目錄
1.背景介紹
主要應用場景在物聯網中，底端設備注冊報文的上報，需要對報文的有效載荷(data)進行CRC16的復驗，驗證與設備端的CRC校驗是否相等，如果相等，報文有效，設備上報就會注冊成功，不是第一次則會刷新心跳時間，避免通信中斷告警。設備的報文結果以及設備的CRC16位置如下：
平台端需要重新對注冊包內容（不包含設備的CRC計算字節）進行CRC校驗計算，與設備端的CRC校驗對比。如果相等，則平台端的CRC校驗成功。
備注：本文的CRC校驗全部指CRC16的校驗。
2. CRC校驗的三種方法
本文側重測試CRC的性能，不講CRC校驗的原理，因為CRC只是個校驗數據准確性的工具，而且每個報文（不單單心跳報文），還有AI,DI,DO,AO，告警報文等都需要校驗，因此，執行CRC程序段的性能顯得尤為重要。
如果讀者對CRC的校驗原理感興趣，請自行網上搜索相關資料進行深入研究，此處不再展開。
2.1. 直接計算CRC校驗
以下代碼已經做過驗證，與設備端的CRC校驗碼相等（協議是基於變種的私有modbus協議），具體校驗步驟可參考如下程序注釋。最終將此類封裝在了Crc16的幫助類里面。
/// <summary>
/// 計算CRC16校驗碼
/// </summary>
/// <param name="value">校驗數據</param>
/// <param name="poly">多項式碼</param>
/// <param name="crcInit">校驗碼初始值</param>
/// <returns></returns>
public static byte[] GetCRC16(byte[] value, ushort poly = 0xA001, ushort crcInit = 0xFFFF)
{
if (value == null || !value.Any())
throw new ArgumentException("生成CRC16的入參有誤");
//運算
ushort crc = crcInit;
for (int i = 0; i < value.Length-2; i++)
{
//Step1.與校驗對象的某字節取異或
crc = (ushort)(crc ^ (value[i]));
for (int j = 0; j < 8; j++)
{ //Step2.==0？右移1比特，否則右移1 bit與多項式異或
crc = (crc & 1) != 0 ? (ushort)((crc >> 1) ^ poly) : (ushort)(crc >> 1);
}
}
byte hi = (byte)((crc & 0xFF00) >> 8); //高位置
byte lo = (byte)(crc & 0x00FF); //低位置
//byte[] buffer = new byte[value.Length + 2];
//value.CopyTo(buffer, 0);
//buffer[buffer.Length - 1] = hi;
//buffer[buffer.Length - 2] = lo;
//return buffer;
byte[] returnVal = new byte[2];
returnVal[1] = hi;//CRC高位
returnVal[0] = lo;//CRC低位
return returnVal;
}
2.2. 查短表法計算CRC16校驗
查短表法計算CRC16，性能佳，而且只需很小內存空間.
