clhs.h

Go to the documentation of this file.

/******************************************************************************
 *
 * Project: sgs
 * Purpose: C++ implementation of CLHS
 * Author: Joseph Meyer
 * Date: November, 2025
 *
 ******************************************************************************/

 
#include <iostream>
#include <random>
 
#include "utils/access.h"
#include "utils/existing.h"
#include "utils/helper.h"
#include "utils/raster.h"
#include "utils/vector.h"
 
#include <boost/unordered/unordered_flat_set.hpp>
#include <mkl.h>
#include "oneapi/dal.hpp"
#include <xoshiro.h>
 
#define MILLION 1000000
 
typedef oneapi::dal::homogen_table              DALHomogenTable;
 
namespace sgs {
namespace clhs {

template <typename T>
struct Point {
    T *p_features = nullptr;
    int x = -1;
    int y = -1;
};

template <typename T>
inline size_t 
getQuantile(T val, std::vector<T>& quantiles) {
    auto it = std::lower_bound(quantiles.begin(), quantiles.end(), val);
    return (it == quantiles.end()) ?
        quantiles.size() :
        std::distance(quantiles.begin(), it);
}

template <typename T>
class CLHSDataManager {
    private:
    std::vector<T> features;
    std::vector<int> x;
    std::vector<int> y;
    size_t fi; //features index
    size_t count;
    int64_t size;
    uint64_t ucount;
 
    //for existing sample points
    std::vector<T> efeatures;
    std::vector<int> ex;
    std::vector<int> ey;
    size_t efi = 0;
    size_t ecount = 0;
 
    std::vector<std::vector<T>> corr;
 
    int nFeat;
    int nSamp;
 
    xso::xoshiro_4x64_plus *p_rng = nullptr;
    uint64_t mask = 0;
 
    public:
    CLHSDataManager(int nFeat, int nSamp, xso::xoshiro_4x64_plus *p_rng, size_t existingCount) {
        this->nFeat = nFeat;
        this->nSamp = nSamp;
        this->count = 0;
        this->fi = 0;
        this->size = MILLION;
        this->features.resize(MILLION * nFeat);
        this->x.resize(MILLION);
        this->y.resize(MILLION);
 
        this->p_rng = p_rng;
 
        if (existingCount != 0) {
            this->efeatures.resize(existingCount * static_cast<size_t>(nFeat));
            this->ex.resize(existingCount);
            this->ey.resize(existingCount);
        }
    }

    inline void
    addPoint(T *p_features, int x, int y) {
        for (int f = 0; f < nFeat; f++) {
            features[this->fi] = p_features[f];
            this->fi++;
        }
 
        this->x[this->count] = x;
            this->y[this->count] = y;
        this->count++;
 
        if (this->count == this->size) {
            this->features.resize(this->features.size() + MILLION * this->nFeat);
            this->x.resize(this->x.size() + MILLION);
            this->y.resize(this->y.size() + MILLION);
            this->size += MILLION;
        }   
    }

    inline void
    addExistingPoint(T *p_features, int x, int y) {
        for (int f = 0; f < nFeat; f++) {
            this->efeatures[this->efi] = p_features[f];
            this->efi++;
        }
 
        this->ex[this->ecount] = x;
        this->ey[this->ecount] = y;
        this->ecount++;
    }

    inline void
    finalize(std::vector<std::vector<T>>& corr) {
        if (this->count < static_cast<size_t>(this->nSamp)) {
            throw std::runtime_error("not enough points saved during raster iteration to conduct clhs sampling.");
        }
        
        this->corr = corr;
 
        this->x.resize(this->count);
        this->y.resize(this->count);
        this->features.resize(this->count * nFeat);
        this->ucount = static_cast<uint64_t>(this->count);
 
