[toc]
#1.背景介绍
主要应用场景在物联网中，底端设备注册报文的上报，需要对报文的有效载荷(data)进行CRC16的复验，验证与设备端的CRC校验是否相等，如果相等，报文有效，设备上报就会注册成功，不是第一次则会刷新心跳时间，避免通信中断告警。设备的报文结果以及设备的CRC16位置如下：
平台端需要重新对注册包内容（不包含设备的CRC计算字节）进行CRC校验计算，与设备端的CRC校验对比。如果相等，则平台端的CRC校验成功。
>备注：本文的CRC校验全部指CRC16的校验。
#2. CRC校验的三种方法
本文侧重测试CRC的性能，不讲CRC校验的原理，因为CRC只是个校验数据准确性的工具，而且每个报文（不单单心跳报文），还有AI,DI,DO,AO，告警报文等都需要校验，因此，执行CRC程序段的性能显得尤为重要。
**如果读者对CRC的校验原理感兴趣，请自行网上搜索相关资料进行深入研究，此处不再展开。**
##2.1. 直接计算CRC校验
以下代码已经做过验证，与设备端的CRC校验码相等（协议是基于变种的私有modbus协议），具体校验步骤可参考如下程序注释。最终将此类封装在了Crc16的帮助类里面。
```
///
/// 计算CRC16校验码
///
///
校验数据
///
多项式码
///
校验码初始值
///
public static byte[] GetCRC16(byte[] value, ushort poly = 0xA001, ushort crcInit = 0xFFFF)
{
if (value == null || !value.Any())
throw new ArgumentException("生成CRC16的入参有误");
//运算
ushort crc = crcInit;
for (int i = 0; i < value.Length-2; i++)
{
//Step1.与校验对象的某字节取异或
crc = (ushort)(crc ^ (value[i]));
for (int j = 0; j < 8; j++)
{ //Step2.==0？右移1比特，否则右移1 bit与多项式异或
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. 查短表法计算CRC16校验
查短表法计算CRC16，性能佳，而且只需很小内存空间.
```
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.查大表法计算CRC16校验
校验结果调了1天没调成功,后面会将测试结果贴出，性能与查短表几乎一样，而且浪费内存，所以没有采用此法。
```
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
};
///
/// 查表法计算CRC16.
///
///
待校验数据
///
数据长度
///
校验值
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;
}
```
#3.三种校验方式的测试方法
##3.1.直接计算CRC校验的时间测试
```
DateTime beforCrc = DateTime.Now;
var CrcValue=CRC16.GetCRC16(validBuff);
DateTime afterCrc = DateTime.Now;
TimeSpan ts = afterCrc.Subtract(beforCrc);
Console.WriteLine("校验结果{1}{2}。直接计算CRC校验总 {0}ms.", ts.TotalMilliseconds,CrcValue[0].ToString("X , CrcValue[1].ToString("X2"));
```
##3.2.查短表计算CRC校验的时间测试
```
beforCrc = DateTime.Now;
var CrcValue_ShotTable=CRC16.CalcCRC16(validBuff);
afterCrc = DateTime.Now;
var ts_table = afterCrc.Subtract(beforCrc);
Console.WriteLine("校验结果{1}。查表计算CRC校验总共花费 ms.", ts_table.TotalMilliseconds, CrcValue_ShotTab ToString("X2"));
```
##3.3.查长表计算CRC校验的时间测试
根据网上的资源，测试计算结果有问题。校验结果昨天调了1天没调成功，而且性能跟查短表几乎一样，还需要占用更多内存，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("校验结果{1}。查长表计算CRC校验总 {0}ms.", ts.TotalMilliseconds, CrcValue_LongTab ToString("X2"));
```
#4.校验结果的测试
##4.1. CRC静态帮助类中的校验结果方法
这里最终是采用2.2. 查短表法计算CRC16校验。通过默认设置模式mode="Table"调用。校验成功返回true，校验失败返回false。
```
///
/// 验证CRC16校验码
///
///
校验数据(包含底端设备上传的CRC校验值)
///
多项式码
///
校验码初始值
///
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的入参有误");
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验证方法的顶层调用
测试CRC验证方法
```
var result =CRC16.CheckCRC16(validBuff);
Console.WriteLine("校验结果{0}。", result);
LoggerHelper.Info("CRC校验结果：" + BitConverter.ToStr(CrcValue));
```
#5. 不同校验方式的性能差异
这里主要是对比2.1与2.2。方法2.3弃用。
```
var diff = ts / ts_table;
Console.WriteLine("直接计算所需时间是查表的的{0}倍", diff);
```
#6. 结果输出
+ 查长表计算CRC与查短表CRC校验计算，性能基本一样，甚至短表性能更佳；
+ 查短表性能是直接计算的3~5倍，所需计算时间是微秒级基本可以忽略不计；
+ 查长表计算校验结果有问题，4F0C为不正确结果，还需要占用更多内存，所以直接pass。
#7.小结
第一次输出性能时间需要比较久，原因是Net Core刚启动完成，需要做的事比较多。
