/* +------------------------------------------------------------------------+
   |                     Mobile Robot Programming Toolkit (MRPT)            |
   |                          https://www.mrpt.org/                         |
   |                                                                        |
   | Copyright (c) 2005-2022, Individual contributors, see AUTHORS file     |
   | See: https://www.mrpt.org/Authors - All rights reserved.               |
   | Released under BSD License. See: https://www.mrpt.org/License          |
   +------------------------------------------------------------------------+ */
#pragma once

// nanoflann library:
#include <mrpt/core/common.h>
#include <mrpt/core/exceptions.h>
#include <mrpt/math/TPoint2D.h>
#include <mrpt/math/TPoint3D.h>

#include <array>
#include <atomic>
#include <memory>  // unique_ptr
#include <mutex>
#include <nanoflann.hpp>

namespace mrpt::math
{
/** \addtogroup kdtree_grp KD-Trees
 *  \ingroup mrpt_math_grp
 *  @{ */

/** Adaptor class providing Nearest Neighbor (NN) searcg via `nanoflann`,
 * making use of the CRTP design pattern.
 *
 * Derived classes must call `kdtree_mark_as_outdated()` when data points change
 * to mark the cached KD-tree (an "index") as invalid, and they must also
 * implement the following interface (note that these are *not* virtual
 * functions due to the usage of CRTP):
 *
 *  \code
 *   // Must return the number of data points
 *   inline size_t kdtree_get_point_count() const { ... }
 *
 *   // Returns the distance between the vector "p1[0:size-1]" and the data
 * point with index "idx_p2" stored in the class:
 *   inline float kdtree_distance(
 *     const float *p1, const size_t idx_p2,size_t size) const { ... }
 *
 *   // Returns the dim'th component of the idx'th point in the class:
 *   inline num_t kdtree_get_pt(const size_t idx, int dim) const { ... }
 *
 *   // Optional bounding-box computation: return false to default to a
 *   // standard bbox computation loop. Return true if the BBOX was already
 *   // computed by the class and returned in "bb" so it can be avoided to
 *   // redo it again.
 *   // Look at bb.size() to find out the expected dimensionality
 *   // (e.g. 2 or 3 for point clouds):
 *   template <class BBOX>
 *   bool kdtree_get_bbox(BBOX &bb) const
 *   {
 *      bb[0].low = ...; bb[0].high = ...;  // "x" limits
 *      return true;
 *   }
 *  \endcode
 *
 * The KD-tree index will be built on demand only upon call of any of the query
 * methods provided by this class.
 *
 * Notice that there is only ONE internal cached KD-tree, so if a method to
 * query a 2D point is called, then another method for 3D points, then again
 * the 2D method, three KD-trees will be built. So, try to group all the calls
 * for a given dimensionality together or build different class instances for
 * queries of each dimensionality.
 *
 * \sa See some of the derived classes for example implementations. See also
 * the documentation of nanoflann
 * \ingroup mrpt_math_grp
 */
template <
	class Derived, typename num_t = float,
	typename metric_t = nanoflann::L2_Simple_Adaptor<num_t, Derived>>
class KDTreeCapable
{
   public:
	// Types ---------------
	using self_t = KDTreeCapable<Derived, num_t, metric_t>;
	// ---------------------

	/// Constructor
	inline KDTreeCapable() = default;
	KDTreeCapable(const KDTreeCapable&) : KDTreeCapable() {}
	KDTreeCapable& operator=(const KDTreeCapable&)
	{
		kdtree_mark_as_outdated();
		return *this;
	}

	/// CRTP helper method
	inline const Derived& derived() const
	{
		return *static_cast<const Derived*>(this);
	}
	/// CRTP helper method
	inline Derived& derived() { return *static_cast<Derived*>(this); }
	struct TKDTreeSearchParams
	{
		TKDTreeSearchParams() = default;
		/** Max points per leaf */
		size_t leaf_max_size = 10;
	};