        //use bit twiddling to fill the mask
        this->mask = static_cast<uint64_t>(this->count);
        this->mask |= this->mask >> 1;
        this->mask |= this->mask >> 2;
        this->mask |= this->mask >> 4;
        this->mask |= this->mask >> 8;
        this->mask |= this->mask >> 16;
        this->mask |= this->mask >> 32;
 
        //resize existing sample vectors 
        this->ex.resize(this->ecount);
        this->ey.resize(this->ecount);
        this->features.resize(this->ecount * nFeat);
    }

    inline uint64_t
    randomIndex() {
        uint64_t index = ((*p_rng)() >> 11) & mask;
 
        while (index >= this->ucount) {
            index = ((*p_rng)() >> 11) & mask;
        }
 
        return index;
    }

    inline void
    getRandomPoint(Point<T>& point) {
        uint64_t index = randomIndex();
 
        point.p_features = this->features.data() + (index * nFeat);
        point.x = x[index];
        point.y = y[index];
    }

    inline T
    quantileObjectiveFunc(std::vector<int>& sampleCountPerQuantile) {
        int retval = 0;
 
        for (const int& count : sampleCountPerQuantile) {
            retval += std::abs(count - 1);
        }
 
        return static_cast<T>(retval);
    }

    inline T
    correlationObjectiveFunc(std::vector<std::vector<T>>& corr) {
        T retval = 0;
 
        for (size_t i = 0; i < this->corr.size(); i++) {
            for (size_t j = 0; j < this->corr[i].size(); j++) {
                retval += std::abs(corr[i][j] - this->corr[i][j]);
            }
        }
    
        return retval;
    }

    inline void
    getExistingFeatures(std::vector<T>& features, std::vector<int>& x, std::vector<int>& y) {
        features.resize(this->efeatures.size());
        x.resize(this->ex.size());
        y.resize(this->ey.size());
 
        std::memcpy(features.data(), this->efeatures.data(), this->efeatures.size() * sizeof(T));
        std::memcpy(x.data(), this->ex.data(), this->ex.size() * sizeof(int));
        std::memcpy(y.data(), this->ey.data(), this->ey.size() * sizeof(int));
    }
};

template <typename T>
inline void
readRaster(
    std::vector<helper::RasterBandMetaData>& bands,
    CLHSDataManager<T>& clhs,
    access::Access& access,
    existing::Existing& existing,
    helper::RandValController& rand,
    GDALDataType type,
    std::vector<std::vector<T>>& quantiles,
    size_t size,
    int width,
    int height,
    int count,
    int nSamp)
{
    std::vector<std::vector<T>> probabilities(count);
    quantiles.resize(count);
 
    for (int i = 0; i < count; i++) {
        probabilities[i].resize(nSamp - 1);
        quantiles[i].resize(nSamp - 1);
 
        for (int j = 0; j < nSamp - 1; j++) {
            probabilities[i][j] = static_cast<T>(j + 1) / static_cast<T>(nSamp);
        }
    }
 
    int xBlockSize = bands[0].xBlockSize;
    int yBlockSize = bands[0].yBlockSize;
 
    int xBlocks = (width + xBlockSize - 1) / xBlockSize;
    int yBlocks = (height + yBlockSize - 1) / yBlockSize;
 
    double deps = .001;
    float seps = .001f;
 
    std::vector<T> corrBuffer(count * xBlockSize * yBlockSize);
    std::vector<std::vector<T>> quantileBuffers(count);
    for (int i = 0; i < count; i++) {
        quantileBuffers[i].resize(xBlockSize * yBlockSize);
    }
 
    if (access.used) {
        access.band.p_buffer = VSIMalloc3(xBlockSize, yBlockSize, access.band.size); 
    }
 
    //create descriptor for correlation matrix streaming calculation with oneDAL
    const auto cor_desc = oneapi::dal::covariance::descriptor{}.set_result_options(oneapi::dal::covariance::result_options::cor_matrix);
    oneapi::dal::covariance::partial_compute_result<> partial_result;
 
