fix(MSHInput): ignore elements with repeated nodes

An element listing the same node twice became a polygon or polyhedron
with a repeated vertex, which is an invalid mesh element. Such elements
are now skipped, with one warning giving how many were ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: BotellaA

src/geode/io/model/msh_input.cpp

@@ -31,6 +31,7 @@
 
 #include <absl/container/flat_hash_map.h>
 #include <absl/container/flat_hash_set.h>
+#include <absl/container/inlined_vector.h>
 #include <absl/strings/ascii.h>
 #include <absl/strings/str_cat.h>
 #include <absl/strings/str_split.h>
@@ -68,6 +69,7 @@
 namespace
 {
     constexpr geode::index_t POINT_TYPE{ 15 };
+    constexpr geode::index_t MAX_ELEMENT_NODES{ 8 };
 
     /// Counts read from the file are not trusted to reserve memory
     constexpr geode::index_t MAX_RESERVE{ 1U << 24U };
@@ -277,6 +279,11 @@ namespace
                 nodes_read && elements_read, nullptr,
                 geode::OpenGeodeException::TYPE::data,
                 "[MSHInput::read_file] Missing $Nodes or $Elements section" );
+            if( nb_degenerate_elements_ != 0 )
+            {
+                geode::Logger::warning( "[MSHInput] ", nb_degenerate_elements_,
+                    " elements with repeated nodes are ignored" );
+            }
         }
 
         void build_geometry()
@@ -708,7 +715,7 @@ namespace
             geode::index_t gmsh_type,
             absl::Span< const std::string_view > node_tokens )
         {
-            component.types.push_back( gmsh_type );
+            absl::InlinedVector< geode::index_t, MAX_ELEMENT_NODES > nodes;
             for( const auto& node : node_tokens )
             {
                 const auto node_tag = geode::string_to_index( node );
@@ -718,8 +725,16 @@ namespace
                     geode::OpenGeodeException::TYPE::data, "[MSHInput] Line ",
                     line_number_, ": element refers to unknown node ",
                     node_tag );
-                component.nodes.push_back( node_id.value() );
+                if( absl::c_linear_search( nodes, node_id.value() ) )
+                {
+                    nb_degenerate_elements_++;
+                    return;
+                }
+                nodes.push_back( node_id.value() );
             }
+            component.types.push_back( gmsh_type );
+            component.nodes.insert(
+                component.nodes.end(), nodes.begin(), nodes.end() );
         }
 
         void read_element( const std::string& line )
@@ -1392,6 +1407,7 @@ namespace
         double version_{ 2 };
         bool has_entities_{ false };
         geode::index_t line_number_{ 0 };
+        geode::index_t nb_degenerate_elements_{ 0 };
         std::vector< geode::Point3D > nodes_;
         NodeTags node_tags_;
         std::array< absl::flat_hash_map< geode::index_t, ComponentElements >,

tests/model/test-msh.cpp

@@ -551,6 +551,22 @@ namespace
         check_unique_vertices( brep );
     }
 
+    void test_degenerate_elements()
+    {
+        write_file( "degenerate_elements_v2.msh",
+            absl::StrCat( V2_HEADER,
+                "$Nodes\n4\n1 0 0 0\n2 1 0 0\n3 0 1 0\n4 1 1 0\n"
+                "$EndNodes\n",
+                "$Elements\n3\n1 2 2 0 1 1 2 3\n2 2 2 0 1 2 4 2\n"
+                "3 1 2 0 2 4 4\n$EndElements\n" ) );
+        const auto brep = geode::load_brep( "degenerate_elements_v2.msh" );
+        geode::OpenGeodeIOModelException::test(
+            first_surface_mesh( brep ).nb_polygons() == 1,
+            "Degenerate polygon should be ignored" );
+        geode::OpenGeodeIOModelException::test( brep.nb_lines() == 0,
+            "Line with only a degenerate edge should be removed" );
+    }
+
     void test_is_loadable()
     {
         write_file( "unsupported_version.msh",
@@ -694,6 +710,7 @@ int main()
         test_empty_components();
         test_corner_with_several_nodes();
         test_unknown_bounding_entities();
+        test_degenerate_elements();
 
         geode::Logger::info( "TEST SUCCESS" );
         return 0;