	/** Parameters to tune KD-tree searches. Refer to nanoflann docs.
	 */
	TKDTreeSearchParams kdtree_search_params;

	/** @name Public utility methods to query the KD-tree
		@{ */

	/** KD Tree-based search for the closest point (only ONE) to some given 2D
	 * coordinates.
	 * This method automatically build the "m_kdtree_data" structure when:
	 *	- It is called for the first time
	 *	- The map has changed
	 *	- The KD-tree was build for 3D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param out_x The X coordinate of the found closest correspondence.
	 * \param out_y The Y coordinate of the found closest correspondence.
	 * \param out_dist_sqr The square distance between the query and the
	 *returned point.
	 *
	 * \return The index of the closest point in the map array.
	 *  \sa kdTreeClosestPoint3D, kdTreeTwoClosestPoint2D
	 */
	inline size_t kdTreeClosestPoint2D(
		float x0, float y0, float& out_x, float& out_y,
		float& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_2D();  // First: Create the 2D KD-Tree if required
		if (!m_kdtree2d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		const size_t knn = 1;  // Number of points to retrieve
		size_t ret_index;
		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&ret_index, &out_dist_sqr);

		const std::array<num_t, 2> query_point{{x0, y0}};
		m_kdtree2d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());

		// Copy output to user vars:
		out_x = derived().kdtree_get_pt(ret_index, 0);
		out_y = derived().kdtree_get_pt(ret_index, 1);

		return ret_index;
		MRPT_END
	}

	/// \overload
	inline size_t kdTreeClosestPoint2D(
		float x0, float y0, float& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_2D();  // First: Create the 2D KD-Tree if required
		if (!m_kdtree2d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		const size_t knn = 1;  // Number of points to retrieve
		size_t ret_index;
		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&ret_index, &out_dist_sqr);

		const std::array<num_t, 2> query_point{{x0, y0}};
		m_kdtree2d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());

		return ret_index;
		MRPT_END
	}

	/// \overload
	inline size_t kdTreeClosestPoint2D(
		const TPoint2D& p0, TPoint2D& pOut, float& outDistSqr) const
	{
		float dmy1, dmy2;
		size_t res =
			kdTreeClosestPoint2D(d2f(p0.x), d2f(p0.y), dmy1, dmy2, outDistSqr);
		pOut.x = dmy1;
		pOut.y = dmy2;
		return res;
	}

	/** Like kdTreeClosestPoint2D, but just return the square error from some
	 * point to its closest neighbor.
	 */
	inline float kdTreeClosestPoint2DsqrError(float x0, float y0) const
	{
		float closerx, closery, closer_dist;
		kdTreeClosestPoint2D(x0, y0, closerx, closery, closer_dist);
		return closer_dist;
	}

	inline float kdTreeClosestPoint2DsqrError(const TPoint2D& p0) const
	{
		return kdTreeClosestPoint2DsqrError(d2f(p0.x), d2f(p0.y));
	}

	/** KD Tree-based search for the TWO closest point to some given 2D
	 *coordinates.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 3D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param out_x1 The X coordinate of the first correspondence.
	 * \param out_y1 The Y coordinate of the first correspondence.
	 * \param out_x2 The X coordinate of the second correspondence.
	 * \param out_y2 The Y coordinate of the second correspondence.
	 * \param out_dist_sqr1 The square distance between the query and the first
	 *returned point.
	 * \param out_dist_sqr2 The square distance between the query and the
	 *second returned point.
	 *
	 *  \sa kdTreeClosestPoint2D
	 */
	inline void kdTreeTwoClosestPoint2D(
		float x0, float y0, float& out_x1, float& out_y1, float& out_x2,
		float& out_y2, float& out_dist_sqr1, float& out_dist_sqr2) const
	{
		MRPT_START
		rebuild_kdTree_2D();  // First: Create the 2D KD-Tree if required
		if (!m_kdtree2d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		const size_t knn = 2;  // Number of points to retrieve
		size_t ret_indexes[2];
		float ret_sqdist[2];
		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&ret_indexes[0], &ret_sqdist[0]);

		const std::array<num_t, 2> query_point{{x0, y0}};
		m_kdtree2d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());