    //create tasks for quantile streaming calculation with MKL
    std::vector<VSLSSTaskPtr> quantileTasks(count);
    int status;
    MKL_INT quant_order_n = quantiles[0].size();
    MKL_INT p = 1;
    MKL_INT n = xBlockSize * yBlockSize;
    MKL_INT nparams = VSL_SS_SQUANTS_ZW_PARAMS_N;
    MKL_INT xstorage = VSL_SS_MATRIX_STORAGE_ROWS;
 
    //MKL functions have different versions for single/double precision floating point data
    if (type == GDT_Float64) {
        for (int i = 0; i < count; i++) {
            //reinterpret cast the pointer for compiler reasons
            status = vsldSSNewTask(
                &quantileTasks[i], 
                &p, 
                &n, 
                &xstorage, 
                reinterpret_cast<double *>(quantileBuffers[i].data()), 
                0, 
                0
            );
        }
    }
    else {
        for (int i = 0; i < count; i++) {
            //reinterpret cast the pointer for compiler reasons
            status = vslsSSNewTask(
                &quantileTasks[i],
                &p,
                &n,
                &xstorage,
                reinterpret_cast<float *>(quantileBuffers[i].data()),
                0,
                0
            );
        }
    }
 
    int8_t *p_access = reinterpret_cast<int8_t *>(access.band.p_buffer);
 
    bool calledEditStreamQuantiles = false;
    void *p_data = reinterpret_cast<void *>(corrBuffer.data());
 
    for (int yBlock = 0; yBlock < yBlocks; yBlock++) {
        for (int xBlock = 0; xBlock < xBlocks; xBlock++) {
            //get block size
            int xValid, yValid;
            bands[0].p_band->GetActualBlockSize(xBlock, yBlock, &xValid, &yValid);
 
            //read bands into memory
            for (int i = 0; i < count ; i++) {
                CPLErr err = bands[i].p_band->RasterIO(
                    GF_Read,
                    xBlock * xBlockSize,
                    yBlock * yBlockSize,
                    xValid,
                    yValid,
                    (void *)((size_t)p_data + i * size),
                    xValid,
                    yValid,
                    type,
                    size * static_cast<size_t>(count),
                    size * static_cast<size_t>(count) * static_cast<size_t>(xBlockSize) 
                );
 
                if (err) {
                    throw std::runtime_error("Error reading data from raster band.");
                }
            }
 
            //calculate rand vals
            rand.calculateRandValues();
 
            //read access band into memory if used
            if (access.used) {
                rasterBandIO(access.band, access.band.p_buffer, xBlockSize, yBlockSize, xBlock, yBlock, xValid, yValid, true, false);
            }
 
            //iterate through pixels
            n = 0;
            for (int y = 0; y < yValid; y++) {
                int index = y * xBlockSize;
                for (int x = 0; x < xValid; x++) {
                    bool isNan = false;
                    T *p_buff = corrBuffer.data() + (index * count);
                    for (int b = 0; b < count; b++) {
                        T val = p_buff[b];
                        isNan = std::isnan(val) || val == bands[b].nan;
 
                        if (isNan) {
                            break;
                        }   
 
                        quantileBuffers[b][n] = val;
                        corrBuffer[n * count + b] = val;
                    }   
 
                    if (!isNan) {
                        n++;
                        
                        bool accessible = !access.used || p_access[index] != 1;
 
