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+/* This file is part of the Gudhi Library. The Gudhi library
+ * (Geometric Understanding in Higher Dimensions) is a generic C++
+ * library for computational topology.
+ *
+ * Author(s): Siargey Kachanovich
+ *
+ * Copyright (C) 2015 INRIA Sophia Antipolis-Méditerranée (France)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef LANDMARK_CHOICE_BY_RANDOM_POINT_H_
+#define LANDMARK_CHOICE_BY_RANDOM_POINT_H_
+
+#include <boost/range/size.hpp>
+
+#include <queue> // for priority_queue<>
+#include <utility> // for pair<>
+#include <iterator>
+#include <vector>
+#include <set>
+
+namespace Gudhi {
+
+namespace witness_complex {
+
+ /**
+ * \ingroup witness_complex
+ * \brief Landmark choice strategy by taking random vertices for landmarks.
+ * \details It chooses nbL distinct landmarks from a random access range `points`
+ * and outputs a matrix {witness}*{closest landmarks} in knn.
+ *
+ * The type KNearestNeighbors can be seen as
+ * Witness_range<Closest_landmark_range<Vertex_handle>>, where
+ * Witness_range and Closest_landmark_range are random access ranges and
+ * Vertex_handle is the label type of a vertex in a simplicial complex.
+ * Closest_landmark_range needs to have push_back operation.
+ */
+
+ template <typename KNearestNeighbours,
+ typename Point_random_access_range>
+ void landmark_choice_by_random_point(Point_random_access_range const &points,
+ int nbL,
+ KNearestNeighbours &knn) {
+ int nbP = boost::size(points);
+ assert(nbP >= nbL);
+ std::set<int> landmarks;
+ int current_number_of_landmarks = 0; // counter for landmarks
+
+ // TODO(SK) Consider using rand_r(...) instead of rand(...) for improved thread safety
+ int chosen_landmark = rand() % nbP;
+ for (current_number_of_landmarks = 0; current_number_of_landmarks != nbL; current_number_of_landmarks++) {
+ while (landmarks.find(chosen_landmark) != landmarks.end())
+ chosen_landmark = rand() % nbP;
+ landmarks.insert(chosen_landmark);
+ }
+
+ int dim = boost::size(*std::begin(points));
+ typedef std::pair<double, int> dist_i;
+ typedef bool (*comp)(dist_i, dist_i);
+ knn = KNearestNeighbours(nbP);
+ for (int points_i = 0; points_i < nbP; points_i++) {
+ std::priority_queue<dist_i, std::vector<dist_i>, comp> l_heap([](dist_i j1, dist_i j2) {
+ return j1.first > j2.first;
+ });
+ std::set<int>::iterator landmarks_it;
+ int landmarks_i = 0;
+ for (landmarks_it = landmarks.begin(), landmarks_i = 0; landmarks_it != landmarks.end();
+ ++landmarks_it, landmarks_i++) {
+ dist_i dist = std::make_pair(euclidean_distance(points[points_i], points[*landmarks_it]), landmarks_i);
+ l_heap.push(dist);
+ }
+ for (int i = 0; i < dim + 1; i++) {
+ dist_i dist = l_heap.top();
+ knn[points_i].push_back(dist.second);
+ l_heap.pop();
+ }
+ }
+ }
+
+} // namespace witness_complex
+
+} // namespace Gudhi
+
+#endif // LANDMARK_CHOICE_BY_RANDOM_POINT_H_