InitializeVariancePartition.hpp

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#ifndef KMEANS_INITIALIZE_VARIANCE_PARTITION_HPP
#define KMEANS_INITIALIZE_VARIANCE_PARTITION_HPP
 
#include <vector>
#include <algorithm>
#include <numeric>
#include <queue>
#include <cstdint>
 
#include "Initialize.hpp"
#include "Matrix.hpp"
 
namespace kmeans {
 
struct InitializeVariancePartitionOptions {
    double size_adjustment = 1;
 
    bool optimize_partition = true;
};
 
namespace InitializeVariancePartition_internal {
 
template<typename Float_>
void compute_welford(Float_ val, Float_& center, Float_& ss, Float_ count) {
    auto cur_center = center;
    Float_ delta = val - cur_center;
    Float_ new_center = cur_center + delta / count;
    center = new_center;
    ss += (val - new_center) * delta;
}
 
template<typename Data_, typename Float_>
void compute_welford(size_t ndim, const Data_* dptr, Float_* center, Float_* dim_ss, Float_ count) {
    for (size_t d = 0; d < ndim; ++d) {
        compute_welford<Float_>(dptr[d], center[d], dim_ss[d], count);
    }
}
 
template<typename Matrix_, typename Index_, typename Float_>
Float_ optimize_partition(
    const Matrix_& data,
    const std::vector<Index_>& current,
    size_t top_dim,
    std::vector<Float_>& value_buffer,
    std::vector<Float_>& stat_buffer)
{
    size_t N = current.size();
    auto work = data.new_extractor(current.data(), N);
    value_buffer.clear();
    for (size_t i = 0; i < N; ++i) {
        auto dptr = work->get_observation();
        value_buffer.push_back(dptr[top_dim]);
    }
 
    std::sort(value_buffer.begin(), value_buffer.end());
 
    stat_buffer.clear();
    stat_buffer.reserve(value_buffer.size() + 1);
    stat_buffer.push_back(0); // first and last entries of stat_buffer are 'nonsense' partitions, i.e., all points in one partition or the other.
 
    // Forward and backward iterations to get the sum of squares at every possible partition point.
    Float_ mean = 0, ss = 0, count = 1;
    for (auto val : value_buffer) {
        compute_welford<Float_>(val, mean, ss, count);
        stat_buffer.push_back(ss);
        ++count;
    }
 
    mean = 0, ss = 0, count = 1;
    auto sbIt = stat_buffer.rbegin() + 1; // skipping the last entry, as it's a nonsense partition.
    for (auto vIt = value_buffer.rbegin(), vEnd = value_buffer.rend(); vIt != vEnd; ++vIt) {
        compute_welford<Float_>(*vIt, mean, ss, count);
        *sbIt += ss;
        ++count;
        ++sbIt;
    }
 
    auto sbBegin = stat_buffer.begin(), sbEnd = stat_buffer.end();
    auto which_min = std::min_element(sbBegin, sbEnd) - sbBegin;
    if (which_min == 0) {
        // Should never get to this point as current.size() > 1,
        // but we'll just add this in for safety's sake.
        return value_buffer[0];
    } else {
        // stat_buffer[i] represents the SS when {0, 1, 2, ..., i-1} goes in
        // the left partition and {i, ..., N-1} goes in the right partition.
        // To avoid issues with ties, we use the average of the two edge
        // points as the partition boundary.
        return (value_buffer[which_min - 1] + value_buffer[which_min]) / 2;
    }
}
 
}
template<typename Index_, typename Data_, typename Cluster_, typename Float_, class Matrix_ = Matrix<Index_, Data_> >
class InitializeVariancePartition final : public Initialize<Index_, Data_, Cluster_, Float_, Matrix_> {
public:
    InitializeVariancePartition(InitializeVariancePartitionOptions options) : my_options(std::move(options)) {}
 
