Polish land and mortgage register number

What it covers

Returns a valid Polish land and mortgage register number (numer księgi wieczystej) with a correct check digit. The four-character court department code is genuine, drawn from a curated table of real courts; only the eight-digit register number is synthetic. Use `court` to restrict the draw to a single court (by code, e.g. WA1M, or by a diacritic-sensitive name substring, e.g. Krakowie) — an unknown court is rejected with a 400. Use `format` to choose slashed (CCCC/NNNNNNNN/K) or compact rendering. Set `invalid=true` to get a number with a deliberately wrong check digit, for testing checksum validators.

How it’s calculated

The Polish land-and-mortgage-register number (numer księgi wieczystej) ends in a single check digit that is a positionally-weighted sum of its twelve significant characters, taken modulo 10.

Significant characters (12): K R 1 P 0 0 1 2 3 4 5 6
Character values: K=21, R=27, 1=1, P=26, 0=0, 0=0, 1=1, 2=2, 3=3, 4=4, 5=5, 6=6
Weights cycling 1,3,7: pos0 K=21x1=21; pos1 R=27x3=81; pos2 1=1x7=7; pos3 P=26x1=26; pos4 0=0x3=0; pos5 0=0x7=0; pos6 1=1x1=1; pos7 2=2x3=6; pos8 3=3x7=21; pos9 4=4x1=4; pos10 5=5x3=15; pos11 6=6x7=42
Sum = 21+81+7+26+0+0+1+6+21+4+15+42 = 224
Check digit = 224 mod 10 = 4

Edge cases

• The court-code portion (CCCC) is alphanumeric, so the check-digit input mixes letters and digits — unlike a purely numeric identifier, the mod-10 sum can be driven by letter values up to 33.
• The check digit is a plain modulo-10 residue, so all ten values 0–9 are reachable and none is reserved or unissued.
• Because letters map with Q and V removed, the value sequence is contiguous (…P=26, R=27, S=28…) — implementers who naively use A=1..Z=26 or keep Q/V will get wrong values.

Gotchas

• The two slashes and the check digit itself are excluded from the sum; only the 12 characters CCCC + NNNNNNNN are weighted. Feeding the slashes or the check digit into the sum gives a wrong result.
• The weight pattern is 1,3,7 applied from the leftmost significant character (the court code), not from the register number — position 0 is the first court-code letter.