Custom traversal example

Orthtree/doc/Orthtree/Orthtree.txt

@@ -148,17 +148,14 @@ In this case, we print out the nodes of the tree without indentation instead.
 
 \cgalExample{Orthtree/Octree_traversal_preorder.cpp}
 
-<!--
-\subsection Section_Orthtree_Custom_Traversal Using a Custom Traversal Method
+\subsection Section_Orthtree_Custom_Traversal Custom Traversal
 
-\b Example
-
-e.g. I've build a tree and I want to print it out, but layer by layer
+Users can define their own traversal methods by creating models of the
+`Traversal` concept. The following example shows how to define a
+custom traversal that only traverses the first branch of the %Octree:
 
 \cgalExample{Orthtree/Octree_traversal_custom.cpp}
 
-!-->
-
 \section Section_Orthtree_Acceleration Acceleration of Common Tasks
 
 Once an %Orthtree is built, its structure can be used to accelerate different tasks.

Orthtree/examples/Orthtree/Octree_traversal_custom.cpp

@@ -1,3 +1,60 @@
-// TODO
+#include <fstream>
+#include <iostream>
 
-int main() { return 0; }
+#include <CGAL/Simple_cartesian.h>
+#include <CGAL/Octree.h>
+#include <CGAL/Point_set_3.h>
+#include <CGAL/Point_set_3/IO.h>
+
+// Type Declarations
+typedef CGAL::Simple_cartesian<double> Kernel;
+typedef Kernel::Point_3 Point;
+typedef CGAL::Point_set_3<Point> Point_set;
+typedef Point_set::Point_map Point_map;
+
+typedef CGAL::Octree<Kernel, Point_set, Point_map> Octree;
+
+struct First_branch_traversal
+{
+  template <typename Node>
+  Node first (Node root) const
+  {
+    return root;
+  }
+
+  template <typename Node>
+  Node next (Node n) const
+  {
+    if (n.is_leaf())
+      return Node();
+    return n[0];
+  }
+};
+
+int main(int argc, char **argv) {
+
+  // Point set will be used to hold our points
+  Point_set points;
+
+  // Load points from a file.
+  std::ifstream stream((argc > 1) ? argv[1] : "data/cube.pwn");
+  stream >> points;
+  if (0 == points.number_of_points()) {
+
+    std::cerr << "Error: cannot read file" << std::endl;
+    return EXIT_FAILURE;
+  }
+  std::cout << "loaded " << points.number_of_points() << " points\n" << std::endl;
+
+  // Create an octree from the points
+  Octree octree(points, points.point_map());
+
+  // Build the octree
+  octree.refine();
+
+  // Print out the first branch using custom traversal
+  for (auto &node : octree.traverse<First_branch_traversal>())
+    std::cout << node << std::endl;
+
+  return EXIT_SUCCESS;
+}