Swedish organisationsnummer

What it covers

Returns a valid Swedish organisationsnummer (legal-entity identifier) with a correct Luhn check digit. Use `format` for the SE VAT number. Set `invalid=true` for a number with a deliberately wrong check digit.

How it’s calculated

The 10th digit of a Swedish organisationsnummer is a Luhn (mod-10) check digit over the nine preceding payload digits.

Payload digits, left to right (indices 0-8): 1, 6, 5, 5, 6, 1, 2, 3, 4.
Process right to left; rightmost (4) is doubled, then alternate.
4 doubled = 8.
3 as-is = 3.
2 doubled = 4.
1 as-is = 1.
6 doubled = 12 -> 12-9 = 3.
5 as-is = 5.
5 doubled = 10 -> 10-9 = 1.
6 as-is = 6.
1 doubled = 2.
Sum = 8 + 3 + 4 + 1 + 3 + 5 + 1 + 6 + 2 = 33.
Check digit = (10 - (33 mod 10)) mod 10 = (10 - 3) mod 10 = 7.

Edge cases

• Zero check digit: when the Luhn residue is 0, the check digit is 0 (the (10 - sum%10) % 10 outer mod handles this). A naive numeric parse that drops a trailing zero would mangle such a number.
• Minimum third digit: d3 = 2 is the smallest legal value (the historic 'month pair' >= 20 that separates organisations from personnummer).
• Entity-type group: d1 ranges 1-9; leading zero is never produced, so the first digit is always significant.

Gotchas

• This is plain Luhn — no per-position weight vector is passed to luhnCheckDigit; the weights are Luhn's implicit alternating 2,1,2,1,... applied from the rightmost payload digit inward.
• The rightmost PAYLOAD digit (d9) is the first to be doubled. Because the check digit is not part of the payload here, doubling starts 'true'; validation over the full 10 digits instead starts 'false' (isValidLuhn).
• The generator's Luhn only checks the checksum; it does NOT verify the entity actually exists in Bolagsverket's register — values are structurally valid but synthetic.