Function luhnValid

Luhn algorithm — ISO/IEC 7812-1.

Used by: credit cards, CA SIN, FR SIREN/SIRET, IT P.IVA, IL TZ/HP, ZA ID, and several IDs that document the algorithm as "Luhn variant".

Algorithm:

1. From the rightmost digit, double every second digit.
2. If doubling produces a 2-digit number, sum its digits (or subtract 9).
3. Sum all digits.
4. Number is valid iff sum is divisible by 10.

Public-domain mathematical algorithm.