A K-D Tree implementation which preforms a nearest neighbor seach on any point type and returns the distance between them in any type using a user-defined distance function.

KDTree.h

/**
 * A K-D Tree implementation which preforms a nearest neighbor seach on any point type and returns the
 * distance between them in any type using a user-defined distance function.
 * 
 * Rather than returning the actual nearest object this class returns the index of the nearest object in
 * the original vector that was passed in.
 *
 * Provided under the MIT License
 * Copyright (c) 2014 Andrew Kallmeyer <fsmv@sapium.net>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#pragma once
#include <vector>

struct Node {
	int index;
	Node* left;
	Node* right;

	Node(int index) : index(index) {}
	bool isLeaf() { return left == nullptr && right == nullptr; }
};

template <typename PointType, typename DistType = double>
class KDTree {
public:
	KDTree(DistType (*getDimVal)(const PointType&, int dim), int dimension = 2);
	~KDTree(void);

	/*
	 * add all entries in the array to the data structure
	 *
	 * Note: If two items have the same coordinates, then do not add the new item
	 *       that has the same coordinates as another item.
	 */
	void build(const std::vector<PointType> &c);
	
	/*
	 * Return a pointer to the entry that is closest to the given coordinates.
	 */
	int getNearest(const PointType &point, DistType *dist) const;

private:
	Node* root;
	std::vector<int> dataIndicies;

	DistType getDist(const PointType &lhs, const PointType &rhs) const;
	bool comp(int dim, const PointType &lhs, const PointType &rhs) const;
	DistType (*getDimVal)(const PointType&, int dim);

	int dimension;

	void destroy(Node* root);

	/*
	 * Recursively builds a kd-tree, n is the length of c
	 */
	Node* build(int start, int n, int depth);

	/*
	 * Recursively finds the nearest neighbor to a given point (x, y) root is the root of the kd-tree to search and currResult is the current closest (arbitrary initial choice)
	 */
	int getNearest(const PointType &point, Node* node, int best, int depth = 0) const;
};

//======= Implementation =======

#include <cmath>
#include <algorithm>
#include <functional>
#include <iterator>

template<typename T, typename V>
KDTree<T, V>::KDTree(V (*getDimVal)(const T&, int dim), int dimension) : getDimVal(getDimVal), dimension(dimension) {}

template<typename T, typename V>
KDTree<T, V>::~KDTree(void) {
	if(root != nullptr) {
		destroy(root);
	}
}

template<typename T, typename V>
void KDTree<T, V>::destroy(Node* root) {
	if(root->left != nullptr){
		destroy(root->left);
	}

	if(root->right != nullptr){
		destroy(root->right);
	}

	delete root;
}

template<typename T, typename V>
void KDTree<T, V>::build(const std::vector<T> &c) {
	data = c;
	dataIndicies.reserve(data.size());
	for(unsigned int i = 0; i < data.size(); ++i) {
		dataIndicies.push_back(i);
	}
	root = build(0, c.size(), 0);
}

template<typename T, typename V>
Node* KDTree<T, V>::build(int start, int n, int depth) {
	Node* result;

	if(n == 0) {
		return nullptr;
	}else if(n == 1) {
		result = new Node(dataIndicies[start]);
		result->left = nullptr;
		result->right = nullptr;
		return result;
	}

	std::sort(dataIndicies.begin() + start, dataIndicies.begin() + start + n, [&](int lhs, int rhs){ return comp(depth % dimension, data[lhs], data[rhs]); });

	const int halfLength = n/2;
	result = new Node(dataIndicies[start + halfLength]);
	result->left = build(start, halfLength, depth + 1);
	result->right = build(start + halfLength + 1, (n % 2 == 0 ? -1 : 0) + halfLength, depth + 1);

	return result;
}

template<typename T, typename V>
int KDTree<T, V>::getNearest(const T &point, V *dist) const {
	int result = getNearest(point, root, root->index);
	*dist = (V) std::sqrt((double) getDist(point, data[result]));
	return result;
}

template<typename T, typename V>
int KDTree<T, V>::getNearest(const T &point, Node* node, int best, int depth) const {
	bool leftOf = comp(depth % dimension, point, data[node->index]);
	Node *childNear = leftOf ? node->left : node->right;
	Node *childFar  = leftOf ? node->right : node->left;

	if(getDist(point, data[node->index]) < getDist(point, data[best])) {
		best = node->index;
	}

	if(childNear != nullptr) {
		best = getNearest(point, childNear, best, depth+1);
	}

	V axisDist = getDimVal(point, depth % dimension) - getDimVal(data[node->index], depth % dimension);
	axisDist *= axisDist; //getDist is a squared distance to save time, this needs to be squared also
	if(axisDist <= getDist(point, data[best])) {
		if(childFar != nullptr) {
			best = getNearest(point, childFar, best, depth+1);
		}
	}

	return best;
}

template<typename T, typename V>
V KDTree<T,V>::getDist(const T &lhs, const T &rhs) const {
	V sum = (V) 0;

	for(int i = 0; i < dimension; ++i) {
		V val = getDimVal(lhs, i) - getDimVal(rhs, i);
		sum += val * val;
	}

	return sum;
}

template<typename T, typename V>
bool KDTree<T,V>::comp(int dim, const T &lhs, const T &rhs) const {
	return getDimVal(lhs, dim) < getDimVal(rhs, dim);
}