using System;

using System.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Collections.Generic;

using System.Diagnostics;

public class Program

{

    private static readonly Regex Base64Regex = new(@"^[a-zA-Z0-9\+/]*={0,3}$", RegexOptions.Compiled);

    private static Random random = new Random();

​

​

    public static void Main()

    {

        // create 1000 random strings, some of which are base64, some are not 

        var strings = new List<string>();

        for(var i = 0; i < 500_000; i++)

        {

            var size = random.Next(5, 50);

            var str = RandomString(size);

            if (random.Next(10) >= 5)

            {

                str = EncodeToBase64(str);

            }

            strings.Add(str);

        }

        Stopwatch stopwatch = new Stopwatch();

        var base64Strings = 0;

        var notBase64Strings = 0;

        stopwatch.Start();

        for (var i = 0; i < strings.Count; i++)

        {

            if (isBase64StringPrefiltered(strings[i]))

            {

                base64Strings++;

            }

            else

            {

                notBase64Strings++;

            }

        }

        stopwatch.Stop();

        Console.WriteLine("Prefiltered Results: " + (stopwatch.ElapsedMilliseconds / 1_000.0) + " seconds elapsed.\n" + base64Strings + " base64 strings " + notBase64Strings + " non-base64 strings"  );

        base64Strings = 0;

        notBase64Strings = 0;

        stopwatch.Reset();

        stopwatch.Start();

        for (var i = 0; i < strings.Count; i++)

        {

            if (isBase64StringUnfiltered(strings[i]))

            {

                base64Strings++;

            }

            else

            {

                notBase64Strings++;

            }

        }

        stopwatch.Stop();

        Console.WriteLine("Unfiltered Results: " + (stopwatch.ElapsedMilliseconds / 1_000.0) + " seconds elapsed.\n" + base64Strings + " base64 strings " + notBase64Strings + " non-base64 strings"  );

    }

    private static string RandomString(int length)

    {

        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 ";

        return new string(Enumerable.Range(1, length).Select(_ => chars[random.Next(chars.Length)]).ToArray());

    }

    private static bool isBase64StringPrefiltered(string input)

    {

        return !(string.IsNullOrWhiteSpace(input)

                || input.Length % 4 != 0

                || input.Contains(" ")

                || input.Contains("\t")

                || input.Contains("\r")

                || input.Contains("\n")

                || !Base64Regex.IsMatch(input));

    }

    private static bool isBase64StringUnfiltered(string input)

    {

        return input.Length % 4 == 0 && Base64Regex.IsMatch(input);

    }

    private static string EncodeToBase64(string input)

    {

        return Convert.ToBase64String(Encoding.UTF8.GetBytes(input));

    }

}