text: /*
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  #include "gmxpre.h"
  
  #include <cmath>
  
  #include "gromacs/commandline/pargs.h"
  #include "gromacs/commandline/viewit.h"
  #include "gromacs/fileio/tpxio.h"
  #include "gromacs/fileio/trxio.h"
  #include "gromacs/fileio/xvgr.h"
  #include "gromacs/gmxana/gmx_ana.h"
  #include "gromacs/gmxana/gstat.h"
  #include "gromacs/math/functions.h"
  #include "gromacs/math/units.h"
  #include "gromacs/math/vec.h"
  #include "gromacs/mdtypes/inputrec.h"
  #include "gromacs/pbcutil/pbc.h"
  #include "gromacs/pbcutil/rmpbc.h"
  #include "gromacs/topology/index.h"
  #include "gromacs/topology/topology.h"
  #include "gromacs/utility/arraysize.h"
  #include "gromacs/utility/cstringutil.h"
  #include "gromacs/utility/fatalerror.h"
  #include "gromacs/utility/smalloc.h"
  
  static void calc_com_pbc(int nrefat, const t_topology* top, rvec x[], t_pbc* pbc, const int index[], rvec xref, PbcType pbcType)
  {
      const real tol = 1e-4;
      gmx_bool   bChanged;
      int        m, j, ai, iter;
      real       mass, mtot;
      rvec       dx, xtest;
  
      /* First simple calculation */
      clear_rvec(xref);
      mtot = 0;
      for (m = 0; (m < nrefat); m++)
      {
          ai   = index[m];
          mass = top->atoms.atom[ai].m;
          for (j = 0; (j < DIM); j++)
          {
              xref[j] += mass * x[ai][j];
          }
          mtot += mass;
      }
      svmul(1 / mtot, xref, xref);
      /* Now check if any atom is more than half the box from the COM */
      if (pbcType != PbcType::No)
      {
          iter = 0;
          do
          {
              bChanged = FALSE;
              for (m = 0; (m < nrefat); m++)
              {
                  ai   = index[m];
                  mass = top->atoms.atom[ai].m / mtot;
                  pbc_dx(pbc, x[ai], xref, dx);
                  rvec_add(xref, dx, xtest);
                  for (j = 0; (j < DIM); j++)
                  {
                      if (std::abs(xtest[j] - x[ai][j]) > tol)
                      {
                          /* Here we have used the wrong image for contributing to the COM */
                          xref[j] += mass * (xtest[j] - x[ai][j]);
                          x[ai][j] = xtest[j];
                          bChanged = TRUE;
                      }
                  }
              }
              if (bChanged)
              {
                  printf("COM: %8.3f  %8.3f  %8.3f  iter = %d\n", xref[XX], xref[YY], xref[ZZ], iter);
              }
              iter++;
          } while (bChanged);
      }
  }
  
  static void spol_atom2molindex(int* n, int* index, const t_block* mols)
  {
      int nmol, i, j, m;
  
      nmol = 0;
      i    = 0;
      while (i < *n)
      {
          m = 0;
          while (m < mols->nr && index[i] != mols->index[m])
          {
              m++;
          }
          if (m == mols->nr)
          {
              gmx_fatal(FARGS,
                        "index[%d]=%d does not correspond to the first atom of a molecule",
                        i + 1,
                        index[i] + 1);
          }
          for (j = mols->index[m]; j < mols->index[m + 1]; j++)
          {
              if (i >= *n || index[i] != j)
              {
                  gmx_fatal(FARGS, "The index group is not a set of whole molecules");
              }
              i++;
          }
          /* Modify the index in place */
          index[nmol++] = m;
      }
      printf("There are %d molecules in the selection\n", nmol);
  
      *n = nmol;
  }
  
  int gmx_spol(int argc, char* argv[])
  {
      t_topology*  top;
      t_atom*      atom;
      t_trxstatus* status;
      int          nrefat, natoms, nf, ntot;
      real         t;
      rvec *       x, xref, trial, dx = { 0 }, dip, dir;
      matrix       box;
  
