Visual Basic

Dim MsgToCalc As String
Dim DataBytes() As Byte
MsgToCalc = Left(msg, Len(msg) - 6)  '6 Because the space and y character befor the checksum is also excluded
CheckSum = Right(msg, 4) '4 Because the checksum is 4 bytes long

'Create an array of DATABYTES for MsgToCalc
ReDim DataBytes(0 To Len(MsgToCalc) - 1)
DataBytes = StrConv(MsgToCalc, vbFromUnicode)

Dim Str16 As String
Dim Total As Long
Dim MyChecksum As Long
Dim SubTotal As Integer
Dim i As Integer
i = 0

' The following is an attempt to do the calculations at this link:
' http://www.cs.tut.fi/~dunaytse/wireshark-2306/lab2.pdf
' Still not sure why they get a negative checksum from 1's compliment

For i = 0 To (Len(MsgToCalc) - 1)
   If i Mod 2 = 0 Then
      Str16 = CStr(Hex$(DataBytes(i))) & CStr(Hex$(DataBytes(i + 1)))
      SubTotal = Val("&H" & Right(Str16, 4))
      Total = Total + SubTotal
      Total = Val("&H" & Right(Hex$(Total), 4))
      MyChecksum = 65535 - Total
   End If
Next i

Dave Kreskowiak wrote:

the most inefficient algorithm I've seen

Dave Kreskowiak wrote:

Now, the last line is removing a compounded total from 65535. Well, in most cases, that's going to be a negative number. That's something that doesn't exist in a checksum.

Public Function crcbits16(ByVal str As String)
        Dim crc As UShort = &HFFFF
        Dim ser_data
        crc = &HFFFF
        Dim i As Long, p As Long
        i = str.Length
        For p = 0 To i - 1
            ser_data = str.Substring(p, 1)
            crc = (crc >> 8) Or (crc << 8)
            crc = crc Xor Asc(ser_data)
            crc = crc Xor (crc And &HFF) >> 4
            crc = crc Xor (crc << 8) << 4
            crc = crc Xor ((crc And &HFF) << 4) << 1
        Next
        Return CRCFormat(Hex(crc))
    End Function

Public Function CRCFormat(ByVal mycrc As String)

        If Len(mycrc) < 4 Then
            mycrc = Replace(Space(4 - Len(mycrc)) & mycrc, " ", "0") '"0" & mycrc
        End If
 End Function

Fatal Error wrote:

crc = (crc >> 8) Or (crc << 8)

Tom deketelaere wrote:

Is there anyway to do this so that I don't have to select the programme the first time

"c:\prog\JORODOCBEHEER\documentbeheer.exe" "ID=eID Arena"