Latvian personas kods

What it covers

Returns a valid Latvian personas kods with a correct check digit. Latvia does not encode sex. Use `standard` for the `legacy` (date-encoded) or `modern` (non-semantic, default) code; the age/birth-date filters apply to `legacy` only. Contradictory combinations are rejected with a 400. Set `invalid=true` for a deliberately wrong check digit.

How it’s calculated

The 11th digit of a Latvian personas kods is a weighted mod-11 check digit whose remainder is offset by 1 and then folded through mod 10 so it always lands on a single digit 0-9.

First ten digits: 311297 (DDMMYY) + 1 (century, 1900s) + 123 (serial) = 3112971123
Digit values: d = [3, 1, 1, 2, 9, 7, 1, 1, 2, 3]
Weights:      W = [10, 5, 8, 4, 2, 1, 6, 3, 7, 9]
Products: 3*10=30, 1*5=5, 1*8=8, 2*4=8, 9*2=18, 7*1=7, 1*6=6, 1*3=3, 2*7=14, 3*9=27
Weighted sum S = 30+5+8+8+18+7+6+3+14+27 = 126
T = 1 + 126 = 127
127 mod 11 = 127 - 121 = 6
6 mod 10 = 6

Edge cases

• Because the final step is (…mod 11) mod 10, the residue 10 collapses to 0 rather than being unissued — so, unlike many mod-11 identifiers (e.g. PL NIP), no ten-digit prefix is ever 'dead'. The check digit is always a valid 0-9.
• Legacy codes carry a semantic 7th digit that marks the birth century: 0 → 1800s, 1 → 1900s, 2 → 2000s (see century.ts). The check-digit weight rule is identical regardless of century.
• Modern (post-2017) codes are non-semantic: they start with 3, encode no birth date, yet use the exact same weights and formula for the check digit.

Gotchas

• The formula is inline in the generator, NOT the shared mod11CheckDigit helper: it is (1 + weightedSum) % 11 % 10. The extra '+1' offset before the modulo is easy to miss and changes every result.
• The weights [10, 5, 8, 4, 2, 1, 6, 3, 7, 9] are position-fixed left-to-right over the first ten digits; they are the same for both legacy and modern standards.
• Digit strings must be handled as strings (leading zeros are significant, e.g. a DDMMYY head like 010100…); converting through Number would corrupt the payload.