Spanish CIF

What it covers

Returns a valid Spanish CIF (tax id for legal entities) with a correct control character. Use `format=vat` for the `ES`-prefixed VAT number. Set `invalid=true` for a CIF with a deliberately wrong control character.

How it’s calculated

The Spanish CIF control character is a Luhn (mod-10) check digit computed over the 7-digit body, rendered as a digit for most entity types and mapped through the letter alphabet "JABCDEFGHI" for entity types P, Q, R, S, W, N.

Body digits: 5 8 8 1 8 5 0. Entity letter A is not in {P,Q,R,S,W,N}, so the control will be a digit.
Process right-to-left; rightmost digit (0) is doubled first, then alternate.
digit 0 (pos7 from left, doubled): 0*2=0 -> 0
digit 5 (pos6, as-is): 5
digit 8 (pos5, doubled): 8*2=16 -> 16-9=7
digit 1 (pos4, as-is): 1
digit 8 (pos3, doubled): 8*2=16 -> 16-9=7
digit 8 (pos2, as-is): 8
digit 5 (pos1, doubled): 5*2=10 -> 10-9=1
Sum = 0+5+7+1+7+8+1 = 29
D = (10 - (29 mod 10)) mod 10 = (10 - 9) mod 10 = 1
Entity type A renders the digit directly, so control character = '1'.

Edge cases

• Entity types P, Q, R, S, W, N always take a LETTER control character (from 'JABCDEFGHI'), never a digit — e.g. control digit D=0 renders as 'J', D=1 as 'A'.
• The 7-digit body may start with 0 (province codes can begin with 0), so the generator draws each of the 7 digits independently 0-9 including the leading one.
• A control digit D=0 maps to the letter 'J' (index 0 of the alphabet), which is why 'J' both appears as a valid entity letter and as a possible control letter.

Gotchas

• The entity letter is NOT fed into the Luhn sum — only the 7 middle digits are. The letter solely determines whether the control is rendered as a digit or as a letter.
• The primitive is plain Luhn: doubled digits over 9 have 9 subtracted (digit-sum), then D = (10 - sum%10) % 10. There is no modulus other than 10 and no external weights array — the doubling pattern comes purely from Luhn's right-to-left alternation over the 7-digit body.
• Because the body is an odd length (7), Luhn's rightmost-first doubling lands on the odd 1-indexed positions from the left; do not assume even positions are doubled.