Lithuanian įmonės kodas

What it covers

Returns a valid Lithuanian įmonės kodas (company code) with a correct two-pass mod-11 check digit. The code is non-semantic. Set `invalid=true` for an įmonės kodas with a deliberately wrong check digit.

How it’s calculated

The 9th digit of a Lithuanian įmonės kodas is a two-pass ISO 7064-style weighted mod-11 check digit over the first eight digits.

Payload digits: d1..d8 = 1,2,3,4,5,6,7,8.
Pass 1 with weights1 = [1,2,3,4,5,6,7,8]: 1×1=1, 2×2=4, 3×3=9, 4×4=16, 5×5=25, 6×6=36, 7×7=49, 8×8=64.
Pass-1 sum = 1+4+9+16+25+36+49+64 = 204.
first = 204 mod 11 = 204 − (11×18=198) = 6.
first = 6 < 10, so the check digit is 6 (second pass not needed).

Edge cases

• When the first pass yields residue 10 (no single-digit answer), the algorithm recomputes with the shifted weights2 = [3,4,5,6,7,8,9,1]. This second-pass weight vector is the first-pass weights rotated: positions 1-7 use 3..9 and position 8 wraps back to 1.
• If BOTH passes yield residue 10, the check digit is forced to 0 (this is the ISO 7064 fallback; the raw '10' is never emitted as a check digit).
• The generator draws the leading digit d1 in the range 1-9 (never 0), so a numeric parse of the 9-digit code keeps all nine digits; digits d2..d8 are drawn freely 0-9.

Gotchas

• The check digit is fully determined by the first eight digits, so it can be any value 0-9 (the 0 case can arise either as a genuine residue-0 or as the both-passes-overflow fallback).
• weights1 and weights2 cover the 8-digit payload only, not the check digit position; the weighted sum runs over exactly eight products in each pass.
• The two-pass scheme means you cannot validate with a single fixed weight vector — a code whose pass-1 residue is 10 will only validate against the pass-2 (or 0) result, never against the pass-1 remainder.