Luhn Numbers

[blindwig]

I've just written 2 functions to work with Luhn numbers. Luhn numbers are (usually) many digits long, and have a built-in checksum. This checksum allows the number to be checked against someone accidently entering it in wrong. Luhn numbers are good for account numbers (in fact the 16-digit credit card numbers issued by MasterCard and Visa are Luhn numbers)

Here's the functions:

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;===============================================================================
;
; Description:    Returns weather a string contains a Luhn number
; Parameter(s):  $s_Num is a string expected to be only digits (0-9)
; Requirement(s):   None
; Return Value(s):  On Success - Returns a boolean True or False
;                               Set @EXTENDED to the Luhn Checksum
;                  On Failure - Return a plain-english description of the error
;                                  @ERROR to:  1 - Input is not a valid numeric string
; Author(s):        Mike Ratzlaff AKA BlindWig
; Note(s):
;
;===============================================================================
Func _LuhnValidate($s_Num)
    If IsString($s_Num) And StringIsDigit($s_Num) Then
        Local $a_Digit = StringSplit($s_Num,'')
        Local $i_Count, $i_State = 0, $i_Temp, $i_CheckSum, $RetVal
        For $i_Count = $a_Digit[0] To 1 Step -1
            If $i_State Then
                $i_Temp = $a_Digit[$i_Count] * 2
                If $i_Temp > 9 Then
                    $i_Temp = Int($i_temp/10) + mod($i_Temp, 10)
                EndIf
            Else
                $i_Temp = $a_Digit[$i_Count]
            EndIf
            $i_CheckSum += $i_Temp
            $i_State = Not $i_State
        Next
        $RetVal = Mod($i_Checksum,10) = 0
        SetExtended($i_CheckSum)
        Return $RetVal
    Else
        SetError(1)
        Return 'Input "' & $s_Num & '" is not a valid numeric string'
    EndIf
EndFunc

;===============================================================================
;
; Description:    Returns a string containing a Luhn number of a given length
; Parameter(s):  $i_Length = number of digits for the Luhn number
; Requirement(s):   Calls _LuhnValidate()
; Return Value(s):  On Success - Returns a Luhn number of the given length
;                  On Failure - Returns '0'
; Author(s):        Mike Ratzlaff AKA BlindWig
; Note(s):
;
;===============================================================================
Func _LuhnGenerate($i_Length)
    Local $i_Count, $s_Num, $i_CheckSum
    If $i_Length >= 2 Then
        For $i_Count = 1 To $i_Length - 1
            $s_Num &= String(Random(0,9,1))
        Next
        _LuhnValidate($s_Num & '0')
        $i_CheckSum = @extended
        $s_Num &= String(10-Mod($i_CheckSum,10))
        Return $s_Num
    Else
        Return '0'
    EndIf
EndFunc

So for example if someone gives you a creditcard number, you can verify that it's a valid account number like this:

_LuhnValidate($s_GivenNumber)

And if you need a random creditcard number (for example, to test an entry form) you can use this:

$s_CCnum = _LuhnGenerate(16)

EDIT

Removed ConsoleWrite() call - that was for debugging, didn't mean to keep it in the final function

Edited April 11, 2006 by blindwig

[Valuater]

Question

$s_Num is a string expected to be only digits (0-9)

So for example if someone gives you a creditcard number, you can verify that it's a valid account number like this:

_LuhnValidate($s_GivenNumber)

with those thoughts and credit cards having 16 digits, how do we use this to check a credit card number??

thx

8)

[greenmachine]

"$s_Num is a string expected to be only digits (0-9)" means you can't have "a" in your string - they can only be digits from 0 to 9. That doesn't mean the length of the string has to be from 0-9 digits long.

[Valuater]

"$s_Num is a string expected to be only digits (0-9)" means you can't have "a" in your string - they can only be digits from 0 to 9. That doesn't mean the length of the string has to be from 0-9 digits long.

Thanks green

8)

[SmOke_N]

This is really good! Wish you were doing this permutation code I can't get to go faster ...

Great work!

[Jango]

I've just written 2 functions to work with Luhn numbers. Luhn numbers are (usually) many digits long, and have a built-in checksum. This checksum allows the number to be checked against someone accidently entering it in wrong. Luhn numbers are good for account numbers (in fact the 16-digit credit card numbers issued by MasterCard and Visa are Luhn numbers)

Thank you for the script it's very usefull to me