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Validating Credit Card Numbers

Validating Credit Card Numbers

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   Const MAX_DIGITS = 20  ' actually don't know any card using more than 16 digits
   
   Dim anDigits(1 To MAX_DIGITS) As Byte
   Dim nDigits As Long
   
   Dim ofsCurrentDigit As Long
   Dim ofsCurrentCharacter As Long
   
   Dim CurrentCharacter As String
   
   Dim Multiplier As Long
   Dim CheckSum As Long
   Dim DigitValue As Long
   
   Dim ValidDigits As String
   
   If Len(Trim$(sCardNo)) < 1 Then
      Result = False
      GoTo Exit_Point
   End If
   
   ValidDigits = "0123456789"
   
   For ofsCurrentCharacter = 1 To Len(sCardNo)
      CurrentCharacter = Mid$(sCardNo, ofsCurrentCharacter, 1)
      If InStr(1, ValidDigits, CurrentCharacter, vbBinaryCompare) Then
         nDigits = nDigits + 1
         If nDigits > MAX_DIGITS Then
            Result = False
            GoTo Exit_Point
         End If
         anDigits(nDigits) = Val(CurrentCharacter)
      End If
   Next ofsCurrentCharacter
   
   CheckSum = anDigits(nDigits)
   
   For ofsCurrentDigit = nDigits - 1 To 1 Step -1
      
      If Multiplier = 2 Then
         Multiplier = 1
      Else
         Multiplier = 2
      End If
      DigitValue = anDigits(ofsCurrentDigit) * Multiplier
      CheckSum = CheckSum + DigitValue
      If DigitValue > 9 Then
         CheckSum = CheckSum - 9
      End If
      
   Next ofsCurrentDigit
   
   Result = ((CheckSum Mod 10) = 0)
   
Exit_Point:
   
   IsValidCreditCardNumber = Result
   Exit Function
   
End Function