Maltese VAT number

What it covers

Returns a valid Maltese VAT number (8 digits with a correct mod-37 check). Use `format=vat` for the `MT`-prefixed form. Set `invalid=true` for a number with a deliberately wrong check.

How it’s calculated

The Maltese VAT number's last two digits are a mod-37 check value: the weighted sum of the six payload digits (weights 3,4,6,7,8,9) is 37-complemented and written as two zero-padded decimal digits.

Payload digits: d1=1, d2=2, d3=3, d4=4, d5=5, d6=6; weights [3,4,6,7,8,9].
Weighted sum = 3*1 + 4*2 + 6*3 + 7*4 + 8*5 + 9*6 = 3 + 8 + 18 + 28 + 40 + 54 = 151.
r = 151 mod 37 = 151 - (37*4=148) = 3.
v = (37 - 3) mod 37 = 34.
Render 34 zero-padded to two digits -> "34".
Verify with full-8-digit rule: weights [3,4,6,7,8,9,10,1] over 1,2,3,4,5,6,3,4 = 151 + 10*3 + 1*4 = 151 + 30 + 4 = 185 = 37*5, and 185 mod 37 = 0. Correct.

Edge cases

• Zero check value: when the weighted sum is already a multiple of 37 (r = 0), v = 0 and the check digits render as "00" — a valid number the generator emits deliberately as an edge case.
• First payload digit is drawn 1-9 (never 0), so the common shape has no leading zero; the generator also emits explicit first-digit-boundary edges with d1 = 1 or 9.

Gotchas

• The check is a single mod-37 VALUE spanning both trailing digits, not two independent per-digit checks. The two digits encode one number 0..36 as 10*d7 + d8, which is why the equivalent full-length rule uses weights 10 and 1 for positions 7 and 8.
• weightedSum sums weight*digit with NO per-product reduction (e.g. 9*6 = 54 stays 54); only the final total is reduced mod 37.
• Because v is at most 36 it never reaches three digits, so the two-digit render never overflows — the code relies on this and never redraws.
• The (37 - r) % 37 form matters only at r = 0: it maps 0 back to 0 rather than to 37, keeping the value in 0..36.