// NY

using System;

using System.Threading;

using System.Threading.Tasks;

using System.Diagnostics;

public class Program

{

    // Data

    static int[] numbers = { 3, 4, 5, 1, 3, 4, 5, 1, 3, 4, 5, 1, 3, 4, 5, 1, 3, 4 };

    // Return factorial

    public static int f(int n) {

        int result = 1;

        while (n > 1) result *= n--;

        Thread.Sleep(10);   // simulate heavy load

        return result;

    }

    // Calculate the sum of factorial the normal way

    public static int GetSumNormal() {

        int sum = 0;

        foreach (int n in numbers) sum += f(n);

        return sum;

    }

    // Calculate the sum of factorial the threaded way

    public static int GetSumThreaded(int threadCount) {

        var tasks = new Task[threadCount];

        int _t_sum = 0;

        int _t_index = 0;

        for (int i = 0; i < threadCount; i++)

            tasks[i] = Task.Run(() => { 

                // Grab the next number until all exhausted

                while (_t_index < numbers.Length) {

                    int index = Interlocked.Increment(ref _t_index) - 1;

                    Interlocked.Add(ref _t_sum, f(numbers[index]));

                }

            });

        // Wait for the job to finish

        Task.WaitAll(tasks);

        return _t_sum;

    }

    public static void Main()

    {

        // Normal time

        Stopwatch stopwatch = Stopwatch.StartNew();

        int sum = GetSumNormal();

        stopwatch.Stop();

        Console.WriteLine($"Normal sum = {sum}, time = {stopwatch.Elapsed} ns");

​

        // Threaded time

        stopwatch.Restart();

        sum = GetSumThreaded(4);

        stopwatch.Stop();

        Console.WriteLine($"Threaded sum = {sum}, time = {stopwatch.Elapsed} ns");

    }

}