Polish PESEL

What it covers

Returns a valid Polish PESEL with a correct checksum. Constrain the person with `sex` and any combination of age/birth-date filters; contradictory combinations are rejected with a 400. Set `invalid=true` to get a PESEL with a deliberately wrong check digit (birth date and sex stay valid), for testing checksum validators.

A PESEL encodes a real birth date, a sex, and an ordinal serial into eleven digits, then closes with a check digit. Because the birth century is folded into the month field, the same two month digits mean different centuries — a PESEL alone tells you the full birth date without a separate century field.

How it’s calculated

The eleventh digit is a weighted mod-10 check digit over the first ten digits.

4×1 = 4
4×3 = 12
0×7 = 0
5×9 = 45
1×1 = 1
4×3 = 12
0×7 = 0
1×9 = 9
3×1 = 3
5×3 = 15
sum = 4+12+0+45+1+12+0+9+3+15 = 101
101 mod 10 = 1
(10 − 1) mod 10 = 9

Edge cases

• The century is carried in the month, not a separate field: month digits have 80 added for 1800–1899, 0 for 1900–1999, 20 for 2000–2099, 40 for 2100–2199, and 60 for 2200–2299. So a January birth reads as month 01, 21, 41, 61, or 81 depending on the century.
• The serial’s last digit sets the sex: odd → male, even → female. Constraining sex therefore constrains the tenth digit’s parity, not a separate flag.
• With invalid=true the check digit is deliberately wrong while the birth date and sex stay valid — so a validator that only checks the date passes it, but one that verifies the checksum rejects it. That’s the point: it isolates checksum-validation bugs.

Gotchas

• A PESEL is not a random 11-digit string — the first ten digits must form a real calendar date after removing the century offset, or no amount of check-digit fixing makes it valid.
• Contradictory age/birth-date constraints (e.g. atAge with a bornOn that disagrees) are rejected with a 400 rather than silently ignored.