cube-game-plus-plus/common/utils/KDTree.cpp

#include <algorithm>

#include "common/utils/KDTree.h"

KDTree::Triangle::Triangle(const Vector3& a, const Vector3& b, const Vector3& c) {
    v[0] = a;
    v[1] = b;
    v[2] = c;
    mid = (a + b + c) * (1.0f / 3.0f);
}

const Array<Vector3, 3>& KDTree::Triangle::data() const {
    return v;
}

const Vector3& KDTree::Triangle::operator[](int index) const {
    return v[index];
}

const Vector3& KDTree::Triangle::getMid() const {
    return mid;
}

KDTree::Node::Node() : splitDim(0), splitValue(0.0f), lessEqual(nullptr), greater(nullptr) {
}

KDTree::KDTree() {
}

KDTree::~KDTree() {
    clean(&root);
}

void KDTree::clean(Node* n) {
    if(n->lessEqual != nullptr) {
        clean(n->lessEqual);
    }
    if(n->greater != nullptr) {
        clean(n->greater);
    }
    delete n->lessEqual;
    delete n->greater;
}

void KDTree::build(std::vector<KDTree::Triangle>& data) {
    build(&root, data);
}

float KDTree::median(std::vector<KDTree::Triangle>& data, int dim) const {
    auto compare = [dim](const Triangle& a, const Triangle & b) {
        return a.getMid()[dim] < b.getMid()[dim];
    };
    size_t length = data.size();
    if((length & 1) == 0) {
        std::nth_element(data.begin(), data.begin() + (length / 2 - 1), data.end(), compare);
        float tmp = data[length / 2 - 1].getMid()[dim];
        std::nth_element(data.begin(), data.begin() + (length / 2), data.end(), compare);
        return (tmp + data[length / 2].getMid()[dim]) / 2;
    }
    std::nth_element(data.begin(), data.begin() + (length / 2), data.end(), compare);
    return data[length / 2].getMid()[dim];
}

void KDTree::build(Node* n, std::vector<KDTree::Triangle>& data) {
    if(data.size() == 0) {
        return;
    } else if(data.size() == 1) {
        n->data.push_back(data[0]);
        return;
    }
    // find min and max coordinates
    Vector3 min = data[0][0];
    Vector3 max = data[0][0];
    for(const Triangle& t : data) {
        for(const Vector3& v : t.data()) {
            min.set(std::min(min[0], v[0]), std::min(min[1], v[1]), std::min(min[2], v[2]));
            max.set(std::max(max[0], v[0]), std::max(max[1], v[1]), std::max(max[2], v[2]));
        }
    }
    // find biggest span and its dimension
    int splitDim = 0;
    float maxSpan = max[0] - min[0];
    for(int i = 1; i < 3; i++) {
        float span = max[i] - min[i];
        if(span > maxSpan) {
            splitDim = i;
            maxSpan = span;
        }
    }
    // assign data to node
    n->splitDim = splitDim;
    n->splitValue = median(data, splitDim);
    // storage for split data
    std::vector<KDTree::Triangle> lessEqualData;
    std::vector<KDTree::Triangle> greaterData;
    // actually split the data
    for(const Triangle& t : data) {
        // count points on each split side
        int lessEqualCounter = 0;
        int greaterCount = 0;
        for(const Vector3& v : t.data()) {
            if(v[n->splitDim] <= n->splitValue) {
                lessEqualCounter++;
            } else {
                greaterCount++;
            }
        }
        // put the data in the correct container
        if(lessEqualCounter == 3) {
            lessEqualData.push_back(t);
        } else if(greaterCount == 3) {
            greaterData.push_back(t);
        } else {
            n->data.push_back(t);
        }
    }
    // recursive calls
    if(lessEqualData.size() > 0) {
        n->lessEqual = new Node();
        build(n->lessEqual, lessEqualData);
    }
    if(greaterData.size() > 0) {
        n->greater = new Node();
        build(n->greater, greaterData);
    }
}