Estonian isikukood

What it covers

Returns a valid Estonian isikukood (personal identification code) with a correct two-pass mod-11 check digit. Constrain with `sex` and age/birth-date filters (the full birth date is encoded). Contradictory combinations are rejected with a 400. Set `invalid=true` for a deliberately wrong check digit.

How it’s calculated

The Estonian isikukood check digit is a two-pass weighted-sum modulo-11 over the first ten digits (G YYMMDD SSS), falling back to a second weight set and finally 0 whenever a pass yields a residue of 10.

Payload (G YYMMDD SSS): G=3 (1900s band, male), YYMMDD=850314, serial=007 → 3850314007.
Digits: [3,8,5,0,3,1,4,0,0,7].
First pass with weights1 [1,2,3,4,5,6,7,8,9,1]: 3×1=3, 8×2=16, 5×3=15, 0×4=0, 3×5=15, 1×6=6, 4×7=28, 0×8=0, 0×9=0, 7×1=7.
Sum = 3+16+15+0+15+6+28+0+0+7 = 90.
90 mod 11 = 90 − 88 = 2. r1 = 2.
2 < 10, so the check digit is 2 — no second pass needed.
Full isikukood: 38503140072.

Edge cases

• The leading digit G is not free: it jointly encodes century and sex — 1/2 = 1800s male/female, 3/4 = 1900s male/female, 5/6 = 2000s male/female (odd = male, even = female). G values 7/8 are reserved for a future century and are never generated here.
• When the first-pass residue is exactly 10 the check digit is NOT 10 (there is no two-digit check); the second weight set [3,4,5,6,7,8,9,1,2,3] is applied instead, and only if that also yields 10 does the check digit become 0.

Gotchas

• The two weight sets are related by a rotation: weights2 is weights1 shifted so it starts at 3 (3,4,5,6,7,8,9,1,2,3). Using the wrong set on the fallback pass produces a subtly wrong-but-plausible check digit.
• The check digit covers exactly the first ten digits including the leading G; it must be recomputed if any of G, the date, or the serial changes.
• toDigits preserves leading zeros — the serial SSS and date fields are treated as a digit string, never parsed through Number, so a payload like 3850314007 keeps its interior zeros in the weighted sum.