		// Copy output to user vars:
		out_x1 = derived().kdtree_get_pt(ret_indexes[0], 0);
		out_y1 = derived().kdtree_get_pt(ret_indexes[0], 1);
		out_dist_sqr1 = ret_sqdist[0];

		out_x2 = derived().kdtree_get_pt(ret_indexes[1], 0);
		out_y2 = derived().kdtree_get_pt(ret_indexes[1], 1);
		out_dist_sqr2 = ret_sqdist[0];

		MRPT_END
	}

	inline void kdTreeTwoClosestPoint2D(
		const TPoint2D& p0, TPoint2D& pOut1, TPoint2D& pOut2,
		float& outDistSqr1, float& outDistSqr2) const
	{
		float dmy1, dmy2, dmy3, dmy4;
		kdTreeTwoClosestPoint2D(
			p0.x, p0.y, dmy1, dmy2, dmy3, dmy4, outDistSqr1, outDistSqr2);
		pOut1.x = static_cast<double>(dmy1);
		pOut1.y = static_cast<double>(dmy2);
		pOut2.x = static_cast<double>(dmy3);
		pOut2.y = static_cast<double>(dmy4);
	}

	/** KD Tree-based search for the N closest point to some given 2D
	 *coordinates.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 3D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param N The number of closest points to search.
	 * \param out_x The vector containing the X coordinates of the
	 *correspondences.
	 * \param out_y The vector containing the Y coordinates of the
	 *correspondences.
	 * \param out_dist_sqr The vector containing the square distance between
	 *the query and the returned points.
	 *
	 * \return The list of indices
	 *  \sa kdTreeClosestPoint2D
	 *  \sa kdTreeTwoClosestPoint2D
	 */
	inline std::vector<size_t> kdTreeNClosestPoint2D(
		float x0, float y0, size_t knn, std::vector<float>& out_x,
		std::vector<float>& out_y, std::vector<float>& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_2D();  // First: Create the 2D KD-Tree if required
		if (!m_kdtree2d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		std::vector<size_t> ret_indexes(knn);
		out_x.resize(knn);
		out_y.resize(knn);
		out_dist_sqr.resize(knn);

		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&ret_indexes[0], &out_dist_sqr[0]);

		const std::array<num_t, 2> query_point{{x0, y0}};
		m_kdtree2d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());

		for (size_t i = 0; i < knn; i++)
		{
			out_x[i] = derived().kdtree_get_pt(ret_indexes[i], 0);
			out_y[i] = derived().kdtree_get_pt(ret_indexes[i], 1);
		}
		return ret_indexes;
		MRPT_END
	}

	inline std::vector<size_t> kdTreeNClosestPoint2D(
		const TPoint2D& p0, size_t N, std::vector<TPoint2D>& pOut,
		std::vector<float>& outDistSqr) const
	{
		std::vector<float> dmy1, dmy2;
		std::vector<size_t> res = kdTreeNClosestPoint2D(
			d2f(p0.x), d2f(p0.y), N, dmy1, dmy2, outDistSqr);
		pOut.resize(dmy1.size());
		for (size_t i = 0; i < dmy1.size(); i++)
		{
			pOut[i].x = static_cast<double>(dmy1[i]);
			pOut[i].y = static_cast<double>(dmy2[i]);
		}
		return res;
	}

