aca-tasks/task1/include/mergesort_mt.h

#include <vector>
#include <span>
#include <thread>
#include <mutex>
#include <functional>

// General purpose mergesorter with multi threading support by Robin Dietzel <robin.dietzel@iem.thm.de>
template<typename T>
class MergeSorterMT {

public:
    template<typename C>
    MergeSorterMT(C cmp, int max_depth) : cmp(cmp), max_depth(max_depth) {
        // Assert that cmp is a function that returns bool and takes two arguments of type T
        static_assert(std::is_same<std::invoke_result_t<C, T, T>, bool>(), "C must be a function that returns a bool");
    }

    // Start sorting process
    auto sort(std::vector<T> &data) -> void {
        // Create span: like a 'view' on the vector -> no unnecessary copies are made when subdividing sorting problem
        std::span<T> sortable(data);
        split(sortable, 0, max_depth);
    }

private:
    // Merge function that merges left & right span into the output span
    // No exclusive access on output is necessary (e.g. via mutex) because all parallel threads work on different parts of output
    auto merge(std::span<T> &output, std::span<T> left, std::span<T> right) -> void {
        // Create buffer, here we need a temporary container where we copy values to, because left and right are a view on parts
        // of output
        std::vector<T> buf;
        buf.reserve(left.size() + right.size());

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

        // Insert from pre sorted half's
        while (l < left.end() && r < right.end()) {
            if (cmp(*l, *r)) {
                buf.insert(o, *l);
                l++;
            } else {
                buf.insert(o, *r);
                r++;
            }
            o++;
        }

        // Fill up with rest of left values
        while (l < left.end()) {
            buf.insert(o, *l);
            o++;
            l++;
        }

        // Fill up with rest of right values
        while (r < right.end()) {
            buf.insert(o, *r);
            o++;
            r++;
        }

        // Completely move buffer to output
        // IMPORTANT: left and right are still a view on the splitted output, that is now sorted
        std::move(buf.begin(), buf.end(), output.begin());
    }

    // Splitup function
    auto split(std::span<T> &data, int depth, const int &mdepth) -> void {

        if (std::distance(data.begin(), data.end()) <= 1) {
            // Quit if only one element 'insortable'
            return;
        } else if (std::distance(data.begin(), data.end()) == 2) {
            // Swap two values dependant on size for small speedup (no call to further split must be made)
            if(cmp(data[1], data[0])) {
                std::swap(data[0], data[1]);
                return;
            }
        }

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

        // Generate left and right view on data (no copies are made here)
        std::span<T> left(data.begin(), mid);
        std::span<T> right(mid, data.end());

        if (depth < mdepth) {
            // Create recursive split functions if maximum depth not reached
            std::thread left_thread([&]() { split(left, depth + 1, mdepth); });
            std::thread right_thread([&]() { split(right, depth + 1, mdepth); });

            // Both threads must join before we could further work on the data viewed
            // by left and right (recursively sorted by the both calls)
            left_thread.join();
            right_thread.join();
        } else {
            // Do normal recursion in a single thread if maximum depth is reached
            split(left, depth + 1, mdepth);
            split(right, depth + 1, mdepth);
        }

        // Merge left and right together before returning
        merge(data, left, right);
        return;
    }


private:
    // Templated comparator function
    std::function<bool(T, T)> cmp;
    // Maximum depth
    const int max_depth;
};