Fix (unintendedly) skipping the next point after erasure of a constraint point

In 'it = set.erase(it)', it is already the next element, so the ++it
of the loop makes the algorithm skip an element, which might have had
to be removed if it also didn't have a corresponding triangulation vertex.

Polyline_simplification_2/test/Polyline_simplification_2/CMakeLists.txt

@@ -36,8 +36,7 @@ endif()
 # Creating entries for all .cpp/.C files with "main" routine
 # ##########################################################
 
+create_single_source_cgal_program( "issue-5774.cpp" )
 create_single_source_cgal_program( "simplify_polygon_test.cpp" )
 
 create_single_source_cgal_program( "simplify_polyline_with_duplicate_points.cpp" )
-
-

Polyline_simplification_2/test/Polyline_simplification_2/issue-5774.cpp

@@ -0,0 +1,78 @@
+#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
+#include <CGAL/Polyline_simplification_2/simplify.h>
+
+#include <iostream>
+#include <string>
+#include <vector>
+
+typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
+typedef CGAL::Point_2<K> Point;
+typedef std::vector<Point> Polyline_2;
+namespace PS = CGAL::Polyline_simplification_2;
+typedef PS::Vertex_base_2<K> Vb;
+typedef CGAL::Constrained_triangulation_face_base_2<K> Fb;
+typedef CGAL::Triangulation_data_structure_2<Vb, Fb> TDS;
+typedef CGAL::Constrained_Delaunay_triangulation_2<K, TDS, CGAL::Exact_predicates_tag> CDT;
+typedef CGAL::Constrained_triangulation_plus_2<CDT> CT;
+typedef PS::Stop_below_count_ratio_threshold Stop;
+typedef PS::Squared_distance_cost Cost;
+
+void print_coords(Polyline_2 polyline) {
+  std::cout << std::endl;
+  std::cout << "Simplified coordinates:" << std::endl << std::endl;
+
+  // Print out the simplified coordinates
+  unsigned int i = 0;
+  for (Point coord : polyline) {
+    std::cout << "[ ";
+    std::cout << coord.x();
+    std::cout << ", ";
+    std::cout << coord.y();
+    std::cout << " ]";
+    if (i != polyline.size() - 1) std::cout << ",";
+    i++;
+  }
+  std::cout << std::endl << std::endl;
+}
+
+void simplify_test() {
+  // Hard code a minimum working example where running PS::simplify results in
+  // self-intersections. There are no self-intersections when {27, 9} is
+  // omitted.
+  std::vector<std::vector<int>> coords = {
+      {64, 20}, {33, 27}, {27, 9}, {33, 18}, {44, 18}, {44, 8},
+      {24, 0},  {0, 13},  {9, 49}, {84, 41}, {83, 29}, {64, 20},
+  };
+  // Simplification outputs:
+  // [ 64, 20 ],[ 27, 9 ],[ 44, 18 ],[ 24, 0 ],
+  // [ 9, 49 ],[ 83, 29 ],[ 64, 20 ],[ 64, 20 ]
+
+  // Create polyline for simplifying later
+  Polyline_2 polyline;
+
+  // Insert coordinates into polyline
+  for (std::vector<int> coord : coords) {
+    Point pt(coord[0], coord[1]);
+    polyline.push_back(pt);
+  }
+
+  // Insert polyline into ct and run simplify()
+  CT ct;
+  ct.insert_constraint(polyline.begin(), polyline.end());
+  PS::simplify(ct, Cost(), Stop(0.2));
+  Polyline_2 polyline_simplified;
+
+  // Transfer simplified coordinates from ct to polyline for easy handling
+  auto cit = ct.constraints_begin();
+  for (auto vit = ct.points_in_constraint_begin(*cit);
+       vit != ct.points_in_constraint_end(*cit); vit++) {
+    polyline_simplified.push_back(*vit);
+  }
+
+  print_coords(polyline_simplified);
+  assert(polyline_simplified.size() == 4);
+}
+
+int main() {
+  simplify_test();
+}

Triangulation_2/include/CGAL/Triangulation_2/internal/Polyline_constraint_hierarchy_2.h

@@ -46,12 +46,13 @@ private:
   class Node {
   public:
     explicit Node(Vertex_handle vh, bool input = false)
-      : vertex_(vh), id(-1), input(input)
+      : vertex_(vh), point_(vertex_->point()), id(-1), input(input)
     {}
-    const Point& point() const { return vertex_->point(); }
+    const Point& point() const { return point_; }
     Vertex_handle vertex() const { return vertex_; }
   private:
     Vertex_handle vertex_;
+    Point point_;
   public:
     int id;
     bool input;
@@ -70,7 +71,7 @@ public:
     >
   {
   public:
-    Point_it() : Vertex_it::iterator_adaptor_() {}
+    Point_it() : Point_it::iterator_adaptor_() {}
     Point_it(typename Vertex_list::all_iterator it) : Point_it::iterator_adaptor_(it) {}
   private:
     friend class boost::iterator_core_access;
@@ -633,10 +634,12 @@ Polyline_constraint_hierarchy_2<T,Compare,Point>::remove_points_without_correspo
 {
   std::size_t n = 0;
   for(Point_it it = points_in_constraint_begin(cid);
-      it != points_in_constraint_end(cid); ++it) {
+      it != points_in_constraint_end(cid);) {
     if(cid.vl_ptr()->is_skipped(it.base())) {
       it = cid.vl_ptr()->erase(it.base());
       ++n;
+    }else{
+      ++it;
     }
   }
   return n;