Saturday, December 22, 2007

CRC Routine with VB

The CRC (Cyclic Redundancy Checksum) algorithm is a highly optimised and powerful way of checking whether a large number of bytes have been modified or not. The algorithm scans through all the bytes and generates a 32 bit number to represent the contents - small changes in the file contents result in large changes in the check sum, and there is a very low chance that two different streams of bytes will have the same CRC.

The implementation here uses a pre-calculated lookup table to do a lot of the heavy work of generating the polynomial. Once this has been calculated then the actual algorithm becomes relatively simple.

Option Explicit

' This code is taken from the VB.NET CRC32 algorithm
' provided by Paul (wpsjr1@succeed.net) - Excellent work!

Private crc32Table() As Long
Private Const BUFFER_SIZE As Long = 8192

Public Function GetByteArrayCrc32(ByRef buffer() As Byte) As Long

Dim crc32Result As Long
crc32Result = &HFFFFFFFF

Dim i As Integer
Dim iLookup As Integer

For i = LBound(buffer) To UBound(buffer)
iLookup = (crc32Result And &HFF) Xor buffer(i)
crc32Result = ((crc32Result And &HFFFFFF00) \ &H100) And 16777215 ' nasty shr 8 with vb :/
crc32Result = crc32Result Xor crc32Table(iLookup)
Next i

GetByteArrayCrc32 = Not (crc32Result)

End Function

Public Function GetFileCrc32(ByRef stream As cBinaryFileStream) As Long

Dim crc32Result As Long
crc32Result = &HFFFFFFFF

Dim buffer(0 To BUFFER_SIZE - 1) As Byte
Dim readSize As Long
readSize = BUFFER_SIZE

Dim count As Integer
count = stream.Read(buffer, readSize)

Dim i As Integer
Dim iLookup As Integer
Dim tot As Integer

Do While (count > 0)
For i = 0 To count - 1
iLookup = (crc32Result And &HFF) Xor buffer(i)
crc32Result = ((crc32Result And &HFFFFFF00) \ &H100) And 16777215 ' nasty shr 8 with vb :/
crc32Result = crc32Result Xor crc32Table(iLookup)
Next i
count = stream.Read(buffer, readSize)
Loop

GetFileCrc32 = Not (crc32Result)

End Function

Private Sub Class_Initialize()

' This is the official polynomial used by CRC32 in PKZip.
' Often the polynomial is shown reversed (04C11DB7).
Dim dwPolynomial As Long
dwPolynomial = &HEDB88320
Dim i As Integer, j As Integer

ReDim crc32Table(256)
Dim dwCrc As Long

For i = 0 To 255
dwCrc = i
For j = 8 To 1 Step -1
If (dwCrc And 1) Then
dwCrc = ((dwCrc And &HFFFFFFFE) \ 2&) And &H7FFFFFFF
dwCrc = dwCrc Xor dwPolynomial
Else
dwCrc = ((dwCrc And &HFFFFFFFE) \ 2&) And &H7FFFFFFF
End If
Next j
crc32Table(i) = dwCrc
Next i

End Sub

1 comment:

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