Added halfedge_merge functionality.

src/polymesh/impl/impl_low_level_api_mutable.hh

@@ -640,6 +640,55 @@ inline void low_level_api_mutable::halfedge_attach(halfedge_index h, vertex_inde
     connect_prev_next(h1, h_next);
 }
 
+inline void low_level_api_mutable::halfedge_merge(halfedge_index h) const{
+    auto v_center = from_vertex_of(h);
+
+    assert(m.handle_of(v_center).adjacent_vertices().size() == 2 && "vertex_from must have valence 2");
+
+    //   |                                   |
+    //   |      h_prev               h       |
+    // v_from --------> v_center ---------> v_to
+    //   |                                   |
+    //   |                                   |
+
+    auto h_prev = prev_halfedge_of(h);
+    auto h_prev_opp = opposite(h_prev);
+    auto h_opp = opposite(h);
+    auto h_prev_prev = prev_halfedge_of(h_prev);
+    auto h_prev_opp_next = next_halfedge_of(h_prev_opp);
+
+    auto f_a = face_of(h);
+    auto f_b = face_of(h_opp);
+
+    //auto v_to = to_vertex_of(h);
+    auto v_from = from_vertex_of(h_prev);
+
+    // Set from vertex
+    to_vertex_of(h_opp) = v_from;
+    // Equivalent to
+    // from_vertex_of(h) = v_from;
+
+    connect_prev_next(h_prev_prev, h);
+    connect_prev_next(h_opp, h_prev_opp_next);
+
+    // fix vertex
+    if(outgoing_halfedge_of(v_from) == h_prev){
+        outgoing_halfedge_of(v_from) = h;
+    }
+
+    // fix adjacent faces
+    if(f_a.is_valid() && halfedge_of(f_a) == h_prev){
+        halfedge_of(f_a) = h;
+    }
+    if(f_b.is_valid() && halfedge_of(f_b) == h_prev_opp){
+        halfedge_of(f_b) = h_opp;
+    }
+
+    // remove
+    set_removed(edge_of(h_prev));
+    set_removed(v_center);
+}
+
 inline void low_level_api_mutable::vertex_collapse(vertex_index v) const
 {
     // isolated vertices are just removed

src/polymesh/impl/impl_ranges.hh

@@ -800,6 +800,12 @@ void halfedge_collection<iterator>::attach(halfedge_handle h, vertex_handle v) c
     low_level_api(this->mesh).halfedge_attach(h.idx, v.idx);
 }
 
+template <class iterator>
+void halfedge_collection<iterator>::merge(halfedge_handle h) const
+{
+    low_level_api(this->mesh).halfedge_merge(h.idx);
+}
+
 template <class iterator>
 void halfedge_collection<iterator>::rotate_next(halfedge_handle h) const
 {

src/polymesh/low_level_api.hh

@@ -247,6 +247,10 @@ public:
     /// attaches a given vertex to the to-vertex of a given half-edge
     void halfedge_attach(halfedge_index h, vertex_index v) const;
 
+    /// merges the given and the previous halfedge.
+    /// The center vertex must have valence 2
+    void halfedge_merge(halfedge_index h) const;
+
     /// collapse a vertex
     void vertex_collapse(vertex_index v) const;
     /// collapse a half-edge

src/polymesh/ranges.hh

@@ -329,6 +329,10 @@ struct halfedge_collection : smart_collection<Mesh*, halfedge_tag, iterator>
     /// Given an isolated vertex v, inserts a self-adjacent edge at the to-vertex to v
     void attach(halfedge_handle h, vertex_handle v) const;
 
+    /// Merges the vertex_from into the vertex_to of this halfedge.
+    /// From_vertex must have valence 2
+    void merge(halfedge_handle h) const;
+
     /// Moves the to-vertex of this half-edge to the same as the next half-edge
     /// Preserves all attributes
     /// NOTE: does not work on boundaries!