>I have this class which returns a CRC32:
>
>
>Imports System.IO
>
>Namespace Framework
>
> Public Class CRC32
>
> Public cMessage As String = ""
> Public cFileName As String = ""
> Public lLogError As Boolean = True
> Public nCRC32 As Integer = 0
> Public oApp As Framework.App
> Private cFileDoesNotExist As String = ""
> Private nBufferSize As Integer = 1024
> Private nCRC32Table() As Integer
> Private nLanguage As Integer = 0
> Private oProcess As Framework.LXProcess
>
> ' This is when we access the data provider in desktop and Web service mode
> Sub New(ByVal toApplication As Framework.App)
> oApp = toApplication
> nLanguage = oApp.nLanguage
> Init()
> Initialize()
> End Sub
>
> ' This is when we access the data provider in a Web mode
> Public Sub New(ByVal toProcess As Framework.LXProcess)
> oProcess = toProcess
> oApp = oProcess.oApp
> nLanguage = oProcess.nLanguage
> Init()
> Initialize()
> End Sub
>
> Private Function Init() As Boolean
>
> ' Based on the language
> Select Case nLanguage
>
> ' English
> Case 1
> cFileDoesNotExist = "The file ##File## does not exist."
>
> ' French
> Case 2
> cFileDoesNotExist = "Le fichier ##File## n'existe pas."
>
> ' Spanish
> Case 3
> cFileDoesNotExist = "The file ##File## does not exist."
>
> ' Portuguese
> Case 4
> cFileDoesNotExist = "The file ##File## does not exist."
>
> End Select
>
> Return True
> End Function
>
> ' This is the official polynomial used by CRC32 in PKZip.
> ' Often the polynomial is shown reversed (04C11DB7).
> ' Initialize
> Private Function Initialize() As Boolean
> Dim lnDWPolynomial As Integer = &HEDB88320
> Dim lnCounter As Integer = 0
> Dim lnCounter2 As Integer = 0
> ReDim nCRC32Table(256)
> Dim lnDWCRC As Integer
>
> For lnCounter = 0 To 255
> lnDWCRC = lnCounter
>
> For lnCounter2 = 8 To 1 Step -1
>
> If (lnDWCRC And 1) Then
> lnDWCRC = ((lnDWCRC And &HFFFFFFFE) \ 2&) And &H7FFFFFFF
> lnDWCRC = lnDWCRC Xor lnDWPolynomial
> Else
> lnDWCRC = ((lnDWCRC And &HFFFFFFFE) \ 2&) And &H7FFFFFFF
> End If
>
> Next
>
> nCRC32Table(lnCounter) = lnDWCRC
> Next
>
> Return True
> End Function
>
> ' This is the main method to call
> Public Function GetCRC() As Boolean
> Dim lcMessage As String = ""
> Dim lnCounter As Integer = 0
> Dim lnDelay As Integer = 250
> Dim lnRetry As Integer = 8
> Dim loStream As Stream = Nothing
>
> ' Reset the values
> cMessage = ""
> nCRC32 = 0
>
> For lnCounter = 1 To lnRetry
>
> Try
>
> ' If the file exists
> If oApp.FileExist(cFileName) Then
> loStream = New FileStream(cFileName, FileMode.Open, FileAccess.Read, FileShare.Read, 8192)
> Else
> lcMessage = oApp.StrTran(cFileDoesNotExist, "##File##", cFileName)
>
> ' If we log the error
> If lLogError Then
>
> ' Get the proper definition as per the current scope
> If oProcess Is Nothing Then
> oApp.ErrorSetup(, "CRC32: " + lcMessage)
> Else
> oProcess.ErrorSetup(, "CRC32: " + lcMessage)
> End If
>
> Else
> Return False
> End If
>
> End If
>
> Exit For
>
> Catch loError As Exception
>
> ' If we have not reached the maximum tries of 8, we retry
> If lnCounter < 8 Then
>
> ' Wait for a timeout before retrying
> System.Threading.Thread.Sleep(lnDelay)
>
> Continue For
> End If
>
> ' If we log the error
> If lLogError Then
>
> ' Get the proper definition as per the current scope
> If oProcess Is Nothing Then
> oApp.ErrorSetup(loError)
> Else
> oProcess.ErrorSetup(loError)
> End If
>
> Else
> cMessage = loError.Message
> End If
>
> End Try
>
> Next
>
> nCRC32 = GetCrc32(loStream)
> loStream.Close()
> Return True
> End Function
>
> Private Function GetCrc32(ByRef toStream As System.IO.Stream) As Integer
> Dim lnCRC32Result As Integer = &HFFFFFFFF
> Dim loBuffer(nBufferSize) As Byte
> Dim lnReadSize As Integer = nBufferSize
> Dim lnCount As Integer = 0
> Dim lnCounter As Integer = 0
> Dim lnLookup As Integer = 0
>
> lnCount = toStream.Read(loBuffer, 0, lnReadSize)
> Do While lnCount > 0
> For lnCounter = 0 To lnCount - 1
> lnLookup = (lnCRC32Result And &HFF) Xor loBuffer(lnCounter)
>
> ' Nasty shr 8 with vb :/
> lnCRC32Result = ((lnCRC32Result And &HFFFFFF00) \ &H100) And 16777215
>
> lnCRC32Result = lnCRC32Result Xor nCRC32Table(lnLookup)
> Next
> lnCount = toStream.Read(loBuffer, 0, lnReadSize)
> Loop
>
> Return Not lnCRC32Result
> End Function
>
> End Class
>
>End Namespace
>
>
>So, on files which are about lower than 5 MB, I get the proper result. For files which are bigger, I get a negative value, which, of course, doesn't match the proper CRC32 of the file. Could it be that I would have to change the declaration to a double instead of an integer, in cases where I have big numbers? I am wondering if this could be the problem.
From the algorithm it looks as if you could easily end up with a negative result regardless of file size.
I don't know the inner workings of the calculation but, since it is essentially a bitwise operation, why do you thing negative int representations are invalid (you don't show any usage of the nCRC32 return value) ?
Also, for larger files why not use a larger buffer ?
See you're still not using OPTION STRICT :-}