    InitializeVariancePartition() = default;
 
private:
    InitializeVariancePartitionOptions my_options;
 
public:
    InitializeVariancePartitionOptions& get_options() {
        return my_options;
    }
 
public:
    Cluster_ run(const Matrix_& data, Cluster_ ncenters, Float_* centers) const {
        auto nobs = data.num_observations();
        size_t ndim = data.num_dimensions();
        if (nobs == 0 || ndim == 0) {
            return 0;
        }
 
        std::vector<std::vector<Index_> > assignments(ncenters);
        assignments[0].resize(nobs);
        std::iota(assignments.front().begin(), assignments.front().end(), 0);
 
        std::vector<std::vector<Float_> > dim_ss(ncenters);
        {
            auto& cur_ss = dim_ss[0];
            cur_ss.resize(ndim);
            std::fill_n(centers, ndim, 0);
            auto matwork = data.new_extractor(0, nobs);
            for (decltype(nobs) i = 0; i < nobs; ++i) {
                auto dptr = matwork->get_observation();
                InitializeVariancePartition_internal::compute_welford(ndim, dptr, centers, cur_ss.data(), static_cast<Float_>(i + 1));
            }
        }
 
        std::priority_queue<std::pair<Float_, Cluster_> > highest;
        auto add_to_queue = [&](Cluster_ i) -> void {
            const auto& cur_ss = dim_ss[i];
            Float_ sum_ss = std::accumulate(cur_ss.begin(), cur_ss.end(), static_cast<Float_>(0));
 
            // Instead of dividing by N and then remultiplying by pow(N, adjustment), we just
            // divide by pow(N, 1 - adjustment) to save some time and precision.
            sum_ss /= std::pow(assignments[i].size(), 1.0 - my_options.size_adjustment);
 
            highest.emplace(sum_ss, i);  
        };
        add_to_queue(0);
 
        std::vector<Index_> cur_assignments_copy;
        std::vector<Float_> opt_partition_values, opt_partition_stats;
 
        for (Cluster_ cluster = 1; cluster < ncenters; ++cluster) {
            auto chosen = highest.top();
            if (chosen.first == 0) {
                return cluster; // no point continuing, we're at zero variance already.
            }
            highest.pop();
 
            auto* cur_center = centers + static_cast<size_t>(chosen.second) * ndim; // cast to size_t to avoid overflow issues.
            auto& cur_ss = dim_ss[chosen.second];
            auto& cur_assignments = assignments[chosen.second];
 
            size_t top_dim = std::max_element(cur_ss.begin(), cur_ss.end()) - cur_ss.begin();
            Float_ partition_boundary;
            if (my_options.optimize_partition) {
                partition_boundary = InitializeVariancePartition_internal::optimize_partition(data, cur_assignments, top_dim, opt_partition_values, opt_partition_stats);
            } else {
                partition_boundary = cur_center[top_dim];
            }
 
            auto* next_center = centers + static_cast<size_t>(cluster) * ndim; // again, size_t to avoid overflow.
            std::fill_n(next_center, ndim, 0);
            auto& next_ss = dim_ss[cluster];
            next_ss.resize(ndim);
            auto& next_assignments = assignments[cluster];
 
            cur_assignments_copy.clear();
            std::fill_n(cur_center, ndim, 0);
            std::fill(cur_ss.begin(), cur_ss.end(), 0);
            auto work = data.new_extractor(cur_assignments.data(), cur_assignments.size());
 
            for (auto i : cur_assignments) {
                auto dptr = work->get_observation(); // make sure this is outside the if(), as it must always be called in each loop iteration to match 'cur_assignments' properly.
                if (dptr[top_dim] < partition_boundary) {
                    cur_assignments_copy.push_back(i);
                    InitializeVariancePartition_internal::compute_welford(ndim, dptr, cur_center, cur_ss.data(), static_cast<Float_>(cur_assignments_copy.size()));
                } else {
                    next_assignments.push_back(i);
                    InitializeVariancePartition_internal::compute_welford(ndim, dptr, next_center, next_ss.data(), static_cast<Float_>(next_assignments.size()));
                }
            }
 
            // If one or the other is empty, then this entire procedure short
            // circuits as all future iterations will just re-select this
            // cluster (which won't get partitioned properly anyway). In the
            // bigger picture, the quick exit out of the iterations is correct
            // as we should only fail to partition in this manner if all points
            // within each remaining cluster are identical.
            if (next_assignments.empty() || cur_assignments_copy.empty()) {
                return cluster;
            }
 
            cur_assignments.swap(cur_assignments_copy);
            add_to_queue(chosen.second);
            add_to_queue(cluster);
        }
 
        return ncenters;
    }
};
 
}
 
#endif