	/** KD Tree-based search for the N closest point to some given 2D
	 *coordinates and returns their indexes.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 3D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param N The number of closest points to search.
	 * \param out_idx The indexes of the found closest correspondence.
	 * \param out_dist_sqr The square distance between the query and the
	 *returned point.
	 *
	 *  \sa kdTreeClosestPoint2D
	 */
	inline void kdTreeNClosestPoint2DIdx(
		float x0, float y0, size_t knn, std::vector<size_t>& out_idx,
		std::vector<float>& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_2D();  // First: Create the 2D KD-Tree if required
		if (!m_kdtree2d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		out_idx.resize(knn);
		out_dist_sqr.resize(knn);
		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&out_idx[0], &out_dist_sqr[0]);

		const std::array<num_t, 2> query_point{{x0, y0}};
		m_kdtree2d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());
		MRPT_END
	}

	inline void kdTreeNClosestPoint2DIdx(
		const TPoint2D& p0, size_t N, std::vector<size_t>& outIdx,
		std::vector<float>& outDistSqr) const
	{
		return kdTreeNClosestPoint2DIdx(
			d2f(p0.x), d2f(p0.y), N, outIdx, outDistSqr);
	}

	/** KD Tree-based search for the closest point (only ONE) to some given 3D
	 *coordinates.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 2D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param z0  The Z coordinate of the query.
	 * \param out_x The X coordinate of the found closest correspondence.
	 * \param out_y The Y coordinate of the found closest correspondence.
	 * \param out_z The Z coordinate of the found closest correspondence.
	 * \param out_dist_sqr The square distance between the query and the
	 *returned point.
	 *
	 * \return The index of the closest point in the map array.
	 *  \sa kdTreeClosestPoint2D
	 */
	inline size_t kdTreeClosestPoint3D(
		float x0, float y0, float z0, float& out_x, float& out_y, float& out_z,
		float& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_3D();  // First: Create the 3D KD-Tree if required
		if (!m_kdtree3d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		const size_t knn = 1;  // Number of points to retrieve
		size_t ret_index;
		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&ret_index, &out_dist_sqr);

		const std::array<num_t, 3> query_point{{x0, y0, z0}};
		m_kdtree3d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());

		// Copy output to user vars:
		out_x = derived().kdtree_get_pt(ret_index, 0);
		out_y = derived().kdtree_get_pt(ret_index, 1);
		out_z = derived().kdtree_get_pt(ret_index, 2);

		return ret_index;
		MRPT_END
	}

	/// \overload
	inline size_t kdTreeClosestPoint3D(
		float x0, float y0, float z0, float& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_3D();  // First: Create the 3D KD-Tree if required
		if (!m_kdtree3d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		const size_t knn = 1;  // Number of points to retrieve
		size_t ret_index;
		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&ret_index, &out_dist_sqr);

		const std::array<num_t, 3> query_point{{x0, y0, z0}};
		m_kdtree3d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());

		return ret_index;
		MRPT_END
	}

	/// \overload
	inline size_t kdTreeClosestPoint3D(
		const TPoint3D& p0, TPoint3D& pOut, float& outDistSqr) const
	{
		float dmy1, dmy2, dmy3;
		size_t res = kdTreeClosestPoint3D(
			d2f(p0.x), d2f(p0.y), d2f(p0.z), dmy1, dmy2, dmy3, outDistSqr);
		pOut.x = static_cast<double>(dmy1);
		pOut.y = static_cast<double>(dmy2);
		pOut.z = static_cast<double>(dmy3);
		return res;
	}

	/** KD Tree-based search for the N closest points to some given 3D
	 *coordinates.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 2D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param z0  The Z coordinate of the query.
	 * \param N The number of closest points to search.
	 * \param out_x The vector containing the X coordinates of the
	 *correspondences.
	 * \param out_y The vector containing the Y coordinates of the
	 *correspondences.
	 * \param out_z The vector containing the Z coordinates of the
	 *correspondences.
	 * \param out_dist_sqr The vector containing the square distance between
	 *the query and the returned points.
	 *
	 *  \sa kdTreeNClosestPoint2D
	 */
	inline void kdTreeNClosestPoint3D(
		float x0, float y0, float z0, size_t knn, std::vector<float>& out_x,
		std::vector<float>& out_y, std::vector<float>& out_z,
		std::vector<float>& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_3D();  // First: Create the 3D KD-Tree if required
		if (!m_kdtree3d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		std::vector<size_t> ret_indexes(knn);
		out_x.resize(knn);
		out_y.resize(knn);
		out_z.resize(knn);
		out_dist_sqr.resize(knn);

