Checksum Calculation

The CheckSum32 function I propose calculates the 32 bits checksum of a string, using a currency variable to avoid overflow errors.

Function CheckSum32(A$) As Long

'Calculates the 32bits checksum of a string

Const MAXULONG = 4294967295# 'Max unsigned long integer
Const MAXLONG = 2147483647 'Max signed long integer
Dim Sum As Currency 'Currency variable type to avoid overflow
Dim i As Integer 'Source string character counter
Dim j As Integer 'Checksum byte counter (byte # = 0..3)

    j = 0

    For i = 1 To Len(A$)

        'Add byte at relevant position in sum

        Sum = Sum + (Asc(Mid$(A$, i, 1)) * 256 ^ j)

        'remove 33d bit if set

        If Sum > MAXULONG Then Sum = Sum - (MAXULONG + 1)

            'skip to next byte in sum

        If j < 3 Then

            j = j + 1

            Else

              j = 0

        End If

    Next

    'Conversion for signed long integer result

    If Sum > MAXLONG Then Sum = Sum - (MAXULONG + 1)

    CheckSum32 = Sum

End Function

Tip Submitted By: Marc Lajus