Polish ID card number

What it covers

Returns a valid Polish ID card number (numer dowodu osobistego) with a correct checksum and an expiration date (a random day less than ten years ahead). Use `format` to choose between compact (`ABA300000`) and spaced (`ABA 300000`) rendering. Set `invalid=true` to get a number with a deliberately wrong check digit, and `expired=true` to get a past expiration date — both for testing validators.

How it’s calculated

The Polish ID-card (dowód osobisty) number carries a single mod-10 check digit sitting at the fourth position, computed from a weighted sum of the three series letters and five trailing digits using the fixed weights 7,3,1,7,3,1,7,3.

Character values: A=10, B=11, A=10; trailing 1=1, 2=2, 3=3, 4=4, 5=5.
Apply weights 7,3,1 to the series: 7*10 = 70, 3*11 = 33, 1*10 = 10.
Skip index 3 (the check slot carries weight null).
Apply weights 7,3,1,7,3 to the five trailing digits: 7*1 = 7, 3*2 = 6, 1*3 = 3, 7*4 = 28, 3*5 = 15.
Weighted sum = 70 + 33 + 10 + 7 + 6 + 3 + 28 + 15 = 172.
Check digit = 172 mod 10 = 2.
Insert at index 3: full number = ABA212345 (compact), or ABA 212345 with a space.

Edge cases

• The three series characters are always letters A-Z in this generator, so they always contribute values in the range 10-35 (never a bare digit) — the letter-to-number mapping (A=10 … Z=35) is essential; treating a series letter as a face-value digit would produce a wrong check digit.
• The check digit occupies index 3 — it is the FIRST of the six digits in the number part, not the last. A reader who appends the check digit to the end would place it wrong.
• Every combination of series letters and trailing digits yields exactly one valid check digit (0-9); the generator has no rejection/retry loop because mod-10 always resolves to a single digit.

Gotchas

• The weight at index 3 (the check slot) is null and skipped; in the source the placeholder character inserted there is a literal '0', but it never enters the sum because its weight is null — do not accidentally give it weight 7.
• This is a plain modulo, not a mod-10 complement (10 - (sum mod 10)) and not Luhn. The residue itself is the answer, so a sum ending in 0 gives check digit 0.
• Character values above 9 (letters) are used raw in the weighted sum — there is no digit-splitting or casting-out as in Luhn; A contributes its full value 10 times the weight.