                        if (existing.used && existing.containsIndex(x + xBlock * xBlockSize, y + yBlock * yBlockSize)) {
                            clhs.addExistingPoint(
                                p_buff,
                                xBlock * xBlockSize + x,
                                yBlock * yBlockSize + y
                            );
                        }
                        else if (accessible && rand.next()) {
                            clhs.addPoint(
                                p_buff,
                                xBlock * xBlockSize + x,
                                yBlock * yBlockSize + y     
                            );
                        }
                    }
                    index++;
                }
            }
 
            if (n == 0) {
                continue;
            }
 
            //MKL functions have different versions for single/double precision floating point data
            if (type == GDT_Float64) {
                if (!calledEditStreamQuantiles) {
                    for (int i = 0; i < count; i++) {
                        //reinterpret cast the pointers for compiler reasons
                        status = vsldSSEditStreamQuantiles(
                            quantileTasks[i],
                            &quant_order_n,
                            reinterpret_cast<double *>(probabilities[i].data()),
                            reinterpret_cast<double *>(quantiles[i].data()),
                            &nparams,
                            &deps
                        );
                    }
                    calledEditStreamQuantiles = true;
                }
                for (int i = 0; i < count; i++) {
                    status = vsldSSCompute(
                        quantileTasks[i],
                        VSL_SS_STREAM_QUANTS,
                        VSL_SS_METHOD_SQUANTS_ZW_FAST
                    );
                }
            }
            else { //type == GDT_Float32
                if (!calledEditStreamQuantiles) {
                    for (int i = 0; i < count; i++) {
                        //reinterpret cast the pointers for compiler reasons
                        status = vslsSSEditStreamQuantiles(
                            quantileTasks[i],
                            &quant_order_n,
                            reinterpret_cast<float *>(probabilities[i].data()),
                            reinterpret_cast<float *>(quantiles[i].data()),
                            &nparams,
                            &seps
                        );
                    }
                    calledEditStreamQuantiles = true;
                }
                for (int i = 0; i < count; i++) {   
                    status = vslsSSCompute(
                        quantileTasks[i],
                        VSL_SS_STREAM_QUANTS,
                        VSL_SS_METHOD_SQUANTS_ZW_FAST
                    );
                }
 
            }
 
            //update correlation matrix calculations
            DALHomogenTable table = DALHomogenTable(corrBuffer.data(), n, count, [](const T *){}, oneapi::dal::data_layout::row_major);
            partial_result = oneapi::dal::partial_compute(cor_desc, partial_result, table); 
        }
    }
 
    if (access.used) {
        VSIFree(access.band.p_buffer);
    }
 
    //calculate and update clhs data manager with correlation matrix
    auto result = oneapi::dal::finalize_compute(cor_desc, partial_result);
    auto correlation = result.get_cor_matrix();
 
    oneapi::dal::row_accessor<const T> acc {correlation};
 
    n = 0;
    std::vector<std::vector<T>> corr(count);
    for (int i = 0; i < count; i++) {
        corr[i].resize(count);
        auto row = acc.pull({i, i + 1});
 
        for (int j = 0; j < count; j++) {
            corr[i][j] = row[j];
        }
 
        status = (type == GDT_Float64) ?
            vsldSSCompute(quantileTasks[i], VSL_SS_STREAM_QUANTS, VSL_SS_METHOD_SQUANTS_ZW) :
            vslsSSCompute(quantileTasks[i], VSL_SS_STREAM_QUANTS, VSL_SS_METHOD_SQUANTS_ZW);
        status = vslSSDeleteTask(&quantileTasks[i]);
    }
 
    clhs.finalize(corr);
}

template <typename T>
inline void
selectSamples(std::vector<std::vector<T>>& quantiles,
          CLHSDataManager<T>& clhs,
          xso::xoshiro_4x64_plus& rng,
          existing::Existing& existing,
          size_t replace,
          size_t iterations,
          size_t nSamp,
          size_t nFeat,
          OGRLayer *p_layer,
          double *GT,
          bool plot,
          std::vector<double>& xCoords,
          std::vector<double>& yCoords)
{
    std::vector<T> features;
    std::vector<int> x;
    std::vector<int> y;
 
    std::vector<int> sampleCountPerQuantile(nFeat * nSamp, 0); //nFeat x nSamp 2d array
    std::vector<int> quantilesOfEachSample(nSamp * nFeat, 0); //nSamp x nFeat 2d array
    