static readonly UInt16[] crcTlb = new UInt16[16]{0x0000, 0xCC01, 0xD801, 0x1400, 0xF001, 0x3C00, 0x2800, 0xE401,
0xA001, 0x6C00, 0x7800, 0xB401, 0x5000, 0x9C01, 0x8801, 0x4400};
public static UInt16 CalcCRC16(byte[] pBuf)
{
byte i = 0, ch = 0;
UInt16 crc = 0xFFFF;
for (i = 0; i < pBuf.Length-2; i++)
{
ch = pBuf[i];
crc = (UInt16)(crcTlb[(ch ^ crc) & 0x0F] ^ (crc >> 4));
crc = (UInt16)(crcTlb[((ch >> 4) ^ crc) & 0x0F] ^ (crc >> 4));
}
crc = (UInt16)((crc & 0xFF) << 8 | (crc >> 8));
return crc;
}
2.3.查大表法計算CRC16校驗
校驗結果調了1天沒調成功,后面會將測試結果貼出，性能與查短表幾乎一樣，而且浪費內存，所以沒有采用此法。
static readonly UInt16[] CRC16Table =new UInt16[256] {
0x0000,0x1021,0x2042,0x3063,0x4084,0x50a5,0x60c6,0x70e7,
0x8108,0x9129,0xa14a,0xb16b,0xc18c,0xd1ad,0xe1ce,0xf1ef,
0x1231,0x0210,0x3273,0x2252,0x52b5,0x4294,0x72f7,0x62d6,
0x9339,0x8318,0xb37b,0xa35a,0xd3bd,0xc39c,0xf3ff,0xe3de,
0x2462,0x3443,0x0420,0x1401,0x64e6,0x74c7,0x44a4,0x5485,
0xa56a,0xb54b,0x8528,0x9509,0xe5ee,0xf5cf,0xc5ac,0xd58d,
0x3653,0x2672,0x1611,0x0630,0x76d7,0x66f6,0x5695,0x46b4,
0xb75b,0xa77a,0x9719,0x8738,0xf7df,0xe7fe,0xd79d,0xc7bc,
0x48c4,0x58e5,0x6886,0x78a7,0x0840,0x1861,0x2802,0x3823,
0xc9cc,0xd9ed,0xe98e,0xf9af,0x8948,0x9969,0xa90a,0xb92b,
0x5af5,0x4ad4,0x7ab7,0x6a96,0x1a71,0x0a50,0x3a33,0x2a12,
0xdbfd,0xcbdc,0xfbbf,0xeb9e,0x9b79,0x8b58,0xbb3b,0xab1a,
0x6ca6,0x7c87,0x4ce4,0x5cc5,0x2c22,0x3c03,0x0c60,0x1c41,
0xedae,0xfd8f,0xcdec,0xddcd,0xad2a,0xbd0b,0x8d68,0x9d49,
0x7e97,0x6eb6,0x5ed5,0x4ef4,0x3e13,0x2e32,0x1e51,0x0e70,
0xff9f,0xefbe,0xdfdd,0xcffc,0xbf1b,0xaf3a,0x9f59,0x8f78,
0x9188,0x81a9,0xb1ca,0xa1eb,0xd10c,0xc12d,0xf14e,0xe16f,
0x1080,0x00a1,0x30c2,0x20e3,0x5004,0x4025,0x7046,0x6067,
0x83b9,0x9398,0xa3fb,0xb3da,0xc33d,0xd31c,0xe37f,0xf35e,
0x02b1,0x1290,0x22f3,0x32d2,0x4235,0x5214,0x6277,0x7256,
0xb5ea,0xa5cb,0x95a8,0x8589,0xf56e,0xe54f,0xd52c,0xc50d,
0x34e2,0x24c3,0x14a0,0x0481,0x7466,0x6447,0x5424,0x4405,
0xa7db,0xb7fa,0x8799,0x97b8,0xe75f,0xf77e,0xc71d,0xd73c,
0x26d3,0x36f2,0x0691,0x16b0,0x6657,0x7676,0x4615,0x5634,
0xd94c,0xc96d,0xf90e,0xe92f,0x99c8,0x89e9,0xb98a,0xa9ab,
0x5844,0x4865,0x7806,0x6827,0x18c0,0x08e1,0x3882,0x28a3,
0xcb7d,0xdb5c,0xeb3f,0xfb1e,0x8bf9,0x9bd8,0xabbb,0xbb9a,
0x4a75,0x5a54,0x6a37,0x7a16,0x0af1,0x1ad0,0x2ab3,0x3a92,
0xfd2e,0xed0f,0xdd6c,0xcd4d,0xbdaa,0xad8b,0x9de8,0x8dc9,
0x7c26,0x6c07,0x5c64,0x4c45,0x3ca2,0x2c83,0x1ce0,0x0cc1,
0xef1f,0xff3e,0xcf5d,0xdf7c,0xaf9b,0xbfba,0x8fd9,0x9ff8,
0x6e17,0x7e36,0x4e55,0x5e74,0x2e93,0x3eb2,0x0ed1,0x1ef0
};
/// <summary>
/// 查表法計算CRC16.
