Add pi calculation using manually spawned threads

Makefile

 # This Makefile compiles and flashes four different programs:
 
-CPPFLAGS=-Wall -O3 -pthread -std=c++11
+CPPFLAGS=-Wall -O3 -pthread -std=c++11 -ffast-math
 
 %: %.cpp
 	g++ $(CPPFLAGS) -o build/$@ $<

pi_threads.cpp

+#include <array>
+#include <chrono>
+#include <iostream>
+#include <numeric>
+#include <thread>
+
+int main() {
+    constexpr double a = 0.5;
+    constexpr double b = 0.75;
+    constexpr double c = 0.25;
+
+    // n_threads * n_iterations_per_thread + 1 must fit in a uint32_t
+    constexpr uint32_t n_threads = 4u;
+    constexpr uint32_t n_iterations_per_thread = 100000000u;
+
+    std::array<std::thread, n_threads> threads;
+    std::array<double, n_threads> thread_results;
+
+    const auto pi_sum = [&](uint32_t index) {
+        double& value = thread_results[index];
+        value = 0;
+        const uint32_t i_begin = n_iterations_per_thread * index + 1;
+        const uint32_t i_end = n_iterations_per_thread * (index + 1);
+        for (uint32_t i = i_begin; i <= i_end; ++i) {
+            value += a / ((i - b) * (i - c));
+        }
+    };
+
+    const auto start_time = std::chrono::steady_clock::now();
+    for (auto i = 0u; i < n_threads; ++i) {
+        threads[i] = std::thread(pi_sum, i);
+    }
+
+    for (auto i = 0u; i < n_threads; ++i) {
+        threads[i].join();
+    }
+
+    const auto end_time = std::chrono::steady_clock::now();
+
+    double pi = std::accumulate(thread_results.begin(), thread_results.end(), double(0));
+
+    std::cout << "Final value: " << pi << '\n';
+    const auto microseconds = std::chrono::duration_cast<std::chrono::microseconds>(end_time - start_time).count();
+    std::cout << "Time (microseconds): " << microseconds << '\n';
+    const auto mega_flops = n_threads * n_iterations_per_thread * 5.0 / microseconds;
+    std::cout << "MFLOPS: " << mega_flops << '\n';
+
+    return 0;
+}
+