    //if there are existing samples, add all of them.
    if (existing.used) {
        clhs.getExistingFeatures(features, x, y);
    }
 
    //Then, remove up to 'replace' number of them which are redundant
    //'redundant' samples are samples which cause over-representation in feature quantiles
    
    size_t neSamples = x.size(); //number of existing samples
    if (neSamples > 0 && replace != 0) {
        for (size_t si = 0; si < neSamples; si++) {
            for (size_t fi = 0; fi < nFeat; fi++) {
                T val = features[(si * nFeat) + fi];
                size_t q = getQuantile<T>(val, quantiles[fi]);
                sampleCountPerQuantile[(fi * nSamp) + q]++;
                quantilesOfEachSample[(si * nFeat) + fi] = q;
            }
        }
 
        while (replace > 0 && neSamples > 0) {
            size_t worstRedundancy = 0;
            size_t worstRedundancyIndex = 0;
 
            //get the sample with the worst redundancy. In other words, the one which is in the
            //most over-represented quantile across all features
            for (size_t si = 0; si < neSamples; si++) {
                size_t curSampleRedundancy = 0;
                for (size_t fi = 0; fi < nFeat; fi++) {
                    size_t q = quantilesOfEachSample[(si * nFeat) + fi];
                    curSampleRedundancy += sampleCountPerQuantile[(fi * nSamp) + q];
                }
                if (curSampleRedundancy > worstRedundancy) {
                    worstRedundancy = curSampleRedundancy;
                    worstRedundancyIndex = si;
                }   
            }
 
            //if the remaining samples are already forming a partial latin hypercube
            //(no more than 1 sample per quantile of each feature)
            //then don't remove any more indices!
            if (worstRedundancy == nFeat) {
                break;
            }
 
            //replace the most redundant sample with the last sample in the vector, and decrease
            //the size of the vector by 1. But first, adjust the values of sampleCountPerQuantile
            size_t si = worstRedundancyIndex;
            for (size_t fi = 0; fi < nFeat; fi++) {
                size_t q = quantilesOfEachSample[(si * nFeat) + fi];
                sampleCountPerQuantile[(fi * nSamp) + q]--;
            }
 
            x[si] = x[neSamples - 1];
            y[si] = y[neSamples - 1];
            std::memcpy(features.data() + (si * nFeat), 
                    features.data() + ((neSamples - 1) * nFeat), 
                    sizeof(T) * nFeat);
            std::memcpy(quantilesOfEachSample.data() + (si * nFeat),
                    quantilesOfEachSample.data() + ((neSamples - 1) * nFeat),
                    sizeof(int) * nFeat);
 
            neSamples--;
            replace--;
        }       
    }
 
    //NOW, the neSamples variable contains the number of existing samples which MUST be kept.
    //This number also happens to be the first index of the remaining space in the vector which
    //may be filled in with non-existing samples. If there were no existing samples this value
    //is 0.
    size_t starti = neSamples;
    boost::unordered::unordered_flat_set<uint64_t> points;
 
    //Add all of the existing samples to the output layer, and add to indices map
    helper::Field fieldExistingTrue("existing", 1);
    for (size_t si = 0; si < neSamples; si++) {
        OGRPoint point = existing.getPoint(x[si], y[si]);
        helper::addPoint(&point, p_layer, &fieldExistingTrue);
 
        if (plot) {
            xCoords.push_back(point.getX());
            yCoords.push_back(point.getY());
        }
 
        points.insert((((uint64_t) x[si]) << 32) | ((uint64_t) y[si]));
    }
 
    //if there are already enough (existing) samples, return and don't add any more
    if (starti >= nSamp) {
        return;
    }
 
    std::uniform_real_distribution<T> dist(0.0, 1.0);
    std::uniform_int_distribution<size_t> indexDist(starti, nSamp - 1);
 
    std::vector<std::vector<T>> corr(nFeat);
    for (int i = 0; i < nFeat; i++) {
        corr[i].resize(nFeat);
    }
 
    features.resize(nSamp * nFeat);
    x.resize(nSamp);
    y.resize(nSamp);
 
