字符串CRC校驗

字符串CRC校驗

 

 

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

 

上邊參考網址內的計算方法親自算一遍就知道CRC怎么使用了（就是上邊的圖片），搜索了一下，大部分都是C語言實現，工作期間需要用Python實現求字符串CRC校驗的函數，使用查表法實現

 

#!/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: ]

 

 

 

 

位移時要注意 python的整數沒有最大值，而C++中不同，左移會將高位移掉，python則一直保留高位，所以通過位0xFFFF與的方式將高位去掉。