crc算法以及实现方法.txt

VB技巧问答10000例,是一个教程

目 前 常 用 的 CRC编 码 有 CRC-12、 CRC-16、 CRC-CCITT、 CRC-32等 。 
    下 面 是 一 个 CRC-32和 CRC-16的 VB实 现 。 
    'Header taken from original C source code: 
    '/* CRC-32 version 1.04 by Craig Bruce, 05-Dec-1994 
    '** 
    '** Based on "File Verification Using CRC" by Mark R. Nelson in Dr. Dobb's 
    '** Journal, May 1992, pp. 64-67. This program DOES generate the same CRC 
    '** values as ZMODEM and PKZIP 
    '** 
    '** v1.00: original release. 
    '** v1.01: fixed printf formats. 
    '** v1.02: fixed something else. 
    '** v1.03: replaced CRC constant table by generator function. 
    '** v1.04: reformatted code, made ANSI C. 05-Dec-1994. 
    '*/ 
    ' 
    'Translated to (Visual) Basic Darren Kelly, 01-Jul-1999 
    'Lines with comments have been added to make the crc code work 
    '02-Jul-1999 CRC Table generation works, compared against c code 
    ' v1.00: Direct translation, modified to VB 
    ' v1.01: Divided into Table and CRC generation procedures, incorporated as module CRC 
    ' Developed in conjunction with <snipped> project 
    ' v1.02: Added support for skipping the last (SkipEnd) bytes 
    Private CrcTable(256) 
    Dim CRC As Long, ToGet As Long 
    Private poly As Long, i As Long, j As Long, mask As Long, TempIntx As 
    Integer 
    Private InByte As Byte, dbl As Double 
     
    Sub CRCGenerateTable() 
    'generate crc table first 
    poly = &HEDB88320 'pre-calculated polynomial used for CCITT CRC-32 calculation 
     
    For i = 0 To 256 
     CRC = i 
     For j = 1 To 8 
     mask = CRC And &H80000000 'if there is a 1 in highest bit... 
     If (CRC And 1) = 1 Then 
     CRC = Int(CRC / 2) Xor poly ' Without Int() returns one 
    higher. 
     Else 
     CRC = (CRC / 2) 
     End If 
     CRC = CRC Xor mask ' ... then remove it 
     Next j 
     CrcTable(i) = CRC 
    Next i 
     
    End Sub 
     
    Function CRCof(ByVal FileNum, Optional ByVal SkipBeginning As Long, 
    Optional ByVal SkipEnd As Long) As Long 'Pass File Number, recieve decimal. Good to convert using Hex$(CRCof) 
     'Add: optional skip bytes at start, skip bytes at end, for files with additional 
     'Headers and... um, footers? 
     
     
    'On Error Resume Next 
     
    dbl = &HFFFFFFFF 'Have to use a double-precision number because basic supports not an unsigned long type. 
    'set up file buffer 
    ToGet = LOF(1) - Loc(1) - SkipEnd 
    While EOF(FileNum) = False And ToGet > 0 'also use counter to skip last SkipEnd bytes 
     Get #FileNum, , InByte ' next version, get using file buffer 
     i = (dbl Xor InByte) And &HFF 
     j = CrcTable(i) 
     If dbl < 0 Then dbl = Int(dbl / 256) Else dbl = Fix(dbl / 256) 
    'division here requires double. also int rounds negative numbers down, fix rounds up 
     dbl = (dbl And &HFFFFFF) Xor j 
     TempIntx = TempIntx + 1 
     
     If TempIntx > 10000 Then 
     TempInt = DoEvents() 
     TempIntx = 0 
     End If 
     
     ToGet = ToGet - 1 
     'if buffer is empty, refill 
    Wend 
     
    'final result 
    CRCof = dbl Xor &HFFFFFFFF 
    'Debug.Print Hex(CRC) 
     
    End Function 
     
    Function ComputeCRC(ByRef inBuf() As Byte, ByVal nStart As Long, 
    ByVal nBytes As Long) As Long 
    '***************************************************** 
    ' Purpose: Calculates the 16-bit CCITT CRC check of 
    ' a sequence of bytes 
    ' Inputs: inBuf() input array of bytes 
    ' nStart starting point in inBuf 
    ' nBytes number of bytes to use 
    ' Returns: The unsigned 16-bit CRC as a long 
    ' Method: Devised by Andy Lowry, Columbia University 
    ' The magic number &H1081 is derived from 
    ' the CRC-CCITT polynomial x^16+x^12+x^5+1 
    '***************************************************** 
     Dim crc As Long 'running CRC value 
     Dim q As Long 'intermediate value 
     Dim c As Long 'byte being used, as a long 
     Dim i As Long 'loop counter 
     
     crc = 0 
     For i = nStart To nStart + nBytes - 1 
     c = inBuf(i) 
     q = (crc Xor c) And &HF& 'low nibble 
     crc = (crc \ &H10&) Xor (q * &H1081&) 
     q = (crc Xor (c \ &H10&)) And &HF& 'high nibble 
     crc = (crc \ &H10&) Xor (q * &H1081&) 
     Next i 
     ComputeCRC = crc 
    End Function 
    如 果 你 想 得 到 C语 言 的 实 现 ， 可 以 参 考 http://tvland.maxinet.com/relling/rae-crc1.htm。 
<END>     
    CRC-CCITT: 
    R(x)=(X^16(X^15+X^14+ ... +1) + X^16*G(x))/P(x) 
    P(x)=X^16+X^12+X^5+1
<END>