From 032f0c882a3470db93de44c9521a2ee47cbf3856 Mon Sep 17 00:00:00 2001
From: nihonium <nihonium@rape.lol>
Date: Sat, 13 May 2023 21:35:33 +0300
Subject: [PATCH] parallel max

---
 .../00problem_paralel_max.cpp                 | 100 ++++++++++++++++++
 1 file changed, 100 insertions(+)
 create mode 100644 term2/seminar01_thread/00problem_paralel_max.cpp

diff --git a/term2/seminar01_thread/00problem_paralel_max.cpp b/term2/seminar01_thread/00problem_paralel_max.cpp
new file mode 100644
index 0000000..59362b8
--- /dev/null
+++ b/term2/seminar01_thread/00problem_paralel_max.cpp
@@ -0,0 +1,100 @@
+/*
+    Задача:
+
+        В данном примере написана функция getMax, которая находит максимум из чисел в векторе.
+        При этом вычисления проходят однопоточно.
+
+
+        Вам нужно написать функцию     
+
+            uint64_t getMaxPar(int n, const std::vector<uint64_t>& v)
+
+        которая будет делать то же самое, но только использовать для этого n потоков.
+*/
+
+
+#include <iostream>
+#include <thread>
+#include <chrono>
+#include <vector>
+#include <random>
+#include <algorithm>
+#include <cstdint>
+#include <functional>
+using std::cout, std::endl, std::size_t;
+using namespace std::chrono_literals;
+
+uint64_t getMax(std::vector<uint64_t>::const_iterator first, std::vector<uint64_t>::const_iterator last)
+{
+    uint64_t res = *first;
+    for (; first != last; first++)
+        if (*first > res)
+            res = *first;
+    return res;
+}
+
+uint64_t getMax(const std::vector<uint64_t>& v) {
+    return getMax(v.begin(), v.end());
+}
+
+struct maxBlock {
+    void operator() (std::vector<uint64_t>::const_iterator first, std::vector<uint64_t>::const_iterator last, uint64_t& res) {
+        res = getMax(first, last); 
+    }
+};
+
+uint64_t getMax(int n, const std::vector<uint64_t>& v) {
+    std::vector<uint64_t> results(n);
+    std::vector<std::thread> threads(n - 1);
+    
+    unsigned long const block_size = v.size() / n;
+
+    auto block_start = v.begin();
+
+    for (int i = 0; i < n - 1; ++i) {
+        auto block_end = block_start;
+        std::advance(block_end, block_size);
+        threads[i] = std::thread(maxBlock(), 
+                        block_start, block_end, std::ref(results[i]));
+        block_start = block_end;
+    }
+    maxBlock() (block_start, v.end(), results[n - 1]);
+
+    std::for_each(threads.begin(), threads.end(), std::mem_fn(&std::thread::join));
+
+    return getMax(results);
+}
+
+
+int main()
+{
+    cout << "Generating numbers!" << endl;
+    std::vector<uint64_t> numbers(5e8);
+    numbers[0] = 123456789;
+    for (size_t i = 1; i < numbers.size(); ++i)
+    {
+        numbers[i] = numbers[i - 1] * i + 1;
+    }
+    cout << "Numbers generated!" << endl;
+
+    auto start = std::chrono::high_resolution_clock::now();
+
+    uint64_t m = getMax(numbers);
+
+    auto end = std::chrono::high_resolution_clock::now();
+
+    cout << "Maximum = " << m << endl;
+    cout << "Time to calclulate max = " << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count()
+         << " milliseconds." << endl;
+
+    /* Parallel */
+    start = std::chrono::high_resolution_clock::now();
+
+    m = getMax(4, numbers);
+
+    end = std::chrono::high_resolution_clock::now();
+
+    cout << "Maximum = " << m << endl;
+    cout << "Time to calclulate max = " << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count()
+         << " milliseconds." << endl;
+}