#checkdigit

///|
fn main {
  // 7992739871 -> check digit 3
  println(luhn_check_digit("7992739871")) // Some(3)
  println(luhn_valid("79927398713")) // true

  // Brand detection and full card check
  println(detect_brand("378282246310005")) // amex
  println(card_valid("378282246310005")) // true

  // IBAN accepts spaces and lowercase
  println(iban_valid("gb82 west 1234 5698 7654 32")) // true
  println(iban_format("GB82WEST12345698765432")) // Some("GB82 WEST 1234 5698 7654 32")

  // Identifiers that carry a check character of their own
  println(cn_id_birthdate("11010519491231002X")) // Some("19491231")
  println(vin_valid("1M8GDM9AXKP042788")) // true
  println(isin_country("US0378331005")) // Some("US")
  println(lei_lou("529900T8BM49AURSDO55")) // Some("5299")

  // Formatting helpers for the identifiers that have a printed form
  println(cpf_format("11144477735")) // Some("111.444.777-35")
  println(cnpj_format("11222333000181")) // Some("11.222.333/0001-81")
  println(uscc_division("91350100M000100Y43")) // Some("350100")

  // Retail barcodes and bibliographic numbers
  println(ean13_valid("4006381333931")) // true
  println(upce_expand("07832309")) // Some("078000003239")
  println(isbn10_to_isbn13("0306406152")) // Some("9780306406157")
}

#What it covers

#Algorithms

• Luhn catches any single-digit error and most adjacent transpositions. Used by payment cards, IMEI, and many national identifiers.
• Verhoeff catches all single-digit errors and all adjacent transpositions, using a multiplication table over the dihedral group D5.
• Damm also catches all single-digit errors and all adjacent transpositions, but needs a single table lookup per digit instead of a position-dependent permutation.

#Identifiers

• Payment cards — brand detection by issuer identification number, then the length rules for that brand, then Luhn. Brands: Visa, Mastercard, American Express, Discover, JCB, Diners Club, UnionPay, Maestro. Overlapping issuer ranges are resolved by testing the most specific range first (Discover's 622126–622925 sits inside UnionPay's 62).
• IBAN — country code, registered length for that country (75 countries), then the mod-97-10 check. Input is normalised, so spaces and lowercase are accepted. Bank account numbers can also be generated: iban_check_digits computes the two check digits for a country code plus BBAN, and iban_assemble returns the complete IBAN.
• IMEI — 15 digits with a Luhn check digit, plus TAC (first 8 digits) and serial number (digits 9–14) extraction.
• Chinese resident identity card — 18 characters: division code, birth date, sequence, then the ISO 7064 MOD 11-2 check character (a digit or X). The birth date is range-checked, so month 13 or day 00 is rejected; month lengths and leap years are deliberately not modelled, and the doc comment says so.
• Aadhaar — 12 digits whose last is a Verhoeff check digit, reusing the Verhoeff module directly.
• VIN — 17 characters excluding I, O and Q, with a modulo-11 check character at position nine that may be X. The positional weight table has a zero in the ninth slot so the check character does not disturb its own sum.
• ISIN — 12 characters: a two-letter country code, nine alphanumeric characters of security identifier, and a Luhn digit computed after every letter has been expanded to its two-digit value.
• LEI — 20 characters: a four-character local operating unit prefix, two reserved zeroes, twelve characters of entity reference, and two check digits at the end. Validation expands letters to 10..35 and requires the whole identifier to be 1 modulo 97, the same scheme IBAN uses.
• Unified social credit identifier — 18 characters: registration authority, entity category, a six-digit division code, a nine-character organisation code, and a check character. The check character is chosen over a 31-character code set, the digits plus the letters minus I, O, S, V and Z — the five that read as another character in handwriting. Position i carries the weight 3^(i-1) mod 31, which spreads a single wrong character across the whole sum instead of shifting it by a constant.
• CPF and CNPJ — the Brazilian individual and company registries. Both are two mod-11 check digits over a weighted prefix, with the second digit covering a prefix one character longer than the first; the two weight tables differ only in where the cycle of nine weights restarts. cpf_format and cnpj_format return the punctuated printed form. All-equal-digit input satisfies the arithmetic but is never issued, so it is rejected.
• BSN — the Dutch citizen service number. This one has no separate check digit: the property holds over all nine digits at once, weighted from nine down to one with the last position negated, and the total must be a multiple of eleven. Negating that last position is what stops a Dutch bank account number, checked with the same weights all positive, from passing as a BSN. The eight-digit form with its leading zero omitted is accepted.
• Codice fiscale — the Italian tax code. Sixteen characters, of which the last is the CIN, computed over the first fifteen with two different value tables: one for the odd positions and one for the even. Validation also checks the required layout, including the twelve month codes, which skip the letters that would spell words, and the omocodia letters that stand in for digits when two people would otherwise share a code.
• Retail barcodes (GS1) — the GTIN family: EAN-8, UPC-A, EAN-13 and GTIN-14 all share one mod-10 check digit computed with the alternating 3-1 weighting read from the right, so gtin_valid accepts any of the four registered lengths. UPC-E is the zero-suppressed form of UPC-A; upce_expand restores the twelve-digit UPC-A, and that expansion is what the UPC-E check digit is verified against.
• ISBN and ISSN — ISBN-10 carries a mod-11 check character that may be X (ten), under weights 10 down to 2. ISBN-13 is an EAN-13 restricted to the 978 and 979 bookland prefixes, so it reuses the GS1 check digit, and isbn10_to_isbn13 converts a ten-character ISBN to its thirteen-character form. ISSN is an eight-character serial number with its own mod-11 check character, verified with weights 8 down to 2.

