpage_neigh[i].size();
  }

  return bytes;
}

/* ----------------------------------------------------------------------
   allocate local atom-based arrays
------------------------------------------------------------------------- */

void FixNeighHistory::grow_arrays(int nmax)
{
  memory->grow(npartner, nmax, "neighbor_history:npartner");
  partner =
      (tagint **) memory->srealloc(partner, nmax * sizeof(tagint *), "neighbor_history:partner");
  valuepartner = (double **) memory->srealloc(valuepartner, nmax * sizeof(double *),
                                              "neighbor_history:valuepartner");
}

/* ----------------------------------------------------------------------
   copy values within local atom-based arrays
------------------------------------------------------------------------- */

void FixNeighHistory::copy_arrays(int i, int j, int /*delflag*/)
{
  // just copy pointers for partner and valuepartner
  // because can't overwrite chunk allocation inside ipage_atom,dpage_atom
  // incoming atoms in unpack_exchange just grab new chunks
  // so are orphaning chunks for migrating atoms
  // OK, because will reset ipage_atom,dpage_atom on next reneighboring

  npartner[j] = npartner[i];
  partner[j] = partner[i];
  valuepartner[j] = valuepartner[i];
}

/* ----------------------------------------------------------------------
   initialize one atom's array values, called when atom is created
------------------------------------------------------------------------- */

void FixNeighHistory::set_arrays(int i)
{
  npartner[i] = 0;
}

/* ----------------------------------------------------------------------
   only called by Comm::reverse_comm_variable for PERPARTNER mode
------------------------------------------------------------------------- */

int FixNeighHistory::pack_reverse_comm_size(int n, int first)
{
  int i, last;
  int dnump1 = dnum + 1;

  int m = 0;
  last = first + n;

  for (i = first; i < last; i++) m += 1 + dnump1 * npartner[i];

  return m;
}

/* ----------------------------------------------------------------------
   two modes: NPARTNER and PERPARTNER
------------------------------------------------------------------------- */

int FixNeighHistory::pack_reverse_comm(int n, int first, double *buf)
{
  int i, k, last;

  int m = 0;
  last = first + n;

  if (commflag == NPARTNER) {
    for (i = first; i < last; i++) { buf[m++] = npartner[i]; }
  } else if (commflag == PERPARTNER) {
    for (i = first; i < last; i++) {
      buf[m++] = npartner[i];
      for (k = 0; k < npartner[i]; k++) {
        buf[m++] = ubuf(partner[i][k]).d;
        memcpy(&buf[m], &valuepartner[i][dnum * k], dnumbytes);
        m += dnum;
      }
    }
  } else
    error->all(FLERR, "Unsupported comm mode in neighbor history");

  return m;
}

/* ----------------------------------------------------------------------
   two modes: NPARTNER and PERPARTNER
------------------------------------------------------------------------- */

void FixNeighHistory::unpack_reverse_comm(int n, int *list, double *buf)
{
  int i, j, k, kk, ncount;

  int m = 0;

  if (commflag == NPARTNER) {
    for (i = 0; i < n; i++) {
      j = list[i];
      npartner[j] += static_cast<int>(buf[m++]);
    }
  } else if (commflag == PERPARTNER) {
    for (i = 0; i < n; i++) {
      j = list[i];
      ncount = static_cast<int>(buf[m++]);
      for (k = 0; k < ncount; k++) {
        kk = npartner[j]++;
        partner[j][kk] = static_cast<tagint>(ubuf(buf[m++]).i);
        memcpy(&valuepartner[j][dnum * kk], &buf[m], dnumbytes);
        m += dnum;
      }
    }
  } else
    error->all(FLERR, "Unsupported comm mode in neighbor history");
}

/* ----------------------------------------------------------------------
   pack values in local atom-based arrays for exchange with another proc
------------------------------------------------------------------------- */

int FixNeighHistory::pack_exchange(int i, double *buf)
{
  int m = 0;
  buf[m++] = npartner[i];
  for (int n = 0; n < npartner[i]; n++) {
    buf[m++] = ubuf(partner[i][n]).d;
    memcpy(&buf[m], &valuepartner[i][dnum * n], dnumbytes);
    m += dnum;
  }
  return m;
}

