add missing mergesort header

/* -*- c++ -*- ----------------------------------------------------------

   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.

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

#ifndef LMP_MERGESORT

#define LMP_MERGESORT

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

// custom upward merge sort implementation which allows to pass a custom

// pointer to the comparison function for access to class instances.

// this avoids having to use global variables.

// part 1. merge two sublists.

static void do_merge(int *idx, int *buf, int llo, int lhi, int rlo, int rhi,

                     void *ptr, int (*comp)(int, int, void *))

{

  int i = llo;

  int l = llo;

  int r = rlo;

  while ((l < lhi) && (r < rhi)) {

    if ((*comp)(buf[l],buf[r],ptr) < 0)

      idx[i++] = buf[l++];

    else idx[i++] = buf[r++];

  }

  while(l < lhi) idx[i++] = buf[l++];

  while(r < rhi) idx[i++] = buf[r++];

}

// part 2: loop over sublists doubling in size with each iteration

static void merge_sort(int *index, int num, void *ptr,

                       int (*comp)(int, int, void *))

{

  if (num < 2) return;

  int *hold = new int[num];

  int i,j,k,m;

  i = 1;

  while (i < num) {

    memcpy(hold,index,sizeof(int)*num);

    for (j=0; j < num-1; j += 2*i) {

      k = j + 2*i;

      if (k > num) k=num;

      m = j+i;

      if (m > num) m=num;

      do_merge(index,hold,j,m,m,k,ptr,comp);

    }

    i *= 2;

  }

  delete[] hold;

}

#endif