Repro test case for suspicion of T-junctions in CGAL corefinement output; probably just "nearly-degenerate" faces though

repro-tjunctions-union.cc

// Copyright 2021 Google LLC.
// SPDX-License-Identifier: Apache-2.0

// Context: https://github.com/openscad/openscad/pull/4117#issuecomment-1053731672

/*
  g++ -stdlib=libc++ -std=c++1y -lgmp -lmpfr repro-tjunctions-union.cc -o repro-tju && ./repro-tju > tjunct.off
*/

#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
#include <CGAL/Polygon_mesh_processing/repair.h>
#include <CGAL/Polygon_mesh_processing/corefinement.h>
#include <CGAL/Polygon_mesh_processing/triangulate_faces.h>
#include <CGAL/Surface_mesh.h>
#include <fstream>

namespace PMP = CGAL::Polygon_mesh_processing;

int main(int argc, char *argv[])
{

  typedef CGAL::Epeck Kernel;
  typedef CGAL::Surface_mesh<CGAL::Point_3<Kernel>> Mesh;
  
  typedef boost::graph_traits<Mesh> GT;
  typedef typename GT::face_descriptor face_descriptor;
  typedef typename GT::halfedge_descriptor halfedge_descriptor;
  typedef typename GT::edge_descriptor edge_descriptor;
  typedef typename GT::vertex_descriptor vertex_descriptor;
  typedef typename CGAL::Kernel_traits<typename Mesh::Point>::Kernel Kernel;

  auto read = [&](auto&mesh, auto source) {
    std::stringstream ss;
    ss << source;
    ss >> mesh;
    assert(CGAL::is_closed(mesh));
    assert(CGAL::is_valid_halfedge_graph(mesh, /* verb */ false));
  };

  Mesh lhs, rhs;

  read(lhs, "OFF\n\
8 12 0\n\
\n\
0.517599 -0.00842033 0.990969\n\
1.51729 0.00902941 0.973517\n\
1.50015 1.00873 0.990967\n\
0.500454 0.99128 1.00842\n\
0.482702 1.00843 0.00872355\n\
1.4824 1.02588 -0.00872886\n\
1.49954 0.0261746 -0.0261786\n\
0.499848 0.00872487 -0.0087262\n\
3  3 0 1\n\
3  7 4 5\n\
3  0 7 6\n\
3  1 6 5\n\
3  2 5 4\n\
3  0 3 4\n\
3  1 2 3\n\
3  5 6 7\n\
3  6 1 0\n\
3  5 2 1\n\
3  4 3 2\n\
3  4 7 0\n\
");

  read(rhs, "OFF\n\
8 12 0\n\
\n\
0.0177517 -0.0171452 0.999695\n\
1.01745 0.00030454 0.982243\n\
1.0003 1.00001 0.999693\n\
0.000606469 0.982556 1.01715\n\
-0.0171452 0.999701 0.0174497\n\
0.98255 1.01715 -2.65809e-06\n\
0.999695 0.0174497 -0.0174524\n\
0 0 0\n\
3  2 3 0\n\
3  7 4 5\n\
3  0 7 6\n\
3  1 6 5\n\
3  3 2 5\n\
3  3 4 7\n\
3  0 1 2\n\
3  5 6 7\n\
3  6 1 0\n\
3  5 2 1\n\
3  5 4 3\n\
3  7 0 3\n\
");

  auto result = PMP::corefine_and_compute_union(lhs, rhs, lhs);
  assert(result);

  // No effect: its already triangular
  //   PMP::triangulate_faces(lhs);

  // Output closed and valid
  assert(CGAL::is_closed(lhs));
  assert(CGAL::is_valid_halfedge_graph(lhs, /* verb */ false));

  // No self-intersections
  std::vector<std::pair<face_descriptor, face_descriptor>> selfIntersectionPairs;
  PMP::self_intersections(lhs.faces(), lhs, back_inserter(selfIntersectionPairs));
  assert(selfIntersectionPairs.empty());

  // No non-manifold vertices detected
  auto addedVertexCount = PMP::duplicate_non_manifold_vertices(lhs);
  assert(addedVertexCount == 0);

  std::cout << lhs;    
}