From 2ec7520d9677cd312dbfafd07d4afc1b7962e38e Mon Sep 17 00:00:00 2001
From: Andreas Fabri <andreas.fabri@geometryfactory.com>
Date: Tue, 5 Aug 2025 15:36:01 +0100
Subject: [PATCH] BGL: Add Face_around_face_circulator

---
 BGL/include/CGAL/boost/graph/iterator.h | 49 ++++++++++++++++++++++++-
 BGL/test/BGL/test_circulator.cpp        | 10 +++++
 2 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/BGL/include/CGAL/boost/graph/iterator.h b/BGL/include/CGAL/boost/graph/iterator.h
index cef4a913cad..e26d7892cbd 100644
--- a/BGL/include/CGAL/boost/graph/iterator.h
+++ b/BGL/include/CGAL/boost/graph/iterator.h
@@ -871,7 +871,54 @@ private:
  */
 template <typename Graph>
 class Face_around_face_circulator
-{};
+#ifndef DOXYGEN_RUNNING
+  : public boost::iterator_adaptor<
+             Face_around_face_circulator<Graph>                    // Derived
+             , Halfedge_around_face_circulator<Graph>                  // Base
+             , typename boost::graph_traits<Graph>::face_descriptor  // Value
+             , Bidirectional_circulator_tag                              // CategoryOrTraversal
+             , typename boost::graph_traits<Graph>::face_descriptor  // Reference
+             >
+#endif
+{
+  internal::Opposite_face<Graph> fct;
+  typedef typename boost::graph_traits<Graph>::halfedge_descriptor halfedge_descriptor;
+
+public:
+#ifndef DOXYGEN_RUNNING
+  typedef std::size_t size_type;
+#endif
+
+  Face_around_face_circulator()
+  {}
+
+  Face_around_face_circulator(halfedge_descriptor h, const Graph& g)
+    : Face_around_face_circulator::iterator_adaptor_(Halfedge_around_face_circulator<Graph>(h,g)), fct(g)
+  {}
+
+#ifndef DOXYGEN_RUNNING
+
+  explicit operator bool() const
+  {
+    return (! (this->base_reference() == nullptr));
+  }
+
+  bool operator== (void*) const
+  {
+    return this->base_reference()== nullptr;
+  }
+
+  bool operator!= (void*) const
+  {
+    return this->base_reference()!= nullptr;
+  }
+
+private:
+  friend class boost::iterator_core_access;
+  typename  boost::graph_traits<Graph>::face_descriptor dereference() const { return fct(*this->base_reference()); }
+#endif
+
+};
 
 /**
  * \ingroup PkgBGLIterators
diff --git a/BGL/test/BGL/test_circulator.cpp b/BGL/test/BGL/test_circulator.cpp
index cd0a18cbbe4..58c56bbd6df 100644
--- a/BGL/test/BGL/test_circulator.cpp
+++ b/BGL/test/BGL/test_circulator.cpp
@@ -18,6 +18,7 @@ typedef GraphTraits::edge_descriptor edge_descriptor;
 typedef GraphTraits::out_edge_iterator out_edge_iterator;
 typedef GraphTraits::in_edge_iterator in_edge_iterator;
 
+typedef CGAL::Face_around_face_circulator<Polyhedron> face_around_face_circulator;
 typedef CGAL::Halfedge_around_face_circulator<Polyhedron> halfedge_around_face_circulator;
 typedef CGAL::Halfedge_around_target_circulator<Polyhedron> halfedge_around_target_circulator;
 typedef CGAL::Vertex_around_target_circulator<Polyhedron> vertex_around_target_circulator;
@@ -32,6 +33,7 @@ typedef CGAL::Vertex_around_target_iterator<Polyhedron> vertex_around_target_ite
 int main(int argc, char* argv[])
 {
 
+  BOOST_CONCEPT_ASSERT((CGAL::Concepts::BidirectionalCirculator<face_around_face_circulator>));
   BOOST_CONCEPT_ASSERT((CGAL::Concepts::BidirectionalCirculator<halfedge_around_face_circulator>));
   BOOST_CONCEPT_ASSERT((CGAL::Concepts::BidirectionalCirculator<halfedge_around_target_circulator>));
   BOOST_CONCEPT_ASSERT((CGAL::Concepts::BidirectionalCirculator<vertex_around_target_circulator>));
@@ -62,6 +64,14 @@ int main(int argc, char* argv[])
     }while(hafc != done);
   }
 
+ {
+    face_around_face_circulator fafc(hd,P), done(fafc);
+
+    do {
+      ++fafc;
+    }while(fafc != done);
+  }
+
   {
     halfedge_around_target_circulator havc(hd,P), done(havc);
     vertex_descriptor vd = target(hd,P);