		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&ret_indexes[0], &out_dist_sqr[0]);

		const std::array<num_t, 3> query_point{{x0, y0, z0}};
		m_kdtree3d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());

		for (size_t i = 0; i < knn; i++)
		{
			out_x[i] = derived().kdtree_get_pt(ret_indexes[i], 0);
			out_y[i] = derived().kdtree_get_pt(ret_indexes[i], 1);
			out_z[i] = derived().kdtree_get_pt(ret_indexes[i], 2);
		}
		MRPT_END
	}

	/** KD Tree-based search for the N closest points to some given 3D
	 *coordinates.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 2D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param z0  The Z coordinate of the query.
	 * \param N The number of closest points to search.
	 * \param out_x The vector containing the X coordinates of the
	 *correspondences.
	 * \param out_y The vector containing the Y coordinates of the
	 *correspondences.
	 * \param out_z The vector containing the Z coordinates of the
	 *correspondences.
	 * \param out_idx The vector containing the indexes of the correspondences.
	 * \param out_dist_sqr The vector containing the square distance between
	 *the query and the returned points.
	 *
	 *  \sa kdTreeNClosestPoint2D
	 */
	inline void kdTreeNClosestPoint3DWithIdx(
		float x0, float y0, float z0, size_t knn, std::vector<float>& out_x,
		std::vector<float>& out_y, std::vector<float>& out_z,
		std::vector<size_t>& out_idx, std::vector<float>& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_3D();  // First: Create the 3D KD-Tree if required
		if (!m_kdtree3d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		out_x.resize(knn);
		out_y.resize(knn);
		out_z.resize(knn);
		out_idx.resize(knn);
		out_dist_sqr.resize(knn);

		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&out_idx[0], &out_dist_sqr[0]);

		const std::array<num_t, 3> query_point{{x0, y0, z0}};
		m_kdtree3d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());

		for (size_t i = 0; i < knn; i++)
		{
			out_x[i] = derived().kdtree_get_pt(out_idx[i], 0);
			out_y[i] = derived().kdtree_get_pt(out_idx[i], 1);
			out_z[i] = derived().kdtree_get_pt(out_idx[i], 2);
		}
		MRPT_END
	}

	inline void kdTreeNClosestPoint3D(
		const TPoint3D& p0, size_t N, std::vector<TPoint3D>& pOut,
		std::vector<float>& outDistSqr) const
	{
		std::vector<float> dmy1, dmy2, dmy3;
		kdTreeNClosestPoint3D(
			d2f(p0.x), d2f(p0.y), d2f(p0.z), N, dmy1, dmy2, dmy3, outDistSqr);
		pOut.resize(dmy1.size());
		for (size_t i = 0; i < dmy1.size(); i++)
		{
			pOut[i].x = static_cast<double>(dmy1[i]);
			pOut[i].y = static_cast<double>(dmy2[i]);
			pOut[i].z = static_cast<double>(dmy3[i]);
		}
	}

	/** KD Tree-based search for all the points within a given radius of some 3D
	 *point.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 2D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param z0  The Z coordinate of the query.
	 * \param maxRadiusSqr The square of the desired search radius.
	 * \param out_indices_dist The output list, with pairs of indeces/squared
	 *distances for the found correspondences.
	 * \return Number of found points.
	 *
	 *  \sa kdTreeRadiusSearch2D, kdTreeNClosestPoint3DIdx
	 */
	inline size_t kdTreeRadiusSearch3D(
		const num_t x0, const num_t y0, const num_t z0,
		const num_t maxRadiusSqr,
		std::vector<std::pair<size_t, num_t>>& out_indices_dist) const
	{
		MRPT_START
		rebuild_kdTree_3D();  // First: Create the 3D KD-Tree if required
		out_indices_dist.clear();
		if (m_kdtree3d_data.m_num_points != 0)
		{
			const num_t xyz[3] = {x0, y0, z0};
			m_kdtree3d_data.index->radiusSearch(
				&xyz[0], maxRadiusSqr, out_indices_dist,
				nanoflann::SearchParams());
		}
		return out_indices_dist.size();
		MRPT_END
	}