    //get first random samples
    int i = starti;
    Point<T> p;
    while (i < nSamp) {
        clhs.getRandomPoint(p);
 
        if (points.contains((((uint64_t) p.x) << 32) | ((uint64_t) p.y))) {
            continue;
        }
 
        x[i] = p.x;
        y[i] = p.y;
        
        for (int f = 0; f < nFeat; f++) {
            T val = p.p_features[f];
            features[(i * nFeat) + f] = val;
 
            int q = getQuantile<T>(val, quantiles[f]);
            sampleCountPerQuantile[(f * nSamp) + q]++;
            quantilesOfEachSample[(i * nFeat) + f] = q;
        }
 
        points.insert((((uint64_t) p.x) << 32) | ((uint64_t) p.y));
        i++;
    }
 
    //define covariance calculation 
    DALHomogenTable table = DALHomogenTable(features.data(), nSamp, nFeat, [](const T *){}, oneapi::dal::data_layout::row_major);
    const auto cor_desc = oneapi::dal::covariance::descriptor{}.set_result_options(oneapi::dal::covariance::result_options::cor_matrix);        
    const auto result = oneapi::dal::compute(cor_desc, table);
    oneapi::dal::row_accessor<const T> acc {result.get_cor_matrix()};
    for (int i = 0; i < nFeat; i++) {
        auto row = acc.pull({i, i + 1});
 
        for (int j = 0; j < nFeat; j++) {
            corr[i][j] = row[j];
        }
    }
 
    double temp = 1;
    double d = temp / static_cast<double>(iterations);
 
    T obj = 0;
    T objQ = clhs.quantileObjectiveFunc(sampleCountPerQuantile);
    T objC = clhs.correlationObjectiveFunc(corr);
 
    obj = objQ + objC;
 
    //features of old (before random new index) index
    std::vector<T> oldf(nFeat);
 
    //begin annealing schedule. If we have a perfect latin hypercube -- or if we pass enough iterations -- stop iterating.
    while (temp > 0 && objQ != 0) {
        size_t i; //the index within the indices, x, y, and features vector so we know what to swap without searching
        if (dist(rng) < 0.5) {
            //50% of the time, choose a random sample to replace
            i = indexDist(rng);
        }
        else {
            //50% of the time, choose the worst sample to replace
 
            //get the sample with the worst redundancy. In other words, the one which is in the
            //most over-represented quantile across all features
            size_t worstRedundancyIndex = 0;
            size_t worstRedundancy = 0;
            for (size_t si = starti; si < nSamp; si++) {
                size_t curSampleRedundancy = 0;
                for (size_t fi = 0; fi < nFeat; fi++) {
                    size_t q = quantilesOfEachSample[(si * nFeat) + fi];
                    curSampleRedundancy += sampleCountPerQuantile[(fi * nSamp) + q];
                }
                if (curSampleRedundancy > worstRedundancy) {
                    worstRedundancy = curSampleRedundancy;
                    worstRedundancyIndex = si;
                }
            }
            i = worstRedundancyIndex;
        }
 
        //move selected replacement to 'oldf' vector to retain the old values in case we revert back
        //to that state
        std::memcpy(oldf.data(), features.data() + (i * nFeat), nFeat * sizeof(T));
 
        //select a new index
        Point<T> p;
        clhs.getRandomPoint(p);
        while (points.contains((((uint64_t) p.x) << 32) | ((uint64_t) p.y))) {
            clhs.getRandomPoint(p);
        }
 
        //move new features into feature vector
        std::memcpy(features.data() + (i * nFeat), p.p_features, nFeat * sizeof(T));
    
