Bulgarian EIK

What it covers

Returns a valid Bulgarian EIK (Единен идентификационен код, the company/Bulstat number) with a correct two-stage check digit. Use `format=vat` for the BG VAT number (`BG` + EIK). Set `invalid=true` for an EIK with a deliberately wrong check digit.

How it’s calculated

The 9th digit of a Bulgarian EIK/BULSTAT is a two-stage weighted-sum modulo-11 check digit over the eight preceding digits, with a fallback second weight set used only when the first stage produces the ambiguous residue 10.

Payload digits: [1, 2, 3, 4, 5, 6, 7, 8].
Stage-one weights: [1, 2, 3, 4, 5, 6, 7, 8].
Products: 1*1=1, 2*2=4, 3*3=9, 4*4=16, 5*5=25, 6*6=36, 7*7=49, 8*8=64.
Stage-one sum = 1+4+9+16+25+36+49+64 = 204.
residueOne = 204 mod 11: 11*18 = 198, 204 - 198 = 6, so residueOne = 6.
residueOne (6) is less than 10, so the check digit is 6. Stage two is not needed.

Edge cases

• Stage two is only ever consulted when the stage-one residue equals exactly 10; for every other payload the check digit is simply the stage-one residue (0-9).
• When the stage-two residue is also 10, the check digit is mapped to 0 rather than being left as a two-character value, keeping the identifier a single 9-digit string.

Gotchas

• The weighted sum here is a plain positional sum (weightedSum) taken directly mod 11 — it is NOT an 11-complement scheme; the residue itself is the check digit, so residue 0 yields check digit 0.
• This generator produces the 9-digit EIK form (legal entities / sole traders). The 13-digit BULSTAT form used by branches and some registrations has its own separate two-weight rule and is not what this generator emits.
• Weights start at 1 (not the position index+? offset seen in some schemes): d1 is multiplied by 1 in stage one and by 3 in stage two.