/// </summary>
/// <param name="dataIn">待校驗數據</param>
/// <param name="length">數據長度</param>
/// <returns>校驗值</returns>
public static UInt16 calCRC16(byte[] dataIn, int length)
{
UInt16 i;
UInt16 nAccum = 0;
for (i = 0; i < length; i++)
nAccum = (UInt16)((nAccum << 8) ^ (UInt16)CRC16Table[(nAccum >> 8) ^ dataIn[i]]);
return nAccum;
}
3.三種校驗方式的測試方法
3.1.直接計算CRC校驗的時間測試
DateTime beforCrc = DateTime.Now;
var CrcValue=CRC16.GetCRC16(validBuff);
DateTime afterCrc = DateTime.Now;
TimeSpan ts = afterCrc.Subtract(beforCrc);
Console.WriteLine("校驗結果{1}{2}。直接計算CRC校驗總 {0}ms.", ts.TotalMilliseconds,CrcValue[0].ToString("X , CrcValue[1].ToString("X2"));
3.2.查短表計算CRC校驗的時間測試
beforCrc = DateTime.Now;
var CrcValue_ShotTable=CRC16.CalcCRC16(validBuff);
afterCrc = DateTime.Now;
var ts_table = afterCrc.Subtract(beforCrc);
Console.WriteLine("校驗結果{1}。查表計算CRC校驗總共花費 ms.", ts_table.TotalMilliseconds, CrcValue_ShotTab ToString("X2"));
3.3.查長表計算CRC校驗的時間測試
根據網上的資源，測試計算結果有問題。校驗結果昨天調了1天沒調成功，而且性能跟查短表幾乎一樣，還需要占用更多內存，所以直接pass。
beforCrc = DateTime.Now;
var CrcValue_LongTable = CRC16.calCRC16(validBuf validBuff.Length-2);
afterCrc = DateTime.Now;
ts = afterCrc.Subtract(beforCrc);
Console.WriteLine("校驗結果{1}。查長表計算CRC校驗總 {0}ms.", ts.TotalMilliseconds, CrcValue_LongTab ToString("X2"));
4.校驗結果的測試
4.1. CRC靜態幫助類中的校驗結果方法
這里最終是采用2.2. 查短表法計算CRC16校驗。通過默認設置模式mode="Table"調用。校驗成功返回true，校驗失敗返回false。
/// <summary>
/// 驗證CRC16校驗碼
/// </summary>
/// <param name="value">校驗數據(包含底端設備上傳的CRC校驗值)</param>
/// <param name="poly">多項式碼</param>
/// <param name="crcInit">校驗碼初始值</param>
/// <returns></returns>
public static bool CheckCRC16(byte[] value, ushort poly = 0xA001, ushort crcInit = 0xFFFF,string mode="Table")
{
if (value == null || !value.Any())
throw new ArgumentException("生成CRC16的入參有誤");
var crc16 = new byte[2];
if (mode == "Table")
{
var result=CalcCRC16(value);
crc16[0] = (byte)(result >> 8);
crc16[1] = (byte)(result);
}
else
{
crc16 = GetCRC16(value, poly, crcInit);
}
if ((value[value.Length - 1] == crc16[crc16.Length - 1]) && (value[value.Length - 2] == crc16[crc16.Length - 2]))
return true;
return false;
}
4.2. CRC驗證方法的頂層調用
測試CRC驗證方法
var result =CRC16.CheckCRC16(validBuff);
Console.WriteLine("校驗結果{0}。", result);
LoggerHelper.Info("CRC校驗結果：" + BitConverter.ToStr(CrcValue));
5. 不同校驗方式的性能差異
這里主要是對比2.1與2.2。方法2.3棄用。
var diff = ts / ts_table;
Console.WriteLine("直接計算所需時間是查表的的{0}倍", diff);
6. 結果輸出
- 查長表計算CRC與查短表CRC校驗計算，性能基本一樣，甚至短表性能更佳；
- 查短表性能是直接計算的3~5倍，所需計算時間是微秒級基本可以忽略不計；
- 查長表計算校驗結果有問題，4F0C為不正確結果，還需要占用更多內存，所以直接pass。
7.小結
第一次輸出性能時間需要比較久，原因是Net Core剛啟動完成，需要做的事比較多。
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