字符串CRC校验

字符串CRC校验

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<pre name="code" class="python"><span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">参考网址http://blog.163.com/du_minchao@126/blog/static/107495394201075114028606/</span>

上边参考网址内的计算方法亲自算一遍就知道CRC怎么使用了（就是上边的图片），搜索了一下，大部分都是C语言实现，工作期间需要用Python实现求字符串CRC校验的函数，使用查表法实现

#!/usr/bin/env python
# -*- coding: utf-8 -*-


POLYNOMIAL = 0x1021
INITIAL_REMAINDER = 0xFFFF
FINAL_XOR_VALUE = 0x0000
WIDTH = 16
TOPBIT = (1 << (WIDTH - 1))
crcTable = {}

def crcInit():
    SHIFT = WIDTH - 8
    for step in range(0, 256):
        remainder = step << SHIFT
        for bit in range(8, 0, -1):
            if remainder & TOPBIT:
                remainder = ((remainder << 1) & 0xFFFF) ^ 0x1021
            else:
                remainder = remainder << 1
        crcTable[step] = remainder

def crcFast(message, nBytes):
    crcInit()
    remainder = 0xFFFF
    data = 0
    byte = 0
    while byte < nBytes:
        data = ord(message[byte]) ^ (remainder >> (WIDTH - 8))
        remainder = crcTable[data] ^ ((remainder << 8)&0xFFFF)
        byte = byte + 1
        
    return hex(remainder)[2: ]

 

 

位移时要注意 python的整数没有最大值，而C++中不同，左移会将高位移掉，python则一直保留高位，所以通过位0xFFFF与的方式将高位去掉。