[io][gmsh] allow granular definitions of geometries to be meshed

One can now set mesh sizes for individual geometries and furthermore,
one can define geometries to be physical or non-physical, depending on
which the interior of the geometries is explicitly meshed or not.

frackit/io/gmshbackend.hh

@@ -24,7 +24,9 @@
 #ifndef FRACKIT_GMSH_FORMAT_BACKEND_HH
 #define FRACKIT_GMSH_FORMAT_BACKEND_HH
 
+#include <array>
 #include <string>
+#include <limits>
 #include <cassert>
 #include <type_traits>
 
@@ -143,7 +145,7 @@ private:
     std::string getGeometryKeyword_(unsigned int geomDim) const
     {
         assert(geomDim <= 3);
-        static const std::vector<std::string> keywords = {
+        static const std::array<std::string, 4> keywords = {
             "Point", "Curve", "Surface", "Volume"
         };
 
@@ -160,6 +162,50 @@ private:
     int gmshVersion_;
 };
 
+namespace Detail {
+
+class MeshSizeVariableNameHelper
+{
+    static constexpr double maxDouble = std::numeric_limits<double>::max();
+    std::array<std::size_t, 4> counts = {0, 0, 0, 0};
+    std::array<double, 4> sizes = {maxDouble, maxDouble, maxDouble, maxDouble};
+    std::array<std::string, 4> names = {"subDomainMeshSize_",
+                                        "entityMeshSize_",
+                                        "intersectionMeshSize_",
+                                        "pointMeshSize_"};
+
+public:
+    void reset()
+    {
+        for (auto& c : counts) c = 0;
+        for (auto& s : sizes) s = maxDouble;
+    }
+
+    bool prepareNew(unsigned int codim, double size)
+    {
+        if (sizes[codim] > size)
+        {
+            sizes[codim] = size;
+            counts[codim]++;
+
+            const auto pos = names[codim].find_last_of("_");
+            const auto suffix = std::to_string(counts[codim]);
+            const auto suffixLength = suffix.size();
+            names[codim].resize(pos+1 + suffixLength);
+            names[codim].replace(pos+1, suffixLength, suffix);
+
+            return true;
+        }
+
+        return false;
+    }
+
+    const std::string& get(unsigned int codim)
+    { return names[codim]; }
+};
+
+} // end namespace Detail
+
 } // end namespace Frackit::Gmsh
 
 #endif // FRACKIT_GMSH_BACKEND_HH