Add BFS trace, index marks for vertices, and construct the cycle

Surface_mesh_topology/include/CGAL/Shortest_noncontractible_cycle.h

@@ -15,14 +15,62 @@ public:
   using Dart_const_handle = typename Gmap::Dart_const_handle;
   using Dart_container = std::vector<Dart_const_handle>;
   using Path = CGAL::Path_on_surface<Gmap>;
+  using Distance_type = int;
 
   Shortest_noncontractible_cycle(Gmap& gmap) :
     m_gmap(gmap) {  }
   
   Path find_cycle(Dart_const_handle root) {
     Dart_container spanning_tree, noncon_edges;
-    BFS(root, spanning_tree);
+    std::vector<Distance_type> distance_from_root;
+    std::vector<size_type> index_marks;
+    std::vector<int> trace_index;
+
+    BFS(root, spanning_tree, distance_from_root, trace_index);
     find_noncon_edges(spanning_tree, noncon_edges);
+    mark_vertices_with_indicies(spanning_tree, index_marks);
+
+    Distance_type min_distance = -1;
+    Dart_const_handle min_noncon_edge;
+    int min_a = -1, min_b = -1;
+    for (auto dh : noncon_edges) {
+      Dart_const_handle a = dh, b = m_gmap.alpha<0>(a);
+      int index_a = 0, index_b = 0;
+      for (int i = 0; i < index_marks.size(); ++i) {
+        if (m_gmap.is_marked(a, index_marks[i])) index_a ^= (1 << i);
+        if (m_gmap.is_marked(b, index_marks[i])) index_b ^= (1 << i);
+      }
+      Distance_type sum_distance = distance_from_root[index_a] + distance_from_root[index_b];
+      if (min_distance < 0 || min_distance > sum_distance) {
+        min_distance = sum_distance;
+        min_noncon_edge = dh;
+        min_a = index_a;
+        min_b = index_b;
+      }
+    }
+
+    Path cycle(m_gmap);
+    if (min_distance < 0) return cycle; // empty cycle;
+    // Trace back the path from `a` to root
+    int ind;
+    do {
+      ind = min_a;
+      cycle.push_back(m_gmap.alpha<0>(spanning_tree[ind]));
+      ind = trace_index[ind];
+    } while (ind != -1);
+    // Reverse: now it is the path from root to `a`
+    cycle.reverse();
+    cycle.push_back(min_noncon_edge);
+    // Trace back the path from `b` to root
+    do {
+      ind = min_b;
+      cycle.push_back(m_gmap.alpha<0>(spanning_tree[ind]));
+      ind = trace_index[ind];
+    } while (ind != -1);
+
+    CGAL_assertion(cycle.is_closed());
+
+    return cycle;
   }
 
 private:
@@ -39,7 +87,8 @@ private:
 
   /// Create a spanning tree using BFS
 
-  void BFS(Dart_const_handle root, Dart_container& spanning_tree) {
+  void BFS(Dart_const_handle root, Dart_container& spanning_tree,
+           std::vector<int>& distance_from_root, std::vector<int>& trace_index) {
     size_type vertex_visited;
     try {
       vertex_visited = m_gmap.get_new_mark();
@@ -47,12 +96,18 @@ private:
       std::cerr << "No more free mark, exit." << std::endl;
       exit(-1);
     }
-    std::queue<Dart_const_handle> q;
+    // The first of pair is a dart of a 0-cell,
-    q.push(root);
+    // the second of pair is the index of the dart leading to this 0-cell in spanning_tree
+    std::queue<std::pair<Dart_const_handle, int> > q;
+
+    // spanning_tree is empty (so far), so the index is -1
+    q.push(std::make_pair(root, -1));
     mark_cell<0>(root, vertex_visited);
-    spanning_tree.push_back(root);
+    distance_from_root.push_back(0);
+    // Note: distance_from_root will have n (= #0-cells) elements while spanning_tree only has n-1
     while (q.size()) {
-      Dart_const_handle u = q.front();
+      Dart_const_handle u = q.front().first;
+      int ind = q.front().second;
       q.pop();
       for (auto it = m_gmap.template one_dart_per_incident_cell<1,0>(u).begin(), 
                 itend = m_gmap.template one_dart_per_incident_cell<1,0>(u).end();
@@ -60,8 +115,15 @@ private:
         Dart_const_handle v = m_gmap.alpha<0>(it);
         if (!m_gmap.is_marked(v, vertex_visited)) {
           mark_cell<0>(v, vertex_visited);
-          q.push(v);
+          if (ind == -1) 
+            distance_from_root.push_back(1);
+          else
+            distance_from_root.push_back(1 + distance_from_root[ind+1]);
           spanning_tree.push_back(it);
+          // `it` will lead to v
+          q.push(make_pair(v, spanning_tree.size() - 1));
+          trace_index.push_back(ind);
+          // v will later be marked with the binary code of spanning_tree.size()
         }
       }
     }
@@ -127,6 +189,26 @@ private:
     m_gmap.free_mark(face_deleted);
   }
 
+  void mark_vertices_with_indicies(const Dart_container& spanning_tree, std::vector<size_type>& index_marks) {
+    int n = spanning_tree.size()+1, bit_count = 0;
+    while (n) ++bit_count, n >> 1;
+    index_marks.resize(bit_count);
+    try {
+      for (int i = 0; i < index_marks.size(); ++i)
+        index_marks[i] = m_gmap.get_new_mark();
+    } catch (typename Gmap::Exception_no_more_available_mark) {
+      std::cerr << "No more free mark, exit." << std::endl;
+      exit(-1);
+    }
+    n = 0;
+    for (auto dh : spanning_tree) {
+      ++n;
+      Dart_const_handle v = m_gmap.alpha<0>(dh);
+      for (int i = 0; i < index_marks.size(); ++i)
+        if (n & (1 << i)) mark_cell<0>(v, index_marks[i]);
+    }
+  }
+
   Gmap m_gmap;
 };