      FILE*       fp;
      int *       isize, nrefgrp;
      int **      index, *molindex;
      char**      grpname;
      real        rmin2, rmax2, rcut, rcut2, rdx2 = 0, rtry2, qav, q, dip2, invbw;
      int         nbin, i, m, mol, a0, a1, a, d;
      double      sdip, sdip2, sinp, sdinp, nmol;
      int*        hist;
      t_pbc       pbc;
      gmx_rmpbc_t gpbc = nullptr;
  
  
      const char* desc[] = {
          "[THISMODULE] analyzes dipoles around a solute; it is especially useful",
          "for polarizable water. A group of reference atoms, or a center",
          "of mass reference (option [TT]-com[tt]) and a group of solvent",
          "atoms is required. The program splits the group of solvent atoms",
          "into molecules. For each solvent molecule the distance to the",
          "closest atom in reference group or to the COM is determined.",
          "A cumulative distribution of these distances is plotted.",
          "For each distance between [TT]-rmin[tt] and [TT]-rmax[tt]",
          "the inner product of the distance vector",
          "and the dipole of the solvent molecule is determined.",
          "For solvent molecules with net charge (ions), the net charge of the ion",
          "is subtracted evenly from all atoms in the selection of each ion.",
          "The average of these dipole components is printed.",
          "The same is done for the polarization, where the average dipole is",
          "subtracted from the instantaneous dipole. The magnitude of the average",
          "dipole is set with the option [TT]-dip[tt], the direction is defined",
          "by the vector from the first atom in the selected solvent group",
          "to the midpoint between the second and the third atom."
      };
  
      gmx_output_env_t* oenv;
      static gmx_bool   bCom   = FALSE;
      static int        srefat = 1;
      static real       rmin = 0.0, rmax = 0.32, refdip = 0, bw = 0.01;
      t_pargs           pa[] = {
          { "-com", FALSE, etBOOL, { &bCom }, "Use the center of mass as the reference position" },
          { "-refat", FALSE, etINT, { &srefat }, "The reference atom of the solvent molecule" },
          { "-rmin", FALSE, etREAL, { &rmin }, "Maximum distance (nm)" },
          { "-rmax", FALSE, etREAL, { &rmax }, "Maximum distance (nm)" },
          { "-dip", FALSE, etREAL, { &refdip }, "The average dipole (D)" },
          { "-bw", FALSE, etREAL, { &bw }, "The bin width" }
      };
  
      t_filenm fnm[] = { { efTRX, nullptr, nullptr, ffREAD },
                         { efTPR, nullptr, nullptr, ffREAD },
                         { efNDX, nullptr, nullptr, ffOPTRD },
                         { efXVG, nullptr, "scdist", ffWRITE } };
  #define NFILE asize(fnm)
  
      if (!parse_common_args(
                  &argc, argv, PCA_CAN_TIME | PCA_CAN_VIEW, NFILE, fnm, asize(pa), pa, asize(desc), desc, 0, nullptr, &oenv))
      {
          return 0;
      }
  
      snew(top, 1);
      // TODO: Only pbcType is used, not the full inputrec.
      t_inputrec  irInstance;
      t_inputrec* ir = &irInstance;
      read_tpx_top(ftp2fn(efTPR, NFILE, fnm), ir, box, &natoms, nullptr, nullptr, top);
  
      /* get index groups */
      printf("Select a group of reference particles and a solvent group:\n");
      snew(grpname, 2);
      snew(index, 2);
      snew(isize, 2);
      get_index(&top->atoms, ftp2fn_null(efNDX, NFILE, fnm), 2, isize, index, grpname);
  
      if (bCom)
      {
          nrefgrp = 1;
          nrefat  = isize[0];
      }
      else
      {
          nrefgrp = isize[0];
          nrefat  = 1;
      }
  
      spol_atom2molindex(&(isize[1]), index[1], &(top->mols));
      srefat--;
  
