From f454510de7ecffd74a720b03b9ad1076cab167e0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Loriot?= <sebastien.loriot@cgal.org>
Date: Wed, 11 Mar 2020 10:33:40 +0100
Subject: [PATCH] if a vertex is on a border, its halfedge is not necessarily
 on the border

---
 .../CGAL/boost/graph/Euler_operations.h       | 38 ++++++++++++------
 BGL/test/BGL/test_Euler_operations.cpp        | 39 +++++++++++++++++++
 2 files changed, 66 insertions(+), 11 deletions(-)

diff --git a/BGL/include/CGAL/boost/graph/Euler_operations.h b/BGL/include/CGAL/boost/graph/Euler_operations.h
index 11dd34b1922..2e4656b691c 100644
--- a/BGL/include/CGAL/boost/graph/Euler_operations.h
+++ b/BGL/include/CGAL/boost/graph/Euler_operations.h
@@ -711,19 +711,35 @@ add_face(const VertexRange& vr, Graph& g)
         break;
 
       case 3: // both are new
-        if (halfedge(v, g) == boost::graph_traits<Graph>::null_halfedge())
         {
-          set_halfedge(v, outer_prev, g);
-          next_cache.push_back(NextCacheEntry(outer_prev, outer_next));
+          // try to pick a border halfedge with v as target
+          halfedge_descriptor hv = halfedge(v, g);
+          if (hv != boost::graph_traits<Graph>::null_halfedge() && !is_border(hv, g))
+          {
+            BOOST_FOREACH(halfedge_descriptor h_around_v, halfedges_around_target(hv, g))
+              if (is_border(h_around_v, g))
+              {
+                hv = h_around_v;
+                break;
+              }
+            if (!is_border(hv, g))
+              hv = boost::graph_traits<Graph>::null_halfedge();
+          }
+
+          if (hv == boost::graph_traits<Graph>::null_halfedge())
+          {
+            set_halfedge(v, outer_prev, g);
+            next_cache.push_back(NextCacheEntry(outer_prev, outer_next));
+          }
+          else
+          {
+            border_prev = hv;
+            border_next = next(border_prev, g);
+            next_cache.push_back(NextCacheEntry(border_prev, outer_next));
+            next_cache.push_back(NextCacheEntry(outer_prev, border_next));
+          }
+          break;
         }
-        else
-        {
-          border_prev = halfedge(v, g);
-          border_next = next(border_prev, g);
-          next_cache.push_back(NextCacheEntry(border_prev, outer_next));
-          next_cache.push_back(NextCacheEntry(outer_prev, border_next));
-        }
-        break;
       }
 
       // set inner link
diff --git a/BGL/test/BGL/test_Euler_operations.cpp b/BGL/test/BGL/test_Euler_operations.cpp
index 3ae73d6c38d..3f394ed85c0 100644
--- a/BGL/test/BGL/test_Euler_operations.cpp
+++ b/BGL/test/BGL/test_Euler_operations.cpp
@@ -402,6 +402,44 @@ test_swap_edges()
   }
 }
 
+template <typename T>
+void
+add_face_bug()
+{
+  typedef boost::graph_traits<T> GT;
+  typedef typename GT::vertex_descriptor vertex_descriptor;
+  typedef typename GT::halfedge_descriptor halfedge_descriptor;
+
+  T g;
+
+  std::vector<vertex_descriptor> vs;
+  vs.push_back( add_vertex(g) ); // Kernel::Point_3(0,1,0)
+  vs.push_back( add_vertex(g) ); // Kernel::Point_3(4,1,0)
+  vs.push_back( add_vertex(g) ); // Kernel::Point_3(5,2,0)
+  vs.push_back( add_vertex(g) ); // Kernel::Point_3(4,0,0)
+
+  CGAL::Euler::add_face(CGAL::make_array(vs[0], vs[1], vs[2]), g);
+  CGAL::Euler::add_face(CGAL::make_array(vs[1], vs[3], vs[2]), g);
+
+  // force vertex halfedge to not be a border halfedge
+  BOOST_FOREACH(vertex_descriptor v, vertices(g))
+  {
+    halfedge_descriptor h = halfedge(v, g);
+    if ( CGAL::is_border(h, g) )
+      set_halfedge(v, prev(opposite(h, g), g), g);
+    assert(target(halfedge(v, g), g)==v);
+  }
+
+  vs.push_back( add_vertex(g) ); // Kernel::Point_3(0,0,0)
+  vs.push_back( add_vertex(g) );  // Kernel::Point_3(1,0,0)
+  CGAL::Euler::add_face(CGAL::make_array(vs[4],vs[5],vs[0]), g);
+
+  vs.push_back( add_vertex(g) ); // Kernel::Point_3(2,0,0)
+  vs.push_back( add_vertex(g) ); // Kernel::Point_3(3,0,0)
+  CGAL::Euler::add_face(CGAL::make_array(vs[6],vs[7],vs[1]), g);
+  CGAL::Euler::add_face(CGAL::make_array(vs[7],vs[3],vs[1]), g);
+}
+
 template <typename Graph>
 void
 test_Euler_operations()
@@ -421,6 +459,7 @@ test_Euler_operations()
   join_split_inverse<Graph>();
   does_satisfy_link_condition<Graph>();
   test_swap_edges<Graph>();
+  add_face_bug<Graph>();
 }
 
 int main()