aca-tasks/task1/mergesort.h

#pragma once

#include <vector>
#include <algorithm>
#include <thread>
#include <mutex>
#include <future>
#include <ranges>
#include <span>

namespace algo {

    class MergeSort_v1 {
    private:
        template<typename Iterator, typename Comparator>
        static auto
        merge(Iterator start, Iterator middle, Iterator end, Comparator cmp, Iterator output_start) -> void {
            Iterator start_m = start;
            Iterator begin = output_start;
            Iterator start2 = middle + 1;

            //merge from input until one half completes
            while (start <= middle && start2 <= end) {
                if (cmp(*start, *start2)) {
                    *output_start = *start;
                    start++;
                } else {
                    *output_start = *start2;
                    start2++;
                }
                output_start++;
            }

            //try to finish first half
            while (start <= middle) {
                *output_start = *start;
                start++;
                output_start++;
            }

            while (start2 <= end) {
                *output_start = *start2;
                start2++;
                output_start++;
            }

            const auto size = std::distance(start_m, end);
            for (auto i = 0; i <= size; i++, start_m++, begin++) {
                *start_m = *begin;
            }

        }

        template<typename Container, typename Iterator, typename Comparator>
        static auto
        ms_split(Container &output_vec, Iterator start, Iterator end, Comparator cmp, Iterator output_start) -> void {
            Iterator mid = start;
            Iterator begin = output_start;

            if (std::distance(start, end) < 1) {
                return;
            } else {
                //move mid iterator litterally to the mid
                std::advance(mid, std::distance(start, end) / 2);
                //sort the first half within an recursion
                ms_split(output_vec, start, mid, cmp, output_start);

                //move output iterator
                std::advance(output_start, std::distance(start, mid + 1));
                //sort the second half within a recursion
                ms_split(output_vec, mid + 1, end, cmp, output_start);

                //merge everything together starting from the complete beginning
                merge(start, mid, end, cmp, begin);
            }
        }

    public:
        template<typename Iterator, typename Comparator>
        static auto sort(Iterator start, Iterator end, Comparator cmp) -> void {
            using valtype = typename std::iterator_traits<Iterator>::value_type;
            std::vector<valtype> temporary_dataset(std::distance(start, end));
            ms_split(temporary_dataset, start, end - 1, cmp, temporary_dataset.begin());
        }
    };

    class MergeSort_v2 {
    private:

        template<typename Container, typename Comparator>
        static auto
        mt_merge(Container left, Container right, Comparator cmp) -> Container {
            //using Iterator = typename std::iterator_traits<Container>::value_type;

            Container output;

            auto lefti = left.begin();
            auto righti = right.begin();

            while (lefti < left.end() && righti < right.end()) {
                if (cmp(*lefti, *righti)) {
                    output.emplace_back(std::move(*lefti));
                    lefti++;
                } else {
                    output.emplace_back(std::move(*righti));
                    righti++;
                }
            }

            while (lefti < left.end()) {
                output.emplace_back(std::move(*lefti));
                lefti++;
            }
            while (righti < right.end()) {
                output.emplace_back(std::move(*righti));
                righti++;
            }

            return output;
        }

        template<typename Iterator, typename Comparator>
        static auto
        merge(Iterator start, Iterator middle, Iterator end, Comparator cmp, Iterator output_start,
              std::recursive_mutex &dataset_guard) -> void {
            Iterator start_m = start;
            Iterator begin = output_start;
            Iterator start2 = middle + 1;

            //merge from input until one half completes
            while (start <= middle && start2 <= end) {
                if (cmp(*start, *start2)) {
                    *output_start = *start;
                    start++;
                } else {
                    *output_start = *start2;
                    start2++;
                }
                output_start++;
            }

            //try to finish first half
            while (start <= middle) {
                *output_start = *start;
                start++;
                output_start++;
            }

            while (start2 <= end) {
                *output_start = *start2;
                start2++;
                output_start++;
            }

            dataset_guard.lock();
            const auto size = std::distance(start_m, end);
            for (auto i = 0; i <= size; i++, start_m++, begin++) {
                *start_m = *begin;
            }
            dataset_guard.unlock();

        }

        template<typename Container, typename Iterator, typename Comparator>
        static auto mt_split(Container &output_vec, Iterator start, Iterator end, Comparator cmp, Iterator output_start,
                             int &nthreads, std::recursive_mutex &dataset_guard, std::mutex &depth_guard) -> void {
            Iterator mid = start;
            Iterator to_start = output_start;

            if (std::distance(start, end) < 1) {
                return;

            }

            bool rem_threads;
            {
                std::lock_guard guard(depth_guard); //RAII guard
                rem_threads = nthreads > 1;
            }

            if (rem_threads) {
                {
                    std::lock_guard guard(depth_guard); //RAII guard
                    nthreads -= 2;
                }

                std::advance(mid, std::distance(start, end) / 2);
                std::thread t1([&]() {
                    mt_split(output_vec, start, mid, cmp, output_start, nthreads, dataset_guard, depth_guard);
                });

                std::advance(output_start, std::distance(start, mid + 1));
                std::thread t2([&]() {
                    mt_split(output_vec, mid + 1, end, cmp, output_start, nthreads, dataset_guard, depth_guard);
                });


                //merge everything together starting from the complete beginning
                t1.join();
                t2.join();
                std::vector<int> left;
                left.assign(start, mid);
                std::vector<int> right;
                right.assign(mid + 1, end);

                mt_merge(left, right, cmp);
            } else {
                //move mid iterator litterally to the mid
                std::advance(mid, std::distance(start, end) / 2);
                //sort the first half within an recursion
                mt_split(output_vec, start, mid, cmp, output_start, nthreads, dataset_guard, depth_guard);

