Add a function to create a LCC from a tet soup

Installation/CHANGES.md

@@ -4,6 +4,9 @@
 
 Release date: December 2026
 
+### [Linear Cell Complex](https://doc.cgal.org/6.3/Manual/packages.html#PkgLinearCellComplex)
+- added `tetrahedron_soup_to_lcc()` to import a tetrahedron soup into a linear cell complex.
+
 ### [2D and 3D Fast Intersection and Distance Computation (AABB Tree)](https://doc.cgal.org/6.3/Manual/packages.html#PkgAABBTree)
 - `CGAL::AABB_tree::build()` now accepts an optional `Concurrency_tag` template parameter (`CGAL::Sequential_tag` by default).
    When `CGAL::Parallel_tag` is specified, the tree construction is performed in parallel.

Linear_cell_complex/doc/Linear_cell_complex/Doxyfile.in

@@ -1,3 +1,4 @@
 @INCLUDE = ${CGAL_DOC_PACKAGE_DEFAULTS}
 
 PROJECT_NAME = "CGAL ${CGAL_DOC_VERSION} - Linear Cell Complex"
+INPUT += ${CGAL_PACKAGE_INCLUDE_DIR}/CGAL/tetrahedron_soup_to_lcc.h

Linear_cell_complex/doc/Linear_cell_complex/PackageDescription.txt

@@ -73,6 +73,7 @@
 - `CGAL::read_plane_graph_in_lcc<LCC>` (formerly `import_from_plane_graph`)
 - `CGAL::triangulation_3_to_lcc<LCC,Triangulation>` (formerly `import_from_triangulation_3`)
 - `CGAL::polyhedron_3_to_lcc<LCC,Polyhedron>` (formerly `import_from_polyhedron_3`)
+- `CGAL::tetrahedron_soup_to_lcc()`
 
 \cgalCRPSubsection{Operations for Linear Cell Complex}
 - `CGAL::compute_normal_of_cell_0<LCC>`

Linear_cell_complex/examples/Linear_cell_complex/CMakeLists.txt

@@ -28,10 +28,12 @@ create_single_source_cgal_program("read_plane_graph_in_lcc_2.cpp")
 create_single_source_cgal_program("voronoi_2.cpp")
 create_single_source_cgal_program("voronoi_3.cpp")
 create_single_source_cgal_program("linear_cell_complex_3_vtk_io.cpp")
+create_single_source_cgal_program("tetsoup_to_lcc.cpp")
 
 create_single_source_cgal_program("draw_linear_cell_complex.cpp")
 if(CGAL_Qt6_FOUND)
   target_link_libraries(draw_linear_cell_complex PRIVATE CGAL::CGAL_Basic_viewer)
   target_link_libraries(linear_cell_complex_3_incremental_builder PRIVATE CGAL::CGAL_Basic_viewer)
   target_link_libraries(linear_cell_complex_3_insert PRIVATE CGAL::CGAL_Basic_viewer)
+  target_link_libraries(tetsoup_to_lcc PRIVATE CGAL::CGAL_Basic_viewer)
 endif()

Linear_cell_complex/examples/Linear_cell_complex/tetsoup_to_lcc.cpp

@@ -0,0 +1,38 @@
+#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
+#include <CGAL/Linear_cell_complex_for_combinatorial_map.h>
+#include <CGAL/tetrahedron_soup_to_lcc.h>
+#include <CGAL/draw_linear_cell_complex.h>
+#include <array>
+
+#include <CGAL/IO/MEDIT.h>
+
+using LCC = CGAL::Linear_cell_complex_for_combinatorial_map<3, 3>;
+using Point_3 = CGAL::Exact_predicates_inexact_constructions_kernel::Point_3;
+
+int main(int argc, char** argv)
+{
+  const std::string filename = (argc > 1) ? argv[1] : CGAL::data_file_path("meshes/elephant.mesh");
+
+  std::vector<Point_3> points;
+  std::vector<std::array<std::size_t,4>> tetras;
+  std::vector<int> subdomains;
+  std::ifstream in(filename);
+  if(!in)
+  {
+    std::cerr << "Cannot open " << filename << std::endl;
+    return 1;
+  }
+
+  CGAL::IO::read_MEDIT(in, points, tetras, subdomains);
+
+  //filter out subdomain 0
+  std::vector<std::array<std::size_t,4>> filtered_tetras;
+  for (std::size_t i=0; i<tetras.size(); ++i)
+    if (subdomains[i]!=0)
+      filtered_tetras.push_back(tetras[i]);
+
+  LCC lcc;
+  CGAL::tetrahedron_soup_to_lcc(points, filtered_tetras, lcc);
+
+  CGAL::draw(lcc);
+}

