QR reference and RF Creditor Reference calculator

How to build a good reference

The reference does not need to be readable by a human: it must be unique and allow your software to find the invoice. A common construction puts a customer number and an invoice number side by side, padded with zeros.

Avoid including a date or an amount: they change, and the reference must stay stable if the invoice is corrected.

How recursive modulo 10 is calculated

The digits are read from left to right with a carry that starts at 0. For each digit, the new carry is read from the table 0, 9, 4, 6, 8, 2, 7, 1, 3, 5 at position (carry + digit) modulo 10. The check digit is (10 - final carry) modulo 10.

table = [0, 9, 4, 6, 8, 2, 7, 1, 3, 5]
carry = 0
for each digit d of the first 26:
    carry = table[(carry + d) mod 10]
check = (10 - carry) mod 10

On the official SIX example, 21 00000 00003 13947 14300 0901, the final carry is 3 and the check digit is 7.