Decimal To Tetrasexagesimal and back extension methods C# (logic from http://stackoverflow.com/a/4964352/3325104)

DecimalTetrasexagesimalExtension

    public static class extensions
    {
        public static char[] baselist = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-".ToCharArray();
        public static StringBuilder toNumericBase64(this int number)
        {
            StringBuilder res = new StringBuilder();

            var r = number % baselist.Length;
            res.Append(baselist[r]);
            int q = (int)Math.Floor((decimal)(number / baselist.Length));

            while (q != 0) {
                r = q % baselist.Length;
                q = (int)Math.Floor((decimal)(q / baselist.Length));
                res.Append(baselist[r]);
            }
            return res;
        }

        public static string toNumericBase10(this string number)
        {
            var numbers = number.Reverse().ToArray();
            int res = Array.IndexOf(baselist, numbers[0]);

            for (int i = 1;i <number.Length;i++) {
                res = baselist.Length * res + Array.IndexOf(baselist, numbers[i]);
            }
            return res.ToString();
        }
    }