Fix crash from perimeter_loop one-point loop

supermerill/SuperSlicer#1623
Fixme: take the first object from this issue project
  and you can see some bad lines at layer 31.

src/libslic3r/ExtrusionEntity.cpp

@@ -388,6 +388,28 @@ void ExtrusionLength::use(const ExtrusionEntityCollection& collection) {
     }
 }
 
+
+void ExtrusionVisitorRecursiveConst::use(const ExtrusionMultiPath& multipath) {
+    for (const ExtrusionPath& path: multipath.paths) {
+        path.visit(*this);
+    }
+}
+void ExtrusionVisitorRecursiveConst::use(const ExtrusionMultiPath3D& multipath3D) {
+    for (const ExtrusionPath3D& path3D : multipath3D.paths) {
+        path3D.visit(*this);
+    }
+}
+void ExtrusionVisitorRecursiveConst::use(const ExtrusionLoop& loop) {
+    for (const ExtrusionPath& path : loop.paths) {
+        path.visit(*this);
+    }
+}
+void ExtrusionVisitorRecursiveConst::use(const ExtrusionEntityCollection& collection) {
+    for (const ExtrusionEntity* entity : collection.entities) {
+        entity->visit(*this);
+    }
+}
+
 //class ExtrusionTreeVisitor : ExtrusionVisitor {
 //public:
 //    //virtual void use(ExtrusionEntity &entity) { assert(false); };

src/libslic3r/ExtrusionEntity.hpp

@@ -606,6 +606,14 @@ public:
     }
 };
 
+class ExtrusionVisitorRecursiveConst : public ExtrusionVisitorConst {
+public:
+    virtual void use(const ExtrusionMultiPath& multipath) override;
+    virtual void use(const ExtrusionMultiPath3D& multipath) override;
+    virtual void use(const ExtrusionLoop& loop) override;
+    virtual void use(const ExtrusionEntityCollection& collection) override;
+};
+
 }
 
 #endif

src/libslic3r/PerimeterGenerator.cpp

@@ -1852,6 +1852,8 @@ PerimeterGenerator::_extrude_and_cut_loop(const PerimeterGeneratorLoop &loop, co
                 for (ExtrusionPath &path : paths) {
                     direction_polyline.points.insert(direction_polyline.points.end(), path.polyline.points.begin(), path.polyline.points.end());
                 }
+                for (int i = 0; i < direction_polyline.points.size() - 1; i++)
+                    assert(direction_polyline.points[i] != direction_polyline.points[i + 1]);
                 direction_polyline.clip_start(SCALED_RESOLUTION);
                 direction_polyline.clip_end(SCALED_RESOLUTION);
                 coord_t dot = direction.dot(Line(direction_polyline.points.back(), direction_polyline.points.front()));
@@ -1950,11 +1952,12 @@ PerimeterGenerator::_traverse_and_join_loops(const PerimeterGeneratorLoop &loop,
             //create new node with recursive ask for the inner perimeter & COPY of the points, ready to be cut
             my_loop.paths.insert(my_loop.paths.begin() + nearest.idx_polyline_outter + 1, my_loop.paths[nearest.idx_polyline_outter]);
 
+            // outer_start == outer_end
             ExtrusionPath *outer_start = &my_loop.paths[nearest.idx_polyline_outter];
             ExtrusionPath *outer_end = &my_loop.paths[nearest.idx_polyline_outter + 1];
             Line deletedSection;
 
-            //cut our polyline
+            //cut our polyline, so outer_start has no common point with outer_end
             //separate them
             size_t nearest_idx_outter = outer_start->polyline.closest_point_index(nearest.outter_best);
             if (outer_start->polyline.points[nearest_idx_outter].coincides_with_epsilon(nearest.outter_best)) {
@@ -2195,6 +2198,16 @@ PerimeterGenerator::_traverse_and_join_loops(const PerimeterGeneratorLoop &loop,
                 my_loop.paths.insert(my_loop.paths.begin() + nearest.idx_polyline_outter + 1, travel_path_begin[i]);
             }
         }
+        //remove one-point extrusion
+        //FIXME prevent this instead of patching here?
+        for (int i = 0; i < my_loop.paths.size(); i++) {
+            if (my_loop.paths[i].polyline.size() < 2) {
+                if (my_loop.paths[i].polyline.size() == 1)
+                    std::cout << "erase one-point extrusion : layer " << this->layer->id() << " " << my_loop.paths[i].polyline.points.front().x() << ":" << my_loop.paths[i].polyline.points.front().y() << "\n";
+                my_loop.paths.erase(my_loop.paths.begin() + i);
+                i--;
+            }
+        }
 
         //update for next loop
         childs.erase(childs.begin() + nearest.idx_children);