cz/dic rejects valid Czech VAT: checkSpecial differs from python-stdnum

[marcos-cuentica]

Root cause: a divergence between stdnum-js and python-stdnum in cz/dic (the special 9-digit case). These VAT numbers are valid per the official EU VIES service, but stdnum-js rejects them.

Examples: CZ687836437, CZ681208919

Details from Claude

Implementations of the Czech special case (9-digit DIČ starting with 6). In cz/dic checkSpecial the JS version includes the leading 6 in the weighted sum (splitAt(value, -1), weights [8,7,6,5,4,3,2,1]), while python-stdnum excludes the first and last digits (digits 2..8, weights [8,7,6,5,4,3,2]). With the python slicing, 687836437 validates and 687836438 (wrong check digit) is still rejected.

Follow-up: fixing only the slice isn't enough. The final formula
(8 - (10 - check) % 11) % 10 relies on Python's always-non-negative modulo.
In JS % takes the dividend's sign, so when (10 - check) % 11 > 8 the result
is negative and the check digit is wrong.

Example: CZ681208919 (valid per VIES) → intermediate 8 - 9 = -1;
Python -1 % 10 = 9 (correct), JS -1 % 10 = -1 (rejected).

Complete fix: const digit = ((8 - ((10 - sum) % 11)) % 10 + 10) % 10;

Version: 1.12.3

[iwian]

Can confirm this issue also affects Czech VAT-group IDs.

For example, CZ699005400 is a valid VAT-group ID used by Nemocnice České Budějovice and other members of the same VAT group, but stdnum.CZ.dic.validate('CZ699005400') returns InvalidChecksum.

The Czech Tax Administration defines VAT-group IDs using the format CZ699nnnnnk, where k is the check digit:
https://financnisprava.gov.cz/assets/cs/prilohy/d-seznam-dani/Infor_skupinova_reg.pdf

The organization publishes the same VAT ID here:
https://www.nemcb.cz/kontakty/telefonni-seznam/

Using the checksum calculation from python-stdnum correctly validates CZ699005400.