      /* initialize reading trajectory:                         */
      natoms = read_first_x(oenv, &status, ftp2fn(efTRX, NFILE, fnm), &t, &x, box);
  
      rcut = 0.99 * std::sqrt(max_cutoff2(ir->pbcType, box));
      if (rcut == 0)
      {
          rcut = 10 * rmax;
      }
      rcut2 = gmx::square(rcut);
      invbw = 1 / bw;
      nbin  = static_cast<int>(rcut * invbw) + 2;
      snew(hist, nbin);
  
      rmin2 = gmx::square(rmin);
      rmax2 = gmx::square(rmax);
  
      nf    = 0;
      ntot  = 0;
      sdip  = 0;
      sdip2 = 0;
      sinp  = 0;
      sdinp = 0;
  
      molindex = top->mols.index;
      atom     = top->atoms.atom;
  
      gpbc = gmx_rmpbc_init(&top->idef, ir->pbcType, natoms);
  
      /* start analysis of trajectory */
      do
      {
          /* make molecules whole again */
          gmx_rmpbc(gpbc, natoms, box, x);
  
          set_pbc(&pbc, ir->pbcType, box);
          if (bCom)
          {
              calc_com_pbc(nrefat, top, x, &pbc, index[0], xref, ir->pbcType);
          }
  
          for (m = 0; m < isize[1]; m++)
          {
              mol = index[1][m];
              a0  = molindex[mol];
              a1  = molindex[mol + 1];
              for (i = 0; i < nrefgrp; i++)
              {
                  pbc_dx(&pbc, x[a0 + srefat], bCom ? xref : x[index[0][i]], trial);
                  rtry2 = norm2(trial);
                  if (i == 0 || rtry2 < rdx2)
                  {
                      copy_rvec(trial, dx);
                      rdx2 = rtry2;
                  }
              }
              if (rdx2 < rcut2)
              {
                  hist[static_cast<int>(std::sqrt(rdx2) * invbw) + 1]++;
              }
              if (rdx2 >= rmin2 && rdx2 < rmax2)
              {
                  unitv(dx, dx);
                  clear_rvec(dip);
                  qav = 0;
                  for (a = a0; a < a1; a++)
                  {
                      qav += atom[a].q;
                  }
                  qav /= (a1 - a0);
                  for (a = a0; a < a1; a++)
                  {
                      q = atom[a].q - qav;
                      for (d = 0; d < DIM; d++)
                      {
                          dip[d] += q * x[a][d];
                      }
                  }
                  for (d = 0; d < DIM; d++)
                  {
                      dir[d] = -x[a0][d];
                  }
                  for (a = a0 + 1; a < a0 + 3; a++)
                  {
                      for (d = 0; d < DIM; d++)
                      {
                          dir[d] += 0.5 * x[a][d];
                      }
                  }
                  unitv(dir, dir);
  
                  svmul(gmx::c_enm2Debye, dip, dip);
                  dip2 = norm2(dip);
                  sdip += std::sqrt(dip2);
                  sdip2 += dip2;
                  for (d = 0; d < DIM; d++)
                  {
                      sinp += dx[d] * dip[d];
                      sdinp += dx[d] * (dip[d] - refdip * dir[d]);
                  }
  
                  ntot++;
              }
          }
          nf++;
  
      } while (read_next_x(oenv, status, &t, x, box));
  
      gmx_rmpbc_done(gpbc);
  
      /* clean up */
      sfree(x);
      close_trx(status);
  
      fprintf(stderr, "Average number of molecules within %g nm is %.1f\n", rmax, static_cast<real>(ntot) / nf);
      if (ntot > 0)
      {
          sdip /= ntot;
          sdip2 /= ntot;
          sinp /= ntot;
          sdinp /= ntot;
          fprintf(stderr,
                  "Average dipole:                               %f (D), std.dev. %f\n",
                  sdip,
                  std::sqrt(sdip2 - gmx::square(sdip)));
          fprintf(stderr, "Average radial component of the dipole:       %f (D)\n", sinp);
          fprintf(stderr, "Average radial component of the polarization: %f (D)\n", sdinp);
      }
  
      fp   = xvgropen(opt2fn("-o", NFILE, fnm), "Cumulative solvent distribution", "r (nm)", "molecules", oenv);
      nmol = 0;
      for (i = 0; i <= nbin; i++)
      {
          nmol += hist[i];
          fprintf(fp, "%g %g\n", i * bw, nmol / nf);
      }
      xvgrclose(fp);
  
      do_view(oenv, opt2fn("-o", NFILE, fnm), nullptr);
  
      return 0;
  }
  