短网址计算

        string chars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

        private void button1_Click(object sender, EventArgs e)
        {
            int i = int.Parse(textBox1.Text);
            textBox2.Text = EncodeNum(i);
            textBox3.Text = DecodeNum(textBox2.Text).ToString();
        }

        /// <summary>
        /// 转码
        /// </summary>
        /// <param name="integer"></param>
        /// <returns></returns>
        private string EncodeNum(int integer)
        {
            double number = integer;
            int hex = chars.Length;
            StringBuilder sb = new StringBuilder();
            do
            {
                //计算指数
                int exp = -1;
                double value = 0;
                while (value < number)
                {
                    exp++;
                    value = Math.Pow(hex, exp + 1);
                }
                //计算对应字符
                double num = number / Math.Pow(hex, exp);
                int index = Convert.ToInt32(Math.Floor(num));
                sb.Append(chars.Substring(index, 1));
                number = number % Math.Pow(hex, exp);
            } while (number > 0);


            return sb.ToString();
        }

        /// <summary>
        /// 解码
        /// </summary>
        /// <param name="s"></param>
        /// <returns></returns>
        private int DecodeNum(string s)
        {
            double x = 0;
            int hex = chars.Length;
            for (int i = 0; i < s.Length; i++)
            {
                int exp = s.Length - 1 - i;
                int num = chars.IndexOf(s.Substring(i, 1));
                x += num * Math.Pow(hex, exp);
            }

            return Convert.ToInt32(x);
        }