From bbf0c0346f48e68b2abb10d8f3105a5879b3004a Mon Sep 17 00:00:00 2001
From: robtor <robtor2000@web.de>
Date: Wed, 1 Nov 2023 12:32:52 +0100
Subject: [PATCH] MT impl WIP

---
 task1/main.cpp    |  6 ++++--
 task1/mergesort.h | 16 ++++++++--------
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/task1/main.cpp b/task1/main.cpp
index 6297ca5..0a31d51 100644
--- a/task1/main.cpp
+++ b/task1/main.cpp
@@ -62,14 +62,16 @@ auto main(int argc, char *argv[]) -> int {
         auto delay_ms = std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1);
         fmt::print("Sorted {} entries within {} ms in sequential\n", dataset_seq.size(), delay_ms.count());
 
+
+        static constexpr int nthreads = 16;
         t1 = std::chrono::high_resolution_clock::now();
         algo::MergeSort_v2::sort(dataset_par.begin(), dataset_par.end(), [](int32_t a, int32_t b) {
             return (a > b);
-        }, 16);
+        }, nthreads);
         t2 = std::chrono::high_resolution_clock::now();
 
         delay_ms = std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1);
-        fmt::print("Sorted {} entries within {} ms in parallel using {} threads\n", dataset_seq.size(), delay_ms.count(), 16);
+        fmt::print("Sorted {} entries within {} ms in parallel using {} threads\n", dataset_seq.size(), delay_ms.count(), nthreads);
 
         auto eq = (dataset_seq == dataset_par);
         fmt::print("Equality: {}\n", eq);
diff --git a/task1/mergesort.h b/task1/mergesort.h
index 18d4ef7..18377aa 100644
--- a/task1/mergesort.h
+++ b/task1/mergesort.h
@@ -83,10 +83,9 @@ namespace algo {
 
     class MergeSort_v2 {
     private:
-        //Caution: non thread-safe, must be executed with additional protection (e.g. lock_guard)
         template<typename Iterator, typename Comparator>
         static auto
-        merge(Iterator start, Iterator middle, Iterator end, Comparator cmp, Iterator output_start) -> void {
+        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;
@@ -116,20 +115,23 @@ namespace algo {
                 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 ms_split(Container &output_vec, Iterator start, Iterator end, Comparator cmp, Iterator output_start,
-                             int &nthreads, std::mutex &dataset_guard, std::mutex &depth_guard) -> void {
+                             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) {
@@ -151,8 +153,7 @@ namespace algo {
                     //merge everything together starting from the complete beginning
                     t1.join();
                     t2.join();
-                    const std::lock_guard<std::mutex> lock(dataset_guard);
-                    merge(start, mid, end, cmp, begin);
+                    merge(start, mid, end, cmp, begin, dataset_guard);
                 } else {
                     //move mid iterator litterally to the mid
                     std::advance(mid, std::distance(start, end) / 2);
@@ -165,8 +166,7 @@ namespace algo {
                     ms_split(output_vec, mid + 1, end, cmp, output_start, nthreads, dataset_guard, depth_guard);
 
                     //merge everything together starting from the complete beginning
-                    const std::lock_guard<std::mutex> lock(dataset_guard);
-                    merge(start, mid, end, cmp, begin);
+                    merge(start, mid, end, cmp, begin, dataset_guard);
                 }
             }
         }
@@ -176,7 +176,7 @@ namespace algo {
         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::mutex dataset_guard;
+            std::recursive_mutex dataset_guard;
             std::mutex depth_guard;
 
             ms_split(temporary_dataset, start, end - 1, cmp, temporary_dataset.begin(), nthreads, dataset_guard, depth_guard);