CRC16计算

Question

有没有一种快速的方法来计算字符串上的CRC16，从这个表开始

NSArray* table16 = [NSArray arrayWithObjects:@0x0000, @0xC0C1, @0xC181, @0x0140, @0xC301, @0x03C0, @0x0280, @0xC241,@0xC601, @0x06C0, @0x0780, @0xC741, @0x0500, @0xC5C1, @0xC481, @0x0440,@0xCC01, @0x0CC0, @0x0D80, @0xCD41, @0x0F00, @0xCFC1, @0xCE81, @0x0E40,@0x0A00, @0xCAC1, @0xCB81, @0x0B40, @0xC901, @0x09C0, @0x0880, @0xC841,@0xD801, @0x18C0, @0x1980, @0xD941, @0x1B00, @0xDBC1, @0xDA81, @0x1A40,@0x1E00, @0xDEC1, @0xDF81, @0x1F40, @0xDD01, @0x1DC0, @0x1C80, @0xDC41,@0x1400, @0xD4C1, @0xD581, @0x1540, @0xD701, @0x17C0, @0x1680, @0xD641,@0xD201, @0x12C0, @0x1380, @0xD341, @0x1100, @0xD1C1, @0xD081, @0x1040,@0xF001, @0x30C0, @0x3180, @0xF141, @0x3300, @0xF3C1, @0xF281, @0x3240,@0x3600, @0xF6C1, @0xF781, @0x3740, @0xF501, @0x35C0, @0x3480, @0xF441,@0x3C00, @0xFCC1, @0xFD81, @0x3D40, @0xFF01, @0x3FC0, @0x3E80, @0xFE41,@0xFA01, @0x3AC0, @0x3B80, @0xFB41, @0x3900, @0xF9C1, @0xF881, @0x3840,@0x2800, @0xE8C1, @0xE981, @0x2940, @0xEB01, @0x2BC0, @0x2A80, @0xEA41,@0xEE01, @0x2EC0, @0x2F80, @0xEF41, @0x2D00, @0xEDC1, @0xEC81, @0x2C40,@0xE401, @0x24C0, @0x2580, @0xE541, @0x2700, @0xE7C1, @0xE681, @0x2640,@0x2200, @0xE2C1, @0xE381, @0x2340, @0xE101, @0x21C0, @0x2080, @0xE041,@0xA001, @0x60C0, @0x6180, @0xA141, @0x6300, @0xA3C1, @0xA281, @0x6240,@0x6600, @0xA6C1, @0xA781, @0x6740, @0xA501, @0x65C0, @0x6480, @0xA441,@0x6C00, @0xACC1, @0xAD81, @0x6D40, @0xAF01, @0x6FC0, @0x6E80, @0xAE41,@0xAA01, @0x6AC0, @0x6B80, @0xAB41, @0x6900, @0xA9C1, @0xA881, @0x6840,@0x7800, @0xB8C1, @0xB981, @0x7940, @0xBB01, @0x7BC0, @0x7A80, @0xBA41,@0xBE01, @0x7EC0, @0x7F80, @0xBF41, @0x7D00, @0xBDC1, @0xBC81, @0x7C40,@0xB401, @0x74C0, @0x7580, @0xB541, @0x7700, @0xB7C1, @0xB681, @0x7640,@0x7200, @0xB2C1, @0xB381, @0x7340, @0xB101, @0x71C0, @0x7080, @0xB041,@0x5000, @0x90C1, @0x9181, @0x5140, @0x9301, @0x53C0, @0x5280, @0x9241,@0x9601, @0x56C0, @0x5780, @0x9741, @0x5500, @0x95C1, @0x9481, @0x5440,@0x9C01, @0x5CC0, @0x5D80, @0x9D41, @0x5F00, @0x9FC1, @0x9E81, @0x5E40,@0x5A00, @0x9AC1, @0x9B81, @0x5B40, @0x9901, @0x59C0, @0x5880, @0x9841,@0x8801, @0x48C0, @0x4980, @0x8941, @0x4B00, @0x8BC1, @0x8A81, @0x4A40,@0x4E00, @0x8EC1, @0x8F81, @0x4F40, @0x8D01, @0x4DC0, @0x4C80, @0x8C41,@0x4400, @0x84C1, @0x8581, @0x4540, @0x8701, @0x47C0, @0x4680, @0x8641,@0x8201, @0x42C0, @0x4380, @0x8341, @0x4100, @0x81C1, @0x8081, @0x4040,nil];

谢谢。

Answer 1

这是一个使用所反映的多项式0x8005一次计算一个字节的16位CRC的表。完成该图还需要两条信息，即初始CRC寄存器内容和最终CRC异或结果。

在此list of 16-bit CRCs中，有四个可能使用该表的CRC定义：

width=16 poly=0x8005 init=0x0000 refin=true refout=true xorout=0x0000 check=0xbb3d name="ARC"
width=16 poly=0x8005 init=0x0000 refin=true refout=true xorout=0xffff check=0x44c2 name="CRC-16/MAXIM"
width=16 poly=0x8005 init=0xffff refin=true refout=true xorout=0xffff check=0xb4c8 name="CRC-16/USB"
width=16 poly=0x8005 init=0xffff refin=true refout=true xorout=0x0000 check=0x4b37 name="MODBUS"

您将注意到，所有对象都具有相同的多边形，并且都是refin和refout true (反射)，但它们具有不同的init和xorout值。您要查找的CRC可能是其中之一，也可能是不在表中的其他CRC。

要使用该表计算CRC，您可以使用如下代码(在C中)：

crc = init;
while (len--)
    crc = (crc >> 8) ^ table[(crc ^ *data++) & 0xff];
crc ^= xorout;