	/** KD Tree-based search for all the points within a given radius of some 2D
	 *point.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 3D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param maxRadiusSqr The square of the desired search radius.
	 * \param out_indices_dist The output list, with pairs of indeces/squared
	 *distances for the found correspondences.
	 * \return Number of found points.
	 *
	 *  \sa kdTreeRadiusSearch3D, kdTreeNClosestPoint2DIdx
	 */
	inline size_t kdTreeRadiusSearch2D(
		const num_t x0, const num_t y0, const num_t maxRadiusSqr,
		std::vector<std::pair<size_t, num_t>>& out_indices_dist) const
	{
		MRPT_START
		rebuild_kdTree_2D();  // First: Create the 2D KD-Tree if required
		out_indices_dist.clear();
		if (m_kdtree2d_data.m_num_points != 0)
		{
			const num_t xyz[2] = {x0, y0};
			m_kdtree2d_data.index->radiusSearch(
				&xyz[0], maxRadiusSqr, out_indices_dist,
				nanoflann::SearchParams());
		}
		return out_indices_dist.size();
		MRPT_END
	}

	/** KD Tree-based search for the N closest point to some given 3D
	 *coordinates and returns their indexes.
	 *  This method automatically build the "m_kdtree_data" structure when:
	 *		- It is called for the first time
	 *		- The map has changed
	 *		- The KD-tree was build for 2D.
	 *
	 * \param x0  The X coordinate of the query.
	 * \param y0  The Y coordinate of the query.
	 * \param z0  The Z coordinate of the query.
	 * \param N The number of closest points to search.
	 * \param out_idx The indexes of the found closest correspondence.
	 * \param out_dist_sqr The square distance between the query and the
	 *returned point.
	 *
	 *  \sa kdTreeClosestPoint2D,  kdTreeRadiusSearch3D
	 */
	inline void kdTreeNClosestPoint3DIdx(
		float x0, float y0, float z0, size_t knn, std::vector<size_t>& out_idx,
		std::vector<float>& out_dist_sqr) const
	{
		MRPT_START
		rebuild_kdTree_3D();  // First: Create the 3D KD-Tree if required
		if (!m_kdtree3d_data.m_num_points)
			THROW_EXCEPTION("There are no points in the KD-tree.");

		out_idx.resize(knn);
		out_dist_sqr.resize(knn);
		nanoflann::KNNResultSet<num_t> resultSet(knn);
		resultSet.init(&out_idx[0], &out_dist_sqr[0]);

		const std::array<num_t, 3> query_point{{x0, y0, z0}};
		m_kdtree3d_data.index->findNeighbors(
			resultSet, &query_point[0], nanoflann::SearchParams());
		MRPT_END
	}

	inline void kdTreeNClosestPoint3DIdx(
		const TPoint3D& p0, size_t N, std::vector<size_t>& outIdx,
		std::vector<float>& outDistSqr) const
	{
		kdTreeNClosestPoint3DIdx(
			d2f(p0.x), d2f(p0.y), d2f(p0.z), N, outIdx, outDistSqr);
	}

	inline void kdTreeEnsureIndexBuilt3D() { rebuild_kdTree_3D(); }
	inline void kdTreeEnsureIndexBuilt2D() { rebuild_kdTree_2D(); }

