Steam Workshop Skyrim: CKM 2 BSA+ESP

using System;
using System.IO;

namespace ckmExtractor
{
    class Program
    {
        private static string FileName = "";

        private static void Main(string[] args)
        {
            if (args.Length > 0)
                FileName = args[0];

            while (string.IsNullOrWhiteSpace(FileName) || !FileName.EndsWith(".ckm"))
            {
                Console.WriteLine("Specify CKM file (you may obtain it at http://steamworkshopdownloader.com/):");
                FileName = Console.ReadLine();
            }
            Console.WriteLine("Starting extraction ...");
            Extract(FileName);
            Console.WriteLine("OK!");
            Console.ReadLine();
        }

        public static void Extract(string fileName)
        {
            var fs = new StreamReader(fileName);
            var br = new BinaryReader(fs.BaseStream);
            var len = br.ReadInt32();
            Console.WriteLine("BSA length is " + len);
            Console.WriteLine("Extracting bsa ...");
            cpyfile(br, Path.ChangeExtension(fileName, "bsa"), len);
            len = br.ReadInt32();
            if (len == 0)
                len = (int) (new FileInfo(FileName).Length - br.BaseStream.Position);
            else
                Console.WriteLine("! Unexpected non-zero value. Don`t trust this program`s results, report author about this error !");

            Console.WriteLine("ESP length is " + len);
            Console.WriteLine("Extracting esp ...");
            cpyfile(br, Path.ChangeExtension(fileName, "esp"), len);

            fs.Close();
        }

        public static void cpyfile(BinaryReader from, string fileName, int bytes)
        {
            var fs = new StreamWriter(fileName);
            var bw = new BinaryWriter(fs.BaseStream);
            int readBytes = 0;
            do
            {
                var bts = from.ReadBytes(Math.Min(4096, bytes - readBytes));
                readBytes += bts.Length;
                bw.Write(bts);
            } while (readBytes < bytes);
            fs.Close();
        }

    }
}