Luhn Algorithm in VBA (for example for use in Excel)

gistfile1.vb

' Version 1.0.0
' You may use these functions directly in Excel: "=luhnCheck(A55)"


' probably only needed internally
Function luhnSum(InVal As String) As Integer
    Dim evenSum As Integer
    Dim oddSum As Integer
         
    evenSum = 0
    oddSum = 0
     
    Dim strLen As Integer
    strLen = Len(InVal)
     
    Dim i As Integer
    For i = strLen To 1 Step -1
        Dim digit As Integer
        digit = CInt(Mid(InVal, i, 1))
         
        If ((i Mod 2) = 0) Then
            oddSum = oddSum + digit
        Else
            digit = digit * 2
             
            If (digit > 9) Then
                digit = digit - 9
            End If
             
            evenSum = evenSum + digit
        End If
    Next i
     
    luhnSum = (oddSum + evenSum)
End Function

' for the curious
Function luhnCheckSum(InVal As String)
    luhnCheckSum = luhnSum(InVal) Mod 10
End Function

' true/false check
Function luhnCheck(InVal As String)
    luhnCheck = (luhnSum(InVal) Mod 10) = 0
End Function

' returns a number which, appended to the InVal, turns the composed number into a valid luhn number
Function luhnNext(InVal As String)
    Dim luhnCheckSumRes
    luhnCheckSumRes = luhnCheckSum(InVal)
     
    If (luhnCheckSumRes = 0) Then
        luhnNext = 0
    Else
        luhnNext = ((10 - luhnCheckSumRes))
    End If
End Function

There are a few problems with this implementation, though If ((i Mod 2) = 0) Then is not necessarily wrong.

First, when calculating the sum, we use i going from length down to 1. This means the first position is considered odd or even based on the length of InVal. First position should always be 1, odd.

(This would explain why If ((i Mod 2) <> 0) Then may yield correct result for some, it depends on the length of your numbers)

My solution:

   For i = 0 To strLen - 1 Step 1
        Dim digit As Integer
        digit = CInt(Mid(InVal, strLen - i, 1))

If ((i Mod 2) = 0) Then is unchanged, as we start at pos 1 when i = 0.

Also when calculating the next value, first position is considered to be held by the value you are trying to calculate.
Ie, if you want to calculate the check value x of 123456, then you need to think of the number as 123456x where x is position 1.
The code does not take this in to account.
Changing luhnCheckSumRes = luhnCheckSum(InVal) to luhnCheckSumRes = luhnCheckSum(InVal*10) in Function luhnNext(InVal As String)
seems to work.

Hello,
Using Excel 2016, Getting #VALUE! with Function luhnNext(InVal As String),
after I change the code to luhnCheckSumRes = luhnCheckSum(InVal*10) (suggested by Voidfae) and result of InVal*10 has more than 15 digits

I'm new to Excel & VBA & programming but a google search shows me that's because of floating point?

After a few try, since Function luhnNext(InVal As String) is As String,
instead of change the line to luhnCheckSumRes = luhnCheckSum(InVal*10),
I'm keeping it untouched, but add a new InVal = InVal + "0" line above that, seems working now.
Is this the right way to do it?

The doubling is alternating, so why not just use a flag to alternate. This way it doesn't matter if the length of InVal is even or odd

    Dim i As Integer
    Dim ToDouble As Boolean
    For i = strLen To 1 Step -1
        Dim digit As Integer
        digit = CInt(Mid(InVal, i, 1))

        ToDouble  = Not ToDouble 
 
        If Not ToDouble Then
            oddSum = oddSum + digit
        Else
            digit = digit * 2