/* ----------------------------------------------------------------------
   unpack values in local atom-based arrays from exchange with another proc
------------------------------------------------------------------------- */

int FixNeighHistory::unpack_exchange(int nlocal, double *buf)
{
  // allocate new chunks from ipage_atom,dpage_atom for incoming values

  int m = 0;
  npartner[nlocal] = static_cast<int>(buf[m++]);
  maxpartner = MAX(maxpartner, npartner[nlocal]);
  partner[nlocal] = ipage_atom->get(npartner[nlocal]);
  valuepartner[nlocal] = dpage_atom->get(dnum * npartner[nlocal]);
  for (int n = 0; n < npartner[nlocal]; n++) {
    partner[nlocal][n] = static_cast<tagint>(ubuf(buf[m++]).i);
    memcpy(&valuepartner[nlocal][dnum * n], &buf[m], dnumbytes);
    m += dnum;
  }
  return m;
}

/* ----------------------------------------------------------------------
   Use write_restart to invoke pre_exchange
------------------------------------------------------------------------- */

void FixNeighHistory::write_restart(FILE *fp)
{
  // Call pre-exchange to copy updated history in page file
  // back into per-atom arrays prior to packing restart data

  pre_exchange();
  if (comm->me == 0) {
    int size = 0;
    fwrite(&size, sizeof(int), 1, fp);
  }
}

/* ----------------------------------------------------------------------
   pack values in local atom-based arrays for restart file
------------------------------------------------------------------------- */

int FixNeighHistory::pack_restart(int i, double *buf)
{
  int m = 1;
  buf[m++] = npartner[i];
  for (int n = 0; n < npartner[i]; n++) {
    buf[m++] = ubuf(partner[i][n]).d;
    memcpy(&buf[m], &valuepartner[i][dnum * n], dnumbytes);
    m += dnum;
  }
  // pack buf[0] this way because other fixes unpack it
  buf[0] = m;
  return m;
}

/* ----------------------------------------------------------------------
   unpack values from atom->extra array to restart the fix
------------------------------------------------------------------------- */

void FixNeighHistory::unpack_restart(int nlocal, int nth)
{
  // ipage_atom = nullptr if being called from granular pair style init()

  if (ipage_atom == nullptr) allocate_pages();

  // skip to Nth set of extra values
  // unpack the Nth first values this way because other fixes pack them

  double **extra = atom->extra;

  int m = 0;
  for (int i = 0; i < nth; i++) m += static_cast<int>(extra[nlocal][m]);
  m++;

  // allocate new chunks from ipage_atom,dpage_atom for incoming values

  npartner[nlocal] = static_cast<int>(extra[nlocal][m++]);
  maxpartner = MAX(maxpartner, npartner[nlocal]);
  partner[nlocal] = ipage_atom->get(npartner[nlocal]);
  valuepartner[nlocal] = dpage_atom->get(dnum * npartner[nlocal]);
  for (int n = 0; n < npartner[nlocal]; n++) {
    partner[nlocal][n] = static_cast<tagint>(ubuf(extra[nlocal][m++]).i);
    memcpy(&valuepartner[nlocal][dnum * n], &extra[nlocal][m], dnumbytes);
    m += dnum;
  }
}

/* ----------------------------------------------------------------------
   maxsize of any atom's restart data
------------------------------------------------------------------------- */

int FixNeighHistory::maxsize_restart()
{
  // maxpartner_all = max # of touching partners across all procs

  int maxpartner_all;
  MPI_Allreduce(&maxpartner, &maxpartner_all, 1, MPI_INT, MPI_MAX, world);
  return (dnum + 1) * maxpartner_all + 2;
}

/* ----------------------------------------------------------------------
   size of atom nlocal's restart data
------------------------------------------------------------------------- */

int FixNeighHistory::size_restart(int nlocal)
{
  return (dnum + 1) * npartner[nlocal] + 2;
}