Proper Luhn AddCheckDigit for any length #

CREATE FUNCTION [dbo].[AddLuhnCheckDigit] (@core varchar(50) )
RETURNS int
-- =============================================
-- Author: Rempe, Bruce D. (brempe@iDataDesigns.com)
-- Create date: 10/10/2006
-- Description:Proper Luhn check-digit calculator
-- for varying length core numbers.
--
-- =============================================
AS
BEGIN

declare @core_wrk_char varchar(2), @core_wrk_int int, @wrk_total int, @placeholder int

SET @placeholder = LEN(@core)
SET @wrk_total = 0
 
While @placeholder > 0
Begin
 Set @core_wrk_char = SUBSTRING ( @core , @placeholder , 1 ) 
 If ((@placeholder - len(@core)) % 2) = 0
 Begin
set @core_wrk_int = Cast(@core_wrk_char as int)
set @core_wrk_int = @core_wrk_int * 2
set @core_wrk_char = Cast(@core_wrk_int as varchar(2))
if Len(@core_wrk_char) = 2
set @wrk_total = @wrk_total + (Cast(Left(@core_wrk_char,1) as int) + Cast(Right(@core_wrk_char,1) as int))
else
set @wrk_total = @wrk_total + Cast(@core_wrk_char as int)
 End
 Else
 Begin
  set @wrk_total = @wrk_total + Cast(@core_wrk_char as int)
 End
 set @placeholder = @placeholder - 1
End
 
Set @core_wrk_char = Cast(10 - Cast(Right(Cast(@wrk_total as varchar(2)),1) as int) as varchar(2))
Set @core = @core + RIGHT(RTRIM(@core_wrk_char),1)

return @core
--
-- to return only the check-digit use
-- "return right(rtrim(@core_wrk_char),1)"
-- instead.
--

END