                //move output iterator
                std::advance(output_start, std::distance(start, mid + 1));
                //sort the second half within a recursion
                mt_split(output_vec, mid + 1, end, cmp, output_start, nthreads, dataset_guard, depth_guard);

                std::vector<int> left;
                left.assign(start, mid);
                std::vector<int> right;
                right.assign(mid + 1, end);

                mt_merge(left, right, cmp);
            }
        }

        template<typename Container, typename Iterator, typename Comparator>
        static auto ms_split(Container &output_vec, Iterator start, Iterator end, Comparator cmp, Iterator output_start,
                             int &nthreads, std::recursive_mutex &dataset_guard, std::mutex &depth_guard) -> void {
            Iterator mid = start;
            Iterator begin = output_start;

            if (std::distance(start, end) < 1) {
                //Quit on smalles list size (one element is always sorted)
                return;
            } else {
                if (nthreads > 1) {
                    depth_guard.lock();
                    nthreads -= 2;
                    depth_guard.unlock();
                    //move mid iterator litterally to the mid
                    std::advance(mid, std::distance(start, end) / 2);
                    //sort the first half within an recursion

                    std::thread t1([&]() {
                        ms_split(output_vec, start, mid, cmp, output_start, nthreads, dataset_guard, depth_guard);
                    });

                    //move output iteratoroutput_vec, start, mid, cmp, output_start
                    std::advance(output_start, std::distance(start, mid + 1));
                    //sort the second half within a recursion
                    std::thread t2([&]() {
                        ms_split(output_vec, mid + 1, end, cmp, output_start, nthreads, dataset_guard, depth_guard);
                    });


                    //merge everything together starting from the complete beginning
                    t1.join();
                    t2.join();
                    merge(start, mid, end, cmp, begin, dataset_guard);
                } else {
                    //move mid iterator litterally to the mid
                    std::advance(mid, std::distance(start, end) / 2);
                    //sort the first half within an recursion
                    ms_split(output_vec, start, mid, cmp, output_start, nthreads, dataset_guard, depth_guard);

                    //move output iterator
                    std::advance(output_start, std::distance(start, mid + 1));
                    //sort the second half within a recursion
                    ms_split(output_vec, mid + 1, end, cmp, output_start, nthreads, dataset_guard, depth_guard);

                    //merge everything together starting from the complete beginning
                    merge(start, mid, end, cmp, begin, dataset_guard);
                }
            }
        }

    public:
        template<typename Iterator, typename Comparator>
        static auto sort(Iterator start, Iterator end, Comparator cmp, int nthreads) -> void {
            using valtype = typename std::iterator_traits<Iterator>::value_type;
            std::vector<valtype> temporary_dataset(std::distance(start, end));
            std::recursive_mutex dataset_guard;
            std::mutex depth_guard;

            mt_split(temporary_dataset, start, end - 1, cmp, temporary_dataset.begin(), nthreads, dataset_guard,
                     depth_guard);
        }
    };

    class MergeSort_mt {

        template<typename T, typename Comparator>
        static auto
        merge(typename std::vector<T>::iterator mid, std::vector<T> &left, std::vector<T> &right,
              Comparator cmp, std::mutex &mut) {

            auto l = left.begin();
            auto r = right.begin();

            while (l < left.end() && r < right.end()) {
                if (cmp(*l, *r)) {
                    *mid = *l;
                    l++;
                } else {
                    *mid = *r;
                    r++;
                }
                mid++;
            }
            while (l < left.end()) {
                *mid = *l;
                mid++;
                l++;
            }
            while (r < right.end()) {
                *mid = *r;
                mid++;
                r++;
            }

        }

        template<typename T, typename Comparator>
        static auto split(std::vector<T> &data, std::vector<T> &output, Comparator cmp, int depth, int &num_threads,
                          std::mutex &mut) {
            if (data.size() <= 1) {
                return;
            }

            auto mid = data.begin();
            std::advance(mid, std::distance(data.begin(), data.end()) / 2);


            std::vector<T> left(data.begin(), mid);
            std::vector<T> right(mid, data.end());

            if (depth < num_threads) {
                std::thread left_thread([&]() { split(left, output, cmp, depth + 1, num_threads, mut); });
                std::thread right_thread([&]() { split(right, output, cmp, depth + 1, num_threads, mut); });

                left_thread.join();
                right_thread.join();
            } else {
                split(left, output, cmp, depth + 1, num_threads, mut);
                split(right, output, cmp, depth + 1, num_threads, mut);
            }


            mid = output.begin();
            std::advance(mid, std::distance(output.begin(), output.end()) / 2);

            std::vector<T> buf;
            buf.reserve(data.size());
            merge(buf.begin(), left, right, cmp, mut);
            {
                std::lock_guard lock(mut);
                auto bufi = buf.begin();
                for(auto &element : output) {
                    element = *bufi;
                    bufi++;
                }

            }
        }

    public:
        template<typename T, typename Comparator>
        static auto
        sort(std::vector<T> &data, Comparator cmp, int num_threads = std::thread::hardware_concurrency()) -> void {
            std::vector<T> local_result = data;
            std::mutex local_result_lock;

            split(data, local_result, cmp, 0, num_threads, local_result_lock);
            data = local_result;
        }
    };
}