Slovak IČO

What it covers

Returns a valid Slovak IČO (identifikačné číslo organizácie) with a correct mod-11 check digit. Set `invalid=true` for an IČO with a deliberately wrong check digit, or `edge=true` for valid edge cases.

How it’s calculated

The Slovak IČO's 8th digit is a weighted mod-11 check digit over the 7 leading digits, read as the 11-complement of the weighted sum modulo 11, then reduced modulo 10 so it is always a single digit.

Weights [8,7,6,5,4,3,2] against digits [1,2,3,4,5,6,7].
Products: 8*1=8, 7*2=14, 6*3=18, 5*4=20, 4*5=20, 3*6=18, 2*7=14.
Weighted sum S = 8+14+18+20+20+18+14 = 112.
base = 112 mod 11 = 2 (since 11*10 = 110, 112-110 = 2).
raw = 11 - 2 = 9.
check = 9 mod 10 = 9.

Edge cases

• Leading-zero IČOs are valid: the first payload digit may be 0 (the edge generator emits these deliberately), so an IČO parsed as a number and reformatted can lose its leading zero and drop below 8 characters.
• A check digit of 0 is legitimate and occurs whenever the 11-complement raw value is 10 or 11 (both reduce to 0 under the mod-10 rule).

Gotchas

• The overflow rule is mod10, not 'invalid': under 11-complement the raw result can be 10 or 11, and both are folded to a real digit (10->0, 11->1) rather than being treated as unissued. There is therefore always a single-digit answer and no residue is rejected — the code's guard for undefined is unreachable.
• The complement is taken before the mod-10 reduction (check = (11 - (S mod 11)) mod 10), not after; reversing the two operations gives wrong results for the overflow cases.