/*
 * Created by SharpDevelop.
 * User: Admin
 * Date: 11/10/2021
 * Time: 6:28 AM
 * 
 * To change this template use Tools | Options | Coding | Edit Standard Headers.
 */
using System;
using System.Diagnostics;

namespace SimplePrimes
{
	class Program
	{
		public static void Main(string[] args)
		{
			//bool[] nums;
			byte[] nums;
			int n, m, lim;
			Stopwatch elapsed;
			string yorn;
			
			Console.WriteLine("This program will search for prime numbers using a simple algorithm . . .");
			Console.Write("Enter max limit # and press ENTER (leave blank for default): ");
			lim = Convert.ToInt32("0" + Console.ReadLine());  // WE HAVE TO PUT THE 0 IN FRONT TO FORCE STUPID C# TO NOT HAVE ERROR IF INPUT IS BLANK...
			// C# SHOULD RETURN 0 FOR A BLANK STRING AND ONLY EXCEPTION ON NULL STRING, BUT I DIDN'T DESIGN THIS :P
			if (lim == 0)
				lim = 100000000;
			
			elapsed = new Stopwatch();
			elapsed.Start();
			//nums = new bool[lim + 1];
			nums = new byte[lim + 1];
			
			for (n = 2; n < Math.Sqrt(lim); n++)
			{
				if (nums[n] == 0)
				{
					m = n * n;
					while (m <= lim)
					{
						nums[m] = 1;
						m += n;
					}
				}
			}
			
			elapsed.Stop();
			Console.WriteLine("It took " + (elapsed.ElapsedMilliseconds / 1000.0).ToString() + " seconds to complete.");
			Console.Write("Do you want to print the results (Y/N)? ");
			yorn = Console.ReadLine();
			
			if (yorn == "Y" || yorn == "y")
			{
				m = 0;
				for (n = 2; n <= lim; n++)
				{
					if(nums[n] == 0)
					{
						Console.WriteLine(n.ToString());
						m++;
					}
				}
				Console.WriteLine("Total primes = " + m.ToString());
				Console.WriteLine("");
				Console.Write("Press any key to end . . . ");
				Console.ReadKey(true);
			}
		}
	}
}