Czech IČO

What it covers

Returns a valid Czech IČO (business identification number) with a correct mod-11 check digit. Use `format=vat` for the `CZ`-prefixed VAT number (DIČ). Set `invalid=true` for an IČO with a deliberately wrong check digit.

How it’s calculated

The Czech IČO's eighth digit is a weighted mod-11 check digit over the first seven digits, using descending weights 8..2, the 11-complement of the sum modulo 11, and a mod-10 reduction of the overflow residues.

Payload digits: 1,2,3,4,5,6,7 with weights 8,7,6,5,4,3,2.
Weighted products: 8*1=8, 7*2=14, 6*3=18, 5*4=20, 4*5=20, 3*6=18, 2*7=14.
Sum = 8+14+18+20+20+18+14 = 112.
base = 112 mod 11 = 2 (since 11*10 = 110, remainder 2).
raw = 11 - 2 = 9.
checkDigit = 9 mod 10 = 9.

Edge cases

• Leading-zero IČOs are valid: the first payload digit may be 0, so the number is genuinely 8 characters and must not be parsed as a shorter integer (the edge generator emits these deliberately).
• A check digit of 0 is valid and occurs when raw = 10 (base = 1); the mod-10 rule maps it cleanly to 0.
• The mod-10 overflow rule means no residue is ever 'unissued' for this scheme — every seven-digit payload yields exactly one single-digit check digit (complement raw 11 -> 1, raw 10 -> 0).

Gotchas

• The overflow rule is mod-10, NOT the 'invalid' rule used by DK CVR / FI Y-tunnus / PL NIP: do not treat residue 10 as unissued here. Complement raw 11 maps to 1 and raw 10 maps to 0.
• Weights are applied to the payload only (7 weights for 7 payload digits); the 8th digit is the result, not an input to weightedSum.
• The VAT/DIČ form is the same digits with a 'CZ' prefix (format: 'vat') — the checksum is computed on the 8 digits, not on the prefixed string.