gnuplot_io.C

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// The libMesh Finite Element Library.
// Copyright (C) 2002-2025 Benjamin S. Kirk, John W. Peterson, Roy H. Stogner

// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.

// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

// C++ includes
#include <fstream>
#include <sstream>
#include <map>

// Local includes
#include "libmesh/elem.h"
#include "libmesh/libmesh_logging.h"
#include "libmesh/mesh_base.h"
#include "libmesh/gnuplot_io.h"

namespace libMesh
{

GnuPlotIO::GnuPlotIO(const MeshBase & mesh_in,
                     const std::string & title,
                     int mesh_properties)
  :
  MeshOutput<MeshBase> (mesh_in),
  _title(title)
{
  _grid       = (mesh_properties & GRID_ON);
  _png_output = (mesh_properties & PNG_OUTPUT);
}

void GnuPlotIO::write(const std::string & fname)
{
  this->write_solution(fname);
}

void GnuPlotIO::write_nodal_data (const std::string & fname,
                                  const std::vector<Number> & soln,
                                  const std::vector<std::string> & names)
{
  LOG_SCOPE("write_nodal_data()", "GnuPlotIO");
  this->write_solution(fname, &soln, &names);
}




void GnuPlotIO::write_solution(const std::string & fname,
                               const std::vector<Number> * soln,
                               const std::vector<std::string> * names)
{
  // Even when writing on a serialized DistributedMesh, we expect
  // non-proc-0 help with calls like n_active_elem
  // libmesh_assert_equal_to (this->mesh().processor_id(), 0);

  const MeshBase & the_mesh = MeshOutput<MeshBase>::mesh();

  dof_id_type n_active_elem = the_mesh.n_active_elem();

  if (this->mesh().processor_id() == 0)
    {
      std::stringstream data_stream_name;
      data_stream_name << fname << "_data";
      const std::string data_file_name = data_stream_name.str();

      // This class is designed only for use with 1D meshes
      libmesh_assert_equal_to (the_mesh.mesh_dimension(), 1);

      // Make sure we have a solution to plot
      libmesh_assert ((names != nullptr) && (soln != nullptr));

      // Create an output stream for script file
      std::ofstream out_stream(fname.c_str());

      // Make sure it opened correctly
      if (!out_stream.good())
        libmesh_file_error(fname.c_str());

      // The number of variables in the equation system
      const unsigned int n_vars =
        cast_int<unsigned int>(names->size());

      // Write header to stream
      out_stream << "# This file was generated by gnuplot_io.C\n"
                 << "# Stores 1D solution data in GNUplot format\n"
                 << "# Execute this by loading gnuplot and typing "
                 << "\"call '" << fname << "'\"\n"
                 << "reset\n"
                 << "set title \"" << _title << "\"\n"
                 << "set xlabel \"x\"\n"
                 << "set xtics nomirror\n";

      // Loop over the elements to find the minimum and maximum x values,
      // and also to find the element boundaries to write out as xtics
      // if requested.
      Real x_min=0., x_max=0.;

      // construct string for xtic positions at element edges
      std::stringstream xtics_stream;

      unsigned int count = 0;

      for (const auto & el : the_mesh.active_element_ptr_range())
        {
          // if el is the left edge of the mesh, print its left node position
          if (el->neighbor_ptr(0) == nullptr)
            {
              x_min = (el->point(0))(0);
              xtics_stream << "\"\" " << x_min << ", \\\n";
            }
          if (el->neighbor_ptr(1) == nullptr)
            {
              x_max = (el->point(1))(0);
            }
          xtics_stream << "\"\" " << (el->point(1))(0);

          if (count+1 != n_active_elem)
            {
              xtics_stream << ", \\\n";
            }
          count++;
        }

      out_stream << "set xrange [" << x_min << ":" << x_max << "]\n";

      if (_grid)
        out_stream << "set x2tics (" << xtics_stream.str() << ")\nset grid noxtics noytics x2tics\n";

      if (_png_output)
        {
          out_stream << "set terminal png\n";
          out_stream << "set output \"" << fname << ".png\"\n";
        }

      out_stream << "plot "
                 << axes_limits
                 << " \"" << data_file_name << "\" using 1:2 title \"" << (*names)[0]
                 << "\" with lines";
      if (n_vars > 1)
        {
          for (unsigned int i=1; i<n_vars; i++)
            {
              out_stream << ", \\\n\"" << data_file_name << "\" using 1:" << i+2
                         << " title \"" << (*names)[i] << "\" with lines";
            }
        }

      out_stream.close();


      // Create an output stream for data file
      std::ofstream data(data_file_name.c_str());

      libmesh_error_msg_if(!data.good(), "ERROR: opening output data file " << data_file_name);

      // get ordered nodal data using a map
      std::map<Real, std::vector<Number>> node_map;

      for (const auto & elem : the_mesh.active_element_ptr_range())
        for (const auto & node : elem->node_ref_range())
          {
            dof_id_type global_id = node.id();

            std::vector<Number> values;
            for (unsigned int c=0; c<n_vars; c++)
              values.push_back((*soln)[global_id*n_vars + c]);

            node_map[the_mesh.point(global_id)(0)] = values;
          }

      for (const auto & pr : node_map)
        {
          const std::vector<Number> & values = pr.second;

          data << pr.first << "\t";

          for (const auto & val : values)
            data << val << "\t";

          data << "\n";
        }

      data.close();
    }
}

} // namespace libMesh