Mark Hensler — Thu, 19 Feb 2004 07:37:35 +0000

I've just finished a few tests, and I believe I have a working function now. Here's the code:

Public Function ChecksumString(ByVal strData As String)
    Dim check As Integer
    Dim sum As Integer
    Dim i As Byte
    Dim chars() As Byte
    Dim char As Byte
    
    'base value for Check
    check = &HA55A
    sum = 0
    
    chars = StrConv(strData, vbFromUnicode)
    
    For i = 0 To UBound(chars)
        char = chars(i)
        
        sum = sum + (char Xor (i And &HFF))
        sum = sum And &HFFFF
        
        check = check + sum
        check = (((check And &HFFFE) \ 2) And &H7FFF) Or ((check And 1) * &H8000)
    Next
    
    ChecksumString = check
End Function

'
The last hurdle was the fact that check was a signed integer, and the integer division is an arithmatic shift right rather than a logic shift. (I haven't found a way to make unsigned integers in VB.)

Mark Hensler — Wed, 18 Feb 2004 18:13:09 +0000

This is what I have so far. Neither method is producing the correct results (but both the same wrong result at least). I think it's getting close, but it's hard to tell.

I'm questioning this line of code:
check = ((check \ 2) And &H7FFF) Or ((check And 1) * &H8000)

Using a test string "55", this VB function returns hex E999. The correct checksum is hex 6998. The difference being hex 8001... the first and last bit.

Public Function ChecksumString(ByVal strData As String)
    Dim check As Integer
    Dim sum As Integer
    Dim i As Byte
    Dim chars() As Byte
    Dim char As Byte
    
    'just for use while designing function
    Dim method
    method = 1
    
    'base value for Check
    check = &HA55A
    sum = 0
    
    chars = StrConv(strData, vbFromUnicode)
    
    If method = 1 Then
        For i = 0 To UBound(chars)
            char = chars(i)
            MsgBox (Chr(char) & " " & Hex(char))
            
            sum = sum + (char Xor (i And &HFF))
            sum = sum And &HFFFF
            
            check = check + sum
            check = ((check \ 2) And &H7FFF) Or ((check And 1) * &H8000)
        Next
    Else
        For i = 0 To Len(strData) - 1
            char = CByte(Asc(Mid(strData, i + 1, 1)))
            MsgBox (Chr(char) & " " & Hex(char))
        
            sum = sum + (char Xor (i And &HFF))
            sum = sum And &HFFFF
        
            check = check + sum
            check = ((check \ 2) And &H7FFF) Or ((check And 1) * &H8000)
        Next
    End If
    
    ChecksumString = check
End Function

Greg K — Wed, 18 Feb 2004 16:57:03 +0000

I know VB pretty well and could probably help you if you reposted the code and added in some comments as to what it is actually doing in C, as I'm not good with C.

-Greg