Swedish personnummer

What it covers

Returns a valid Swedish personnummer with a correct Luhn check digit. Constrain with `sex` and age/birth-date filters (the full birth date is encoded). Contradictory combinations are rejected with a 400. Use `format` for the short/long rendering and `kind` for a samordningsnummer. Set `invalid=true` for a deliberately wrong check digit.

How it’s calculated

The 10th digit of a Swedish personnummer is a Luhn (mod-10) check digit computed over the nine preceding significant digits YYMMDDNNN.

Significant payload digits = head 900514 + serial 123 = 900514123 (9 digits).
Scan right to left with doubling starting TRUE at the rightmost digit:
pos '3' (double): 3*2=6 -> 6
pos '2' (single): 2
pos '1' (double): 1*2=2 -> 2
pos '4' (single): 4
pos '1' (double): 1*2=2 -> 2
pos '5' (single): 5
pos '0' (double): 0*2=0 -> 0
pos '0' (single): 0
pos '9' (double): 9*2=18 -> 18-9=9
Sum = 6+2+2+4+2+5+0+0+9 = 30
Check digit = (10 - (30 mod 10)) mod 10 = (10 - 0) mod 10 = 0

Edge cases

• Samordningsnummer (coordination number): the day-of-month field has 60 added to it before the checksum is computed, so the same Luhn algorithm runs over the offset day (e.g. day 14 becomes 74).
• The 9th significant digit (last digit of the serial NNN) encodes sex by parity: even = female, odd = male. When a sex is forced the generator draws it as nextInt(0,4)*2 + parity to guarantee the right parity, then still runs the plain Luhn over all nine digits.

Gotchas

• Only the nine digits YYMMDDNNN feed the Luhn sum — the two-digit year is used, NOT the full four-digit year and NOT any century/separator marker; the century (+/- for age 100) lives outside the checksummed digits.
• The doubling alternation starts TRUE on the rightmost payload digit (the check-digit generation convention), so it is the 9th significant digit that gets doubled first, not left undoubled.