using System;

using System.Linq;

using System.Security.Cryptography;

using System.Text;

using System.Text.RegularExpressions;

​

​

public class Program

{

    public static void Main()

    {

        var bookId = "323232";

        Console.WriteLine(GenerateFakeIsbnNew(bookId));

        Console.WriteLine(GenerateFakeIsbnReverse(bookId));

    }

        public static string GenerateFakeIsbnReverse(string bookId)

        {

            using (SHA256 sha256 = SHA256.Create())

            {

                byte[] idBytes = Encoding.UTF8.GetBytes(bookId);

                byte[] hashBytes = sha256.ComputeHash(idBytes);

                // Convert the last 8 bytes of the hash to hexadecimal and concatenate 'F' at the start

                //TakeLast() only exists in dotnet core

                return "F" + string.Join("", hashBytes.Reverse().Take(8).Reverse().Select(b => b.ToString("X2")));

​

            }

        }

            public static string GenerateFakeIsbnNew(string bookId)

        {

            using (SHA256 sha256 = SHA256.Create())

            {

                byte[] idBytes = Encoding.UTF8.GetBytes(bookId);

                byte[] hashBytes = sha256.ComputeHash(idBytes);

​

                // Convert the last 8 bytes of the hash to hexadecimal and concatenate 'F' at the start

                return "F" + string.Join("", hashBytes.TakeLast(8).Select(b => b.ToString("X2")));

            }

        }

​

​

}