diff --git a/src/mergesort.h b/src/mergesort.h
new file mode 100644
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--- /dev/null
+++ b/src/mergesort.h
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+/* -*- 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