Romanian CNP

What it covers

Returns a valid Romanian CNP (Cod Numeric Personal) with a correct check digit. Constrain with `sex` and age/birth-date filters (the CNP encodes the full birth date). Contradictory combinations are rejected with a 400. Set `invalid=true` for a deliberately wrong check digit.

How it’s calculated

The 13th digit of a Romanian CNP is a weighted-sum modulo-11 check over the first 12 digits, with the special rule that a remainder of 10 becomes 1.

First 12 digits: 5 0 1 0 2 0 3 1 2 3 4 5
Weights:        2 7 9 1 4 6 3 5 8 2 7 9
Products: 5x2=10, 0x7=0, 1x9=9, 0x1=0, 2x4=8, 0x6=0, 3x3=9, 1x5=5, 2x8=16, 3x2=6, 4x7=28, 5x9=45
Weighted sum = 10+0+9+0+8+0+9+5+16+6+28+45 = 136
136 mod 11 = 136 - (11 x 12=132) = 4
Remainder is 4 (not 10), so the check digit is 4

Edge cases

• Remainder 10 is the only special case: it maps to check digit 1 rather than producing a two-digit result.
• The leading digit S is not the check digit — it encodes both century and sex (1/2=1900s, 3/4=1800s, 5/6=2000s; odd=male, even=female) and still participates in the weighted sum via weight 2.
• Foreigner markers 7/8 (residents) and 9 do not encode a century and are not emitted by this generator, but the check-digit rule itself is identical for them.

Gotchas

• The weight vector is a fixed 12-element constant (2,7,9,1,4,6,3,5,8,2,7,9); it does not repeat a shorter cycle and must be applied left-to-right starting at the very first digit S.
• Digits are handled as a string throughout (leading zeros in YYMMDD and JJ are significant); toDigits converts the 12-character string to numbers only at the arithmetic boundary — never route a CNP through Number.
• The check is a plain remainder, not an 11-complement: e.g. remainder 4 gives check digit 4, not 7.