using System.Collections.Concurrent;
using System.Diagnostics;

namespace Demo
{
    class Program
    {
        static void Main()
        {
            var limit = 2000000;
            var numbers = Enumerable.Range(0, limit).ToList();
            var watch = Stopwatch.StartNew();
            var primeNumbersFromForeach = GetPrimeList(numbers); watch.Stop();
            var watchForParallel = Stopwatch.StartNew();
            var primeNumbersFromParallelForeach = GetPrimeListWithParallel(numbers);
            watchForParallel.Stop();
            Console.WriteLine($"Classical foreach loop | Total prime numbers: {primeNumbersFromForeach.Count} | Time Taken: {watch.ElapsedMilliseconds} ms.");
            Console.WriteLine($"Parallel.ForEach loop | Total prime numbers : {primeNumbersFromParallelForeach.Count} | Time Taken:{watchForParallel.ElapsedMilliseconds} ms.");
            Console.ReadLine();
        }
        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();
        }

        /// <summary>
        /// 判断是否为素数
        /// </summary>
        /// <param name="number"></param>
        /// <returns></returns>
        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;
        }
    }
}