        //recalculate sample count per quantile
        std::vector<int> oldq(nFeat);
        std::vector<int> newq(nFeat);
        for (int f = 0; f < nFeat; f++) {
            //decrement based removal of on old features
            int q = getQuantile(oldf[f], quantiles[f]);
            oldq[f] = q;
            sampleCountPerQuantile[(f * nSamp) + q]--;
 
            //increment based on inputof new features
            q = getQuantile(p.p_features[f], quantiles[f]);
            newq[f] = q;
            sampleCountPerQuantile[(f * nSamp) + q]++;
        }
 
        //recalculate objective function from quantiles
        T newObjQ = clhs.quantileObjectiveFunc(sampleCountPerQuantile);
        
        //recalculate correlation matrix
        const auto result = oneapi::dal::compute(cor_desc, table); // we update the table in place
        oneapi::dal::row_accessor<const T> acc {result.get_cor_matrix()};
        for (int j = 0; j < nFeat; j++) {
            auto row = acc.pull({j, j + 1});
 
            for (int k = 0; k < nFeat; k++) {
                corr[j][k] = row[k];
            }
        }
 
        //recalculate objective function from correlation matrix
        T newObjC = clhs.correlationObjectiveFunc(corr);
 
        T newObj = newObjQ + newObjC;
        T delta = newObj - obj;
 
        bool keep = dist(rng) < std::exp(-1 * delta / temp);
 
        if (keep) {
            //update the new changes
            points.erase((((uint64_t) x[i]) << 32) | ((uint64_t) y[i]));
            points.insert((((uint64_t) p.x) << 32) | ((uint64_t) p.y));
 
            x[i] = p.x;
            y[i] = p.y;
        
            std::memcpy(quantilesOfEachSample.data() + (i * nFeat), newq.data(), sizeof(int) * nFeat);
 
            objC = newObjC;
            objQ = newObjQ;
            obj = newObj;
        }
        else {
            //revert back to old changes
            for (int f = 0; f < nFeat; f++) {
                sampleCountPerQuantile[(f * nSamp) + newq[f]]--;
                sampleCountPerQuantile[(f * nSamp) + oldq[f]]++;
            }
 
            std::memcpy(
                reinterpret_cast<void *>(features.data() + (i * nFeat)),
                reinterpret_cast<void *>(oldf.data()),
                nFeat * sizeof(T)
            );
        }
 
        //update annealing temperature
        temp -= d;
    }
 
    //add samples to output layer
    helper::Field fieldExistingFalse("existing", 0);
    for (int i = starti ; i < nSamp; i++) {
        const auto [xCoord, yCoord] = helper::sample_to_point(GT, x[i], y[i]);
        OGRPoint point = OGRPoint(xCoord, yCoord);
        existing.used ? 
            helper::addPoint(&point, p_layer, &fieldExistingFalse) :
            helper::addPoint(&point, p_layer);
 
        if (plot) {
            xCoords.push_back(xCoord);
            yCoords.push_back(yCoord);
        }
    }
}

std::tuple<std::vector<std::vector<double>>, vector::GDALVectorWrapper *>
clhs(
    raster::GDALRasterWrapper *p_raster, 
    int nSamp,
    int iterations,
    vector::GDALVectorWrapper *p_access,
    std::string layerName,
    double buffInner,
    double buffOuter,
    vector::GDALVectorWrapper *p_existing,
    size_t replace,
    bool plot,
    std::string tempFolder,
    std::string filename)
{
    GDALAllRegister();
 
    int width = p_raster->getWidth();
    int height = p_raster->getHeight();
    int nFeat = p_raster->getBandCount();
    double *GT = p_raster->getGeotransform();
    
    std::vector<double> xCoords, yCoords;
 
    std::vector<helper::RasterBandMetaData> bands(p_raster->getBandCount());
    for (int i = 0; i < nFeat; i++) {
        bands[i].p_band = p_raster->getRasterBand(i);
        bands[i].type = p_raster->getRasterBandType(i);
        bands[i].size = p_raster->getRasterBandTypeSize(i);
        bands[i].nan = bands[i].p_band->GetNoDataValue();
        bands[i].p_band->GetBlockSize(&bands[i].xBlockSize, &bands[i].yBlockSize);
    }
 
