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CRC算法以及实现方法

编号：	QA001309
建立日期：	1999年7月8日	最后修改日期：	2003年11月4日
所属类别：	Visual Basic - 网络与通信

    vb
    Windows98
    请详细介绍CRC算法，以及实现方法。
    (袁文彬)

    限于时间，我无法向你详细地介绍CRC的原理，如果你感兴趣可以看一些通信方面的书，比如曹志刚、钱亚生的《现代通信原理》，在这些书中都详细介绍了CRC的原理和计算方法。目前常用的CRC编码有CRC-12、CRC-16、CRC-CCITT、CRC-32等。
    如果你想得到C语言的实现，可以参考http://tvland.maxinet.com/relling/rae-crc1.htm。
    如果你想得到Delphi语言实现，可以参考
    http://www.efg2.com/Lab/Mathematics/CRC.htm。
    如果你想得到Visual Basic.NET的实现，可以参考《Visual Basic.NET解决方案工具箱--30个.NET实用组件》。该书的代码可以从下载http://support.apress.com/books.asp?bID=1861007396&s=0&Go=Select+Book。
    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

    fulium的意见：
    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

此问题由李海回答。

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