using System;

using System.Collections;

using System.Collections.Generic;

using System.Collections.Concurrent;

using System.Linq;

using System.Diagnostics;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        var limit = 2000000;

        var numbers = Enumerable.Range(0, limit).ToList();

        var watch = Stopwatch.StartNew();

        var primeNumbers = GetPrimeList(numbers);

        watch.Stop();

        var watch2 = Stopwatch.StartNew();

        var primeNumbersFromParallel = GetPrimeListWithParallel(numbers);

        Console.WriteLine($"Using Normal ForLoop, Total Time: {watch.ElapsedMilliseconds}, Total Prime: {primeNumbers.Count}");

        Console.WriteLine($"Using Parallel ForEachLoop, Total Time: {watch2.ElapsedMilliseconds}, Total Prime: {primeNumbersFromParallel.Count}");

    }

    private static IList<int> GetPrimeList(IList<int> numbers) => numbers.Where(IsPrime).ToList();

    private static IList<int> GetPrimeListWithParallel(IList<int> numbers) 

    {

            var primeNumbers = new ConcurrentBag<int>();

            Parallel.ForEach(numbers, number => {

                if(IsPrime(number))

                    primeNumbers.Add(number);

            });

            return primeNumbers.ToList();

    }

    private static bool IsPrime(int number) 

    {

        if(number < 2)

            return false;

        for(var divisor = 2; divisor <= Math.Sqrt(number); divisor++) 

        {

            if(number % divisor == 0)

                return false;

        }

        return true;

    }

}