RE: Modulus 10 formula (REPOST)

["Bob Cozzi" <cozzi@xxxxxxxxx>]

Gosh, is it too early for me to type or what??? :)
-Bob

I've been going back and forth on a Modulus10 routine because, as it turns
out there seems to be two different formulas: IBM and LUHN.
The biggest issue is a subtle one which follows.
With IBM, you always start with the left most digit (highest order digit)
and select the even position digits by moving to the right and multiplying
hem by two.
This seems to be done regardless of the length of the original number;
therefore,  34567 would use 3, 5 and 7, and 4567 would use 4 and 6.
But the LUHN Modulus 10 formula begins with the rightmost digit (lowest
order digit) and selects every other digit moving to the left, and multiples
them by two.
 
So with IBM we would get this (starting on the left):

Start with:  3456
Step 1:  Multiple 3 and 5 by 2 =  6 10 
Step 2: Add them:  6 + 4 + 1 + 0 + 6  = 17
Step 3:  20 - 17 = 3
Check Digit = 3
Result:   3456-3

But with the LUHN formula we would get this:

Start with 3456
Step 1: Multiple 4 and 6 by 2 = 12 and 8
Step 2: Add them:  3 + 8 + 5 + 1 + 2 = 19
Step 3:  20 - 19 = 1
Check Digit = 1
Result:  3456-1


So is the IBM one screwy or are there two of them?

-Bob Cozzi