improve the check for collapsing edges to avoid to introduce non-manifolds into a mesh

src/Mod/Mesh/App/Core/Algorithm.cpp

@@ -1746,6 +1746,24 @@ std::set<unsigned long> MeshRefPointToFacets::NeighbourPoints(const std::vector<
     return nb;
 }
 
+std::set<unsigned long> MeshRefPointToFacets::NeighbourPoints(unsigned long pos) const
+{
+    std::set<unsigned long> p;
+    const std::set<unsigned long>& vf = _map[pos];
+    for (std::set<unsigned long>::const_iterator it = vf.begin(); it != vf.end(); ++it) {
+        unsigned long p1, p2, p3;
+        _rclMesh.GetFacetPoints(*it, p1, p2, p3);
+        if (p1 != pos)
+            p.insert(p1);
+        if (p2 != pos)
+            p.insert(p2);
+        if (p3 != pos)
+            p.insert(p3);
+    }
+
+    return p;
+}
+
 void MeshRefPointToFacets::Neighbours (unsigned long ulFacetInd, float fMaxDist, MeshCollector& collect) const
 {
     std::set<unsigned long> visited;

src/Mod/Mesh/App/Core/Algorithm.h

@@ -375,6 +375,7 @@ public:
     const std::set<unsigned long>& operator[] (unsigned long) const;
     MeshFacetArray::_TConstIterator GetFacet (unsigned long) const;
     std::set<unsigned long> NeighbourPoints(const std::vector<unsigned long>& , int level) const;
+    std::set<unsigned long> NeighbourPoints(unsigned long) const;
     void Neighbours (unsigned long ulFacetInd, float fMaxDist, MeshCollector& collect) const;
     Base::Vector3f GetNormal(unsigned long) const;
     void AddNeighbour(unsigned long, unsigned long);

src/Mod/Mesh/App/Core/Degeneration.cpp

@@ -576,6 +576,13 @@ bool MeshRemoveNeedles::Fixup()
             vf.erase(neighbour);
         ce._changeFacets.insert(ce._changeFacets.begin(), vf.begin(), vf.end());
 
+        // get adjacent points
+        std::set<unsigned long> vv;
+        vv = vf_it.NeighbourPoints(ce._fromPoint);
+        ce._adjacentFrom.insert(ce._adjacentFrom.begin(), vv.begin(),vv.end());
+        vv = vf_it.NeighbourPoints(ce._toPoint);
+        ce._adjacentTo.insert(ce._adjacentTo.begin(), vv.begin(),vv.end());
+
         if (topAlg.IsCollapseEdgeLegal(ce)) {
             topAlg.CollapseEdge(ce);
             for (auto it : ce._removeFacets) {

src/Mod/Mesh/App/Core/Elements.h

@@ -75,6 +75,8 @@ struct MeshExport EdgeCollapse
 {
   unsigned long _fromPoint;
   unsigned long _toPoint;
+  std::vector<unsigned long> _adjacentFrom; // adjacent points to _fromPoint
+  std::vector<unsigned long> _adjacentTo;   // adjacent points to _toPoint
   std::vector<unsigned long> _removeFacets;
   std::vector<unsigned long> _changeFacets;
 };

src/Mod/Mesh/App/Core/TopoAlgorithm.cpp

@@ -955,6 +955,18 @@ bool MeshTopoAlgorithm::CollapseEdge(unsigned long ulFacetPos, unsigned long ulN
 
 bool MeshTopoAlgorithm::IsCollapseEdgeLegal(const EdgeCollapse& ec) const
 {
+    // http://stackoverflow.com/a/27049418/148668
+    // Check connectivity
+    //
+    std::vector<unsigned long> commonPoints;
+    std::set_intersection(ec._adjacentFrom.begin(), ec._adjacentFrom.end(),
+                          ec._adjacentTo.begin(), ec._adjacentTo.end(),
+                          std::back_insert_iterator<std::vector<unsigned long> >(commonPoints));
+    if (commonPoints.size() > 2) {
+        return false;
+    }
+
+    // Check geometry
     std::vector<unsigned long>::const_iterator it;
     for (it = ec._changeFacets.begin(); it != ec._changeFacets.end(); ++it) {
         MeshFacet f = _rclMesh._aclFacetArray[*it];