    //create output dataset before doing anything which will take a long time in case of failure.
    GDALDriver *p_driver = GetGDALDriverManager()->GetDriverByName("MEM");
    if (!p_driver) {
        throw std::runtime_error("unable to create output sample dataset driver.");
    }
    GDALDataset *p_samples = p_driver->Create("", 0, 0, 0, GDT_Unknown, nullptr);
    if (!p_samples) {
        throw std::runtime_error("unable to create output dataset with driver.");
    }
 
    vector::GDALVectorWrapper *p_wrapper = new vector::GDALVectorWrapper(p_samples, std::string(p_raster->getDataset()->GetProjectionRef()));
    OGRLayer *p_layer = p_samples->CreateLayer("samples", p_wrapper->getSRS(), wkbPoint, nullptr);
    if (!p_layer) {
        throw std::runtime_error("unable to create output dataset layer.");
    }
 
    access::Access access(
        p_access,
        p_raster,
        layerName,
        buffInner,
        buffOuter,
        true,
        tempFolder,
        bands[0].xBlockSize,
        bands[0].yBlockSize
    );
 
    existing::Existing existing(
        p_existing,
        p_raster,
        GT,
        width,
        p_layer,
        false,
        xCoords,
        yCoords,
        false   
    );
 
    //fast random number generator using xoshiro256++
    //https://vigna.di.unimi.it/ftp/papers/ScrambledLinear.pdf
    xso::xoshiro_4x64_plus rng;
    uint64_t multiplier = helper::getProbabilityMultiplier(
        width, 
        height, 
        p_raster->getPixelWidth(), 
        p_raster->getPixelHeight(), 
        8, 
        MILLION * 100, 
        false, 
        access.area
    );
    helper::RandValController rand(bands[0].xBlockSize, bands[0].yBlockSize, multiplier, &rng);
 
    //get data type for all bands
    GDALDataType type = GDT_Float32;
    for (const helper::RasterBandMetaData& band : bands) {
        if (band.type == GDT_Float64) {
            type = GDT_Float64;
            break;
        }
    }
 
    if (type == GDT_Float64) {  
        std::vector<std::vector<double>> quantiles;
        
        //create instance of data management class
        CLHSDataManager<double> clhs(nFeat, nSamp, &rng, existing.count());
 
        //read raster, calculating quantiles, correlation matrix, and adding points to sample from.
        readRaster<double>(bands, clhs, access, existing, rand, type, quantiles, sizeof(double), width, height, nFeat, nSamp);
 
        //select samples and add them to output layer
        selectSamples<double>(quantiles, clhs, rng, existing, replace, iterations, nSamp, nFeat, p_layer, GT, plot, xCoords, yCoords);
    }
    else { //type == GDT_Float32    
        std::vector<std::vector<float>> quantiles;
 
        //create instance of data management class
        CLHSDataManager<float> clhs(nFeat, nSamp, &rng, existing.count());
 
        //read raster, calculating quantiles, correlation matrix, and adding points to sample from.
        readRaster<float>(bands, clhs, access, existing, rand, type, quantiles, sizeof(float), width, height, nFeat, nSamp);
 
        //select samples and add them to output layer
        selectSamples<float>(quantiles, clhs, rng, existing, replace, iterations, nSamp, nFeat, p_layer, GT, plot, xCoords, yCoords);
    }
 
    if (filename != "") {
        try {
            p_wrapper->write(filename);
        }
        catch (const std::exception& e) {
            std::cout << "Exception thrown trying to write file: " << e.what() << std::endl;
        }
    }
 
    return {{xCoords, yCoords}, p_wrapper};
}
 
} //namespace clhs
} //namespace sgs