MQTT 协议剩余长度计算C#版

MQTT Version 5.0

中固定头部中byte2 标识包的剩余长度, 单个byte最大能表示127的包长度，最高为标识位，1代表借用后续字节来标识包长度。

 Table 1‑1 Size of Variable Byte Integer

 

 

协议文档给出了算法如下： 

 The algorithm for encoding a non-negative integer (X) into the Variable Byte Integer encoding scheme is as follows:

do
   encodedByte = X MOD 128
   X = X DIV 128
   // if there are more data to encode, set the top bit of this byte
 
   if (X > 0)
      encodedByte = encodedByte OR 128
   endif
   'output' encodedByte
while (X > 0)

 Where MOD is the modulo operator (% in C), DIV is integer division (/ in C), and OR is bit-wise or (| in C).

The algorithm for decoding a Variable Byte Integer type is as follows:   

multiplier = 1
value = 0
do
   encodedByte = 'next byte from stream'
   value += (encodedByte AND 127) * multiplier
   if (multiplier > 128*128*128)
      throw Error(Malformed Variable Byte Integer)
   multiplier *= 128
while ((encodedByte AND 128) != 0)

where AND is the bit-wise and operator (& in C).

When this algorithm terminates, value contains the Variable Byte Integer value.

 利用C# 来写为：

 
        private void btn10toHex_Click(object sender, EventArgs e)
        {
            try
            {
                int num = int.Parse(this.txtDeciValue.Text);
                byte[] bytes = ConvertLength2MqttByte(num);
                string result= BytesConverter.ToHexString(bytes);
                this.txtHexValueCvt.Text = result;
                this.txtHexValue.Text = result;
            }
            catch { }
        }
 
        /// 
        /// 把给定长度转成Mqtt 固定头部剩余长度字节
        /// 
        /// 
        /// 
        byte[] ConvertLength2MqttByte(int length)
        {
            try
            {
                int encodeByte = 0;//需编码输出的数
                int dec = 128;//进制数，二进制为 1000 0000 
                int reminder = length;
                do
                {
                    //取num 的结束字节，高1位是符号位
                    int lastEncodeByte = reminder % dec;
                    reminder = reminder / dec;// 取余数
                    if (reminder > 0)//需向高位进位
                    {
                        lastEncodeByte |= dec;// 高位符号位 填充1                       
                        lastEncodeByte <<= 8;
                        encodeByte <<= 8;//左移动8位，留给低字节
                    }
                    encodeByte |= lastEncodeByte;//填充低字节
                } while (reminder > 0);
 
                byte[] bytes = BitConverter.GetBytes(encodeByte);
                Array.Reverse(bytes);
                List<byte> targetBytes = new List<byte>();
                targetBytes.AddRange(bytes);
                var outputArray = targetBytes.Where(x => x > 0);
                return outputArray.ToArray();
            }
            catch 
            {
                return null;
            }
        }
 
 
        /// 
        /// 把MQTT 剩余长度的16进制格式转化对应的10进制数值
        /// 
        /// 
        /// 
        private void btnHexTo10_Click(object sender, EventArgs e)
        {
            try
            {
                string[] hexRaw = this.txtHexValue.Text.Split(' ');//低位在前             
                int result = 0;
                for (int i = 0; i 
                {
                    int mutiplier = 1;
                    for (int j = 0; j < i; j++)
                    {
                        mutiplier *= 128;
                    }
                    byte num = Convert.ToByte(hexRaw[i], 16);
                    byte temp = (byte)(num << 1);
                    byte leftNum = (byte)(temp >> 1);
                    result += leftNum * mutiplier;
                }
                this.txtDeciCvt.Text = result.ToString();
            }
            catch
            { }
        }