This topic shows you how to compute Crc32 checksum.

Microsoft Visual Basic 5.0Microsoft Visual Basic 4.0 32-bitMicrosoft Visual Basic 4.0 16-bitMicrosoft Visual Basic 3.0

This document has been viewed times since February 25, 1998.
Last modified on October 30, 1998.

'// At top level of a module, always include to be sure that all variables have the right type
Option Explicit
Option Compare Text

'// Then declare this array variable Crc32Table
Private Crc32Table(255) As Long

'// Then all we have to do is writing public functions like these...
Public Function InitCrc32(Optional ByVal Seed As Long = &HEDB88320, Optional ByVal Precondition As Long = &HFFFFFFFF) As Long

   '// Declare counter variable iBytes, counter variable iBits, value variables lCrc32 and lTempCrc32
   Dim iBytes As Integer, iBits As Integer, lCrc32 As Long, lTempCrc32 As Long

   '// Turn on error trapping
   On Error Resume Next

   '// Iterate 256 times
   For iBytes = 0 To 255

      '// Initiate lCrc32 to counter variable
      lCrc32 = iBytes

      '// Now iterate through each bit in counter byte
      For iBits = 0 To 7
         '// Right shift unsigned long 1 bit
         lTempCrc32 = lCrc32 And &HFFFFFFFE
         lTempCrc32 = lTempCrc32 \ &H2
         lTempCrc32 = lTempCrc32 And &H7FFFFFFF

         '// Now check if temporary is less than zero and then mix Crc32 checksum with Seed value
         If (lCrc32 And &H1) <> 0 Then
            lCrc32 = lTempCrc32 Xor Seed
            lCrc32 = lTempCrc32
         End If

      '// Put Crc32 checksum value in the holding array
      Crc32Table(iBytes) = lCrc32

   '// After this is done, set function value to the precondition value
   InitCrc32 = Precondition

End Function

'// The function above is the initializing function, now we have to write the computation function
Public Function AddCrc32(ByVal Item As String, ByVal Crc32 As Long) As Long

   '// Declare following variables
   Dim bCharValue As Byte, iCounter As Integer, lIndex As Long
   Dim lAccValue As Long, lTableValue As Long

   '// Turn on error trapping
   On Error Resume Next

   '// Iterate through the string that is to be checksum-computed
   For iCounter = 1 To Len(Item)

      '// Get ASCII value for the current character
      bCharValue = Asc(Mid$(Item, iCounter, 1))

      '// Right shift an Unsigned Long 8 bits
      lAccValue = Crc32 And &HFFFFFF00
      lAccValue = lAccValue \ &H100
      lAccValue = lAccValue And &HFFFFFF

      '// Now select the right adding value from the holding table
      lIndex = Crc32 And &HFF
      lIndex = lIndex Xor bCharValue
      lTableValue = Crc32Table(lIndex)

      '// Then mix new Crc32 value with previous accumulated Crc32 value
      Crc32 = lAccValue Xor lTableValue

   '// Set function value the the new Crc32 checksum
   AddCrc32 = Crc32

End Function

'// At last, we have to write a function so that we can get the Crc32 checksum value at any time
Public Function GetCrc32(ByVal Crc32 As Long) As Long
   '// Turn on error trapping
   On Error Resume Next

   '// Set function to the current Crc32 value
   GetCrc32 = Crc32 Xor &HFFFFFFFF

End Function

'// And for testing the routines above...
Public Sub Main()

   Dim lCrc32Value As Long

   On Error Resume Next
   lCrc32Value = InitCrc32()
   lCrc32Value = AddCrc32("This is the original message!", lCrc32Value)
   Debug.Print Hex$(GetCrc32(lCrc32Value))

End Sub

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