Linear_cell_complex/include/CGAL/tetrahedron_soup_to_lcc.h

@@ -0,0 +1,125 @@
+// Copyright (c) 2026 CNRS and LIRIS' Establishments (France).
+// All rights reserved.
+//
+// This file is part of CGAL (www.cgal.org)
+//
+// $URL$
+// $Id$
+// SPDX-License-Identifier: LGPL-3.0-or-later OR LicenseRef-Commercial
+//
+// Author(s)     : Guillaume Damiand <guillaume.damiand@liris.cnrs.fr>
+//                 Sebastien Loriot <sebastien.loriot@cgal.org>
+//
+
+#ifndef CGAL_TETRAHEDRON_SOUP_TO_LCC_H
+#define CGAL_TETRAHEDRON_SOUP_TO_LCC_H
+
+#include <boost/container_hash/hash.hpp>
+#include <vector>
+#include <array>
+#include <unordered_map>
+#include <algorithm>
+
+namespace CGAL
+{
+/**
+ * @ingroup PkgLinearCellComplexConstructions
+ *
+ * \brief imports a 3D tetrahedron soup into a linear cell complex.
+ *
+ * Creates a 3D linear cell complex in `lcc` from a set of points and a range
+ * of tetrahedra defined by indices into `points`. The topological 3-combinatorial
+ * map is constructed by sewing adjacent 3-volumes (tetrahedra) along matching 2-faces.
+ *
+ * @tparam LCC a model of the `LinearCellComplex` concept.
+ * @tparam PointRange a model of `RandomAccessContainer` with `Point_3` as value type, being compatible with the point type of LCC.
+ * @tparam TetrahedronRange a model of `ConstRange` where each element is a container of 4 point indices, accessible using `operator[](int)`.
+ *
+ * @param points range of 3D points.
+ * @param tetrahedra range of tetrahedra, where each tetrahedron is represented by 4 indices corresponding to entries in `points`.
+ * @param lcc the target linear cell complex.
+ *
+ * @return a descriptor to a dart of the created complex, or `LCC::null_descriptor` if the input is empty.
+ *
+ * @pre `LCC::dimension >= 3` and `LCC::ambient_dimension == 3`.
+ * @pre Indices in `tetrahedra` must be valid 0-based indices into `points`.
+ *
+ * @sa `CGAL::triangulation_3_to_lcc()`
+ */
+  template <class LCC, class PointRange, class TetrahedronRange>
+  typename LCC::Dart_descriptor
+  tetrahedron_soup_to_lcc(const PointRange& points,
+                          const TetrahedronRange& tetrahedra,
+                          LCC& lcc)
+  {
+    static_assert( LCC::dimension>=3 && LCC::ambient_dimension==3 );
+
+    if (points.empty() || tetrahedra.empty())
+      return LCC::null_descriptor;
+
+    using Dart_descriptor = typename LCC::Dart_descriptor;
+
+    std::vector<typename LCC::Vertex_attribute_descriptor> vertices;
+    vertices.reserve(points.size());
+    for(const auto& pt : points)
+      vertices.push_back(lcc.create_vertex_attribute(pt));
+
+    struct Array_hasher
+    {
+      std::size_t operator()(const std::array<std::size_t, 3>& a) const
+      {
+        std::size_t seed = 0;
+        boost::hash_combine(seed, a[0]);
+        boost::hash_combine(seed, a[1]);
+        boost::hash_combine(seed, a[2]);
+
+        return seed;
+      }
+    };
+
+    std::unordered_map< std::array<std::size_t, 3>, Dart_descriptor, Array_hasher> face_map;
+
+    Dart_descriptor dart = LCC::null_descriptor;
+    for (const auto& t : tetrahedra)
+    {
+      Dart_descriptor res = lcc.make_tetrahedron(vertices[t[0]],
+                                                 vertices[t[1]],
+                                                 vertices[t[2]],
+                                                 vertices[t[3]]);
+
+      if (dart==LCC::null_descriptor) dart = res;
+
+      for (int i=0; i<4; ++i)
+      {
+        Dart_descriptor curr= LCC::null_descriptor;
+        switch (i)
+        {
+          case 0: curr = lcc.template opposite<2>(lcc.next(res)); break;
+          case 1: curr = lcc.template opposite<2>(lcc.previous(res)); break;
+          case 2: curr = lcc.template opposite<2>(res); break;
+          default: curr = res; break;
+        }
+
+        std::array<std::size_t, 3> f{t[(i+1)%4],t[(i+2)%4],t[(i+3)%4]};
+        std::sort(f.begin(), f.end());
+        auto insert_res = face_map.emplace(f, curr);
+        if (!insert_res.second)
+        {
+          Dart_descriptor curr2=insert_res.first->second;
+          while (lcc.vertex_attribute(curr2) !=
+                 lcc.vertex_attribute(lcc.other_extremity(curr)) )
+          {
+            curr2 = lcc.next(curr2);
+          }
+          lcc.template topo_sew<3>(curr, lcc.other_orientation(curr2));
+        }
+      }
+    }
+
+    CGAL_assertion(dart!=LCC::null_descriptor);
+    return dart;
+  }
+} // end of CGAL namespace
+
+
+#endif // CGAL_TETRAHEDRON_SOUP_TO_LCC_H

Stream_support/include/CGAL/IO/MEDIT.h

@@ -46,7 +46,7 @@ bool read_MEDIT(std::istream& is,
   using FT = typename Kernel_traits<Point_3>::Kernel::FT;
   using Surface_patch_index = SurfacePatchIndex_;
   using Facet        = std::array<int, 3>;
-  using Tet_with_ref = std::array<int, 4>;
+  using Tet_with_ref = typename std::iterator_traits<typename CellRange::const_iterator>::value_type;
 
   if(!is)
     return false;