fix(cz/dic): correct special 9-digit DIC check to match python-stdnum (fixes #164) - #169
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fix(cz/dic): correct special 9-digit DIC check to match python-stdnum (fixes #164)#169youdie006 wants to merge 1 commit into
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cz/dic checkSpecial (the special 9-digit DIC case, numbers starting with 6) diverged from the python-stdnum reference (cz/dic.py::calc_check_digit_special) two ways: (1) it included the leading 6 in the weighted sum (splitAt(value,-1) -> 8-digit front, weights [8,7,6,5,4,3,2,1]); the reference sums digits 2..8 only (number[1:8], weights [8,7,6,5,4,3,2]), excluding the leading 6 and the check digit. (2) the final reduction (8 - ((10 - sum) % 11)) % 10 relied on Python's non-negative modulo, but JS % keeps the dividend's sign, so a negative inner value yielded e.g. '-1' and never matched. As a result every special-case Czech VAT was falsely rejected via validate() -> InvalidChecksum. Drop the leading digit from the weighted sum, use weights [8,7,6,5,4,3,2], and compute the final digit with the existing pymod helper. Fixes koblas#164
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Fixes #164.
Root cause
cz/diccheckSpecial(the special 9-digit DIČ case, numbers starting with6) diverged from thepython-stdnumreference (cz/dic.py::calc_check_digit_special) in two ways:6in the weighted sum (splitAt(value, -1)-> 8-digit front, weights[8, 7, 6, 5, 4, 3, 2, 1]). The reference sums digits 2..8 only (number[1:8], weights[8, 7, 6, 5, 4, 3, 2]), excluding both the leading6and the check digit.(8 - ((10 - sum) % 11)) % 10relied on Python's always-non-negative modulo. JS%keeps the dividend's sign, so when the inner value is negative the computed digit becomes e.g."-1"and never matches.As a result every special-case Czech VAT (9-digit DIČ starting with
6) was falsely rejected viavalidate()->InvalidChecksum.Fix
Drop the leading digit from the weighted sum, use weights
[8, 7, 6, 5, 4, 3, 2], and compute the final digit with the existingpymodhelper (Python modulo).Verified valid VATs (accepted per the EU VIES service)
CZ687836437,CZ681208919(from the issue)640903926(python-stdnum's own doctest)699005400(VAT-group ID, format699nnnnnk, reported in the issue comment)Wrong check digit (
687836438) is still rejected, and normal non-special DIČ (e.g.CZ991231123, delegated tocz/rc) is unchanged.Testing
Red/green verified: reverting the fix fails exactly the four valid special VATs; with the fix all pass. Full Jest suite (200 suites / 1641 tests),
tsc --noEmit, and eslint all clean.This contribution was prepared with AI assistance (Claude); the change and tests were reviewed and verified locally.