	/* @} */

   protected:
	/** To be called by child classes when KD tree data changes. */
	inline void kdtree_mark_as_outdated() const
	{
		std::lock_guard<std::mutex> lck(m_kdtree_mtx);
		m_kdtree_is_uptodate = false;
	}

   private:
	/** Internal structure with the KD-tree representation (mainly used to avoid
	 * copying pointers with the = operator) */
	template <int _DIM = -1>
	struct TKDTreeDataHolder
	{
		inline TKDTreeDataHolder() = default;
		/** Copy constructor: It actually does NOT copy the kd-tree, a new
		 * object will be created if required!   */
		inline TKDTreeDataHolder(const TKDTreeDataHolder&) : TKDTreeDataHolder()
		{
		}
		/** Copy operator: It actually does NOT copy the kd-tree, a new object
		 * will be created if required!  */
		inline TKDTreeDataHolder& operator=(const TKDTreeDataHolder& o) noexcept
		{
			if (&o != this) clear();
			return *this;
		}

		/** Free memory (if allocated)  */
		inline void clear() noexcept { index.reset(); }
		using kdtree_index_t = nanoflann::KDTreeSingleIndexAdaptor<
			metric_t, Derived, _DIM, std::size_t /*index*/>;

		/** nullptr or the up-to-date index */
		std::unique_ptr<kdtree_index_t> index;

		/** Dimensionality. typ: 2,3 */
		size_t m_dim = _DIM;
		size_t m_num_points = 0;
	};

	mutable std::mutex m_kdtree_mtx;
	mutable TKDTreeDataHolder<2> m_kdtree2d_data;
	mutable TKDTreeDataHolder<3> m_kdtree3d_data;
	/** whether the KD tree needs to be rebuilt or not. */
	mutable std::atomic_bool m_kdtree_is_uptodate{false};

	/// Rebuild, if needed the KD-tree for 2D (nDims=2), 3D (nDims=3), ...
	/// asking the child class for the data points.
	void rebuild_kdTree_2D() const
	{
		if (m_kdtree_is_uptodate) return;

		std::lock_guard<std::mutex> lck(m_kdtree_mtx);
		using tree2d_t = typename TKDTreeDataHolder<2>::kdtree_index_t;

		if (!m_kdtree_is_uptodate)
		{
			m_kdtree2d_data.clear();
			m_kdtree3d_data.clear();
		}

		if (!m_kdtree2d_data.index)
		{
			// Erase previous tree:
			m_kdtree2d_data.clear();
			// And build new index:
			const size_t N = derived().kdtree_get_point_count();
			m_kdtree2d_data.m_num_points = N;
			m_kdtree2d_data.m_dim = 2;
			if (N)
			{
				m_kdtree2d_data.index = std::make_unique<tree2d_t>(
					2, derived(),
					nanoflann::KDTreeSingleIndexAdaptorParams(
						kdtree_search_params.leaf_max_size));
				m_kdtree2d_data.index->buildIndex();
			}
			m_kdtree_is_uptodate = true;
		}
	}

	/// Rebuild, if needed the KD-tree for 2D (nDims=2), 3D (nDims=3), ...
	/// asking the child class for the data points.
	void rebuild_kdTree_3D() const
	{
		if (m_kdtree_is_uptodate) return;
		std::lock_guard<std::mutex> lck(m_kdtree_mtx);
		using tree3d_t = typename TKDTreeDataHolder<3>::kdtree_index_t;

		if (!m_kdtree_is_uptodate)
		{
			m_kdtree2d_data.clear();
			m_kdtree3d_data.clear();
		}

		if (!m_kdtree3d_data.index)
		{
			// Erase previous tree:
			m_kdtree3d_data.clear();
			// And build new index:
			const size_t N = derived().kdtree_get_point_count();
			m_kdtree3d_data.m_num_points = N;
			m_kdtree3d_data.m_dim = 3;
			if (N)
			{
				m_kdtree3d_data.index = std::make_unique<tree3d_t>(
					3, derived(),
					nanoflann::KDTreeSingleIndexAdaptorParams(
						kdtree_search_params.leaf_max_size));
				m_kdtree3d_data.index->buildIndex();
			}
			m_kdtree_is_uptodate = true;
		}
	}

};	// end of KDTreeCapable

/**  @} */	// end of grouping

}  // namespace mrpt::math
