by Waty Thierry (60 Submissions)
Category: Math/Dates
Compatability: Visual Basic 3.0
Difficulty: Unknown Difficulty
Originally Published: Tue 13th April 1999
Date Added: Mon 8th February 2021
Rating: 
(1 Votes)
Compute Crc32 checksum
VBcoders
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
Else
lCrc32 = lTempCrc32
End If
Next
'// Put Crc32 checksum value in the holding array
Crc32Table(iBytes) = lCrc32
Next
'// 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$(Item, iCounter, 1))
'// Right shift an Unsigned Long 8 bits
lAccValue = Crc32 And &HFFFFFF00
lAccValue = lAccValue \ &H100
lAccValue = lAccValue And &HFFFFFF
'// Now select the right adding value from the holding table
lIndex = Crc32 And &HFF
lIndex = lIndex Xor bCharValue
lTableValue = Crc32Table(lIndex)
'// Then mix new Crc32 value with previous accumulated Crc32 value
Crc32 = lAccValue Xor lTableValue
Next
'// Set function value the the new Crc32 checksum
AddCrc32 = Crc32
End Function
'// At last, we have to write a function so that we can get the Crc32 checksum value at any time
Public Function GetCrc32(ByVal Crc32 As Long) As Long
'// Turn on error trapping
On Error Resume Next
'// Set function to the current Crc32 value
GetCrc32 = Crc32 Xor &HFFFFFFFF
End Function
'// And for testing the routines above...
Public Sub Main()
Dim lCrc32Value As Long
On Error Resume Next
lCrc32Value = InitCrc32()
lCrc32Value = AddCrc32("This is the original message!", lCrc32Value)
Debug.Print Hex$(GetCrc32(lCrc32Value))
End Sub
