260 Timing Code.cpp

#include <iostream>
#include <future>
#include <chrono>
#include <thread>
#include <vector>
#include <mutex>
#include <cmath>
#include <iomanip>

using namespace std;

/*
 *   Slow Leibniz approximation.
 */
double calculate_pi(int terms, int start, int skip)
{
    double sum = 0.0;

    for (int i = start; i < terms; i += skip)
    {
        int sign = pow(-1, i);
        double term = 1.0 / (i * 2 + 1);
        sum += sign * term;
    }

    return sum * 4;
}

int main()
{
    vector<shared_future<double>> futures;

    const int CONCURRENCY = thread::hardware_concurrency();

    auto start = chrono::steady_clock::now();

    for (int i = 0; i < CONCURRENCY; i++)
    {
        shared_future<double> f = async(launch::async, calculate_pi, 1E8, i, CONCURRENCY);
        futures.push_back(f);
    }

    double sum = 0.0;

    for (auto f : futures)
    {
        sum += f.get();
    }

    auto end = chrono::steady_clock::now();

    auto duration = chrono::duration_cast<chrono::milliseconds>(end - start).count();

    cout << "Duration: " << duration << endl;
    cout << setprecision(15) << "PI:  " << M_PI << endl;
    cout << setprecision(15) << "Sum: " << sum << endl;

    return 0;
}