Luhn Mod N in Cache?

Hi all

I'm trying to recreate this method below in Cache/IRIS. 

https://en.wikipedia.org/wiki/Luhn_mod_N_algorithm ISO-7812-1 (LUHN-10)

I've been following the javascript example. It seems simple in theory but i'm struggling with which cache methods to pick.

$LENGTH, $System.SQL.FLOOR() - seem ok

$EXTRACT - I think for .charAt (though may need to correct base)

$FIND - for .indexOf

Any suggestion which would be the best ones for the job?

Thanks

Dan