#Installation

moon add sssssurf/checkdigit

#Command line

moon run cmd/main -- card 378282246310005
moon run cmd/main -- iban "gb82 west 1234 5698 7654 32"
moon run cmd/main -- imei 490154203237518
moon run cmd/main -- luhn 7992739871
moon run cmd/main -- verhoeff 236
moon run cmd/main -- damm 572
moon run cmd/main -- cnid 11010519491231002X
moon run cmd/main -- aadhaar 234123412346
moon run cmd/main -- vin 1M8GDM9AXKP042788
moon run cmd/main -- isin US0378331005
moon run cmd/main -- lei 529900T8BM49AURSDO55
moon run cmd/main -- uscc 91350100M000100Y43
moon run cmd/main -- cpf 11144477735
moon run cmd/main -- cnpj 11222333000181
moon run cmd/main -- bsn 111222333
moon run cmd/main -- cf RSSMRA85T10A562S
moon run cmd/main -- gtin 4006381333931
moon run cmd/main -- upce 07832309
moon run cmd/main -- isbn 9780306406157
moon run cmd/main -- issn 03785955

$ moon run cmd/main -- imei 490154203237518
imei: 490154203237518
tac: 49015420
serial: 323751
check digit: 8
status: valid

$ moon run cmd/main -- lei 529900T8BM49AURSDO55
lei: 529900T8BM49AURSDO55
lou: 5299
expected check digits: 55
status: valid

$ moon run cmd/main -- gtin 4006381333931
gtin: 4006381333931
expected check digit: 1
status: valid

$ moon run cmd/main -- upce 07832309
upce: 07832309
expands to: 078000003239
expected check digit: 9
status: valid

$ moon run cmd/main -- isbn 9780306406157
isbn: 9780306406157
form: isbn-13
expected check digit: 7
status: valid

#与现有库的关系（互补边界）

#Testing

moon test

node scripts/check-docs.mjs

#Development

• docs/reproducible-demo.md — how to build, test and run the project, with the exact commands and their expected output.
• docs/development-retrospective.md — key architectural decisions and their trade-offs, the role of AI tooling in the build, the standards referenced, and the known limitations.
• CHANGELOG.md — what changed in each version.

#Runnable example

moon run cmd/demo

#References

• ISO/IEC 7812 — Identification cards, issuer identification numbers
• ISO 13616 — International Bank Account Number (IBAN), mod-97-10
• 3GPP TS 23.003 — Numbering, addressing and identification (IMEI)
• GB 11643 — 公民身份号码 (Chinese resident identity card number)
• GB 32100 — 法人和其他组织统一社会信用代码编码规则 (unified social credit identifier)
• ISO 3779 — Vehicle Identification Number (VIN), world manufacturer identifier
• ISO 6166 — International Securities Identification Number (ISIN)
• ISO 17442 — Legal Entity Identifier (LEI)
• GS1 General Specifications — GTIN check digit and UPC-E zero suppression
• ISO 2108 — International Standard Book Number (ISBN)
• ISO 3297 — International Standard Serial Number (ISSN)
• UIDAI — Aadhaar number specification (Verhoeff check digit)
• Receita Federal — CPF and CNPJ check-digit specification
• De Nederlandsche Bank — BSN eleven-test (11-proef)
• DM 23/12/1976 — Codice fiscale, CIN computation
• GLEIF — Global LEI registry, api.gleif.org
• H. P. Damm, Totally anti-symmetric quasigroups for all orders n ≠ 2 mod 4
• J. Verhoeff, Error detecting decimal codes

#License