Improve VTK output

-use pvtu files to allow simple parallel output
-remain very naive implementation

CMakeLists.txt

@@ -174,6 +174,9 @@ include_directories(src/output)
 include_directories(src/utils)
 include_directories(src/scheme)
 
+# Pastis generated sources
+include_directories(${PASTIS_BINARY_DIR}/src/utils)
+
 # Pastis tests
 
 set(CATCH_MODULE_PATH "${PASTIS_SOURCE_DIR}/packages/Catch2")

src/output/VTKWriter.hpp

@@ -9,6 +9,7 @@
 #include <IConnectivity.hpp>
 
 #include <ItemValue.hpp>
+#include <Messenger.hpp>
 
 class VTKWriter
 {
@@ -18,6 +19,24 @@ class VTKWriter
   double m_last_time;
   const double m_time_period;
 
+  std::string _getFilenamePVTU()
+  {
+    std::ostringstream sout;
+    sout << m_base_filename;
+    sout << '.' << std::setfill('0') << std::setw(4) << m_file_number << ".pvtu";
+    return sout.str();
+  }
+  std::string _getFilenameVTU(const int& rank_number) const
+  {
+    std::ostringstream sout;
+    sout << m_base_filename;
+    if (parallel::size() > 1) {
+      sout << '-' << std::setfill('0') << std::setw(4) << rank_number;
+    }
+    sout << '.' << std::setfill('0') << std::setw(4) << m_file_number << ".vtu";
+    return sout.str();
+  }
+
  public:
   template <typename MeshType>
   void write(const MeshType& mesh,
@@ -30,9 +49,34 @@ class VTKWriter
     } else {
       return;
     }
-    std::ostringstream sout;
-    sout << m_base_filename << '.' << std::setfill('0') << std::setw(4) << m_file_number << ".vtu" << std::ends;
-    std::ofstream fout(sout.str());
+
+    if (parallel::rank() == 0)
+    { // write PVTK file
+      std::ofstream fout(_getFilenamePVTU());
+      fout << "<?xml version=\"1.0\"?>\n";
+      fout << "<VTKFile type=\"PUnstructuredGrid\">\n";
+      fout << "<PUnstructuredGrid GhostLevel=\"0\">\n";
+
+      fout << "<PPoints>\n";
+      fout << "<PDataArray Name=\"Positions\" NumberOfComponents=\"3\" type=\"Float64\"/>\n";
+      fout << "</PPoints>\n";
+
+      fout << "<PCells>\n";
+      fout << "<PDataArray type=\"Int32\" Name=\"connectivity\" NumberOfComponents=\"1\"/>\n";
+      fout << "<PDataArray type=\"UInt32\" Name=\"offsets\" NumberOfComponents=\"1\"/>\n";
+      fout << "<PDataArray type=\"Int8\" Name=\"types\" NumberOfComponents=\"1\"/>\n";
+      fout << "</PCells>\n";
+
+      for (size_t i_rank=0; i_rank<parallel::size(); ++i_rank) {
+        fout << "<Piece Source=\""<< _getFilenameVTU(i_rank) << "\"/>\n";
+      }
+      fout << "</PUnstructuredGrid>\n";
+      fout << "</VTKFile>\n";
+
+    }
+
+    { // write VTK files
+      std::ofstream fout(_getFilenameVTU(parallel::rank()));
       fout << "<?xml version=\"1.0\"?>\n";
       fout << "<VTKFile type=\"UnstructuredGrid\">\n";
       fout << "<UnstructuredGrid>\n";
@@ -133,7 +177,7 @@ class VTKWriter
       fout << "</Piece>\n";
       fout << "</UnstructuredGrid>\n";
       fout << "</VTKFile>\n";
-
+    }
     m_file_number++;
   }
   VTKWriter(const std::string& base_filename,