BExIS.IO.Transform.Output.ExcelWriter.getColumnIndex C# (CSharp) Метод

        private string getColumnIndex(int index, int offset=1)
        {
            int residual = 0;
            string column = "";
            bool firstRun = true;
            do
            {
                if (firstRun == true)
                {
                    residual = ((index % 26)) + offset;
                    column = alphabet[residual].ToString() + column;
                    index = (index / 26);
                    firstRun = false;
                }
                else
                {
                    residual = ((index % 26)) + offset;
                    column = alphabet[residual - 1].ToString() + column;
                    index = (index / 26);
                }

            } while (index > 0);
            return column;
        }