# dsh-checkdigit

Check-digit mathematics toolbox for [DeepSeek Harness](https://github.com/deepseek-ai/deepseek-harness) (dsh): generate, validate and detect check digits for **11 schemes** — Luhn, Verhoeff, Damm, ISBN-10, ISBN-13, EAN-8, EAN-13, UPC-A, ISIN, CUSIP and IBAN.

> 校验位数学工具箱：生成 / 验证 / 识别 11 种校验位（Luhn、Verhoeff、Damm、ISBN、EAN、UPC、ISIN、CUSIP、IBAN），零运行时依赖、纯本地算术。

## Why

Language models routinely botch check-digit arithmetic: wrong card-number checksums, ISBNs that fail validation, IBANs with impossible check digits. Every algorithm here is implemented from its published specification with **pure integer arithmetic** — deterministic, offline, zero runtime dependencies — and cross-checked against published worked examples (Apple's `US0378331005`, the classic `GB82 WEST 1234 5698 7654 32`, the Luhn textbook example `79927398713`, and more).

## Install

```bash
dsh plugin --profile web add github:TYEclipse/dsh-checkdigit
```

Then add `dsh-checkdigit` to `dsh.profile.bundles` in your profile's `package.json` if it is not auto-activated, and restart dsh.

## Tools

### `checkdigit_generate`

Compute the check digit for a payload and return the complete identifier.

```text
checkdigit_generate(scheme="luhn", payload="7992739871")
→ checkDigit: "3", complete: "79927398713"
```

Supported `scheme` values and their payloads:

| scheme    | payload                             | check digit      | example complete |
|-----------|-------------------------------------|------------------|------------------|
| `luhn`    | digits (any length ≥ 1)             | last digit       | `79927398713` |
| `verhoeff`| digits (any length ≥ 1)             | last digit       | `2363` |
| `damm`    | digits (any length ≥ 1)             | last digit       | `5724` |
| `isbn10`  | 9 digits                            | 10th char (0-9 or X) | `0306406152` |
| `isbn13`  | 12 digits                           | 13th digit       | `9780306406157` |
| `ean8`    | 7 digits                            | 8th digit        | `96385074` |
| `ean13`   | 12 digits                           | 13th digit       | `4006381333931` |
| `upca`    | 11 digits                           | 12th digit       | `036000291452` |
| `isin`    | 2 letters + 9 alphanumerics         | 12th char (digit)| `US0378331005` |
| `cusip`   | 8 chars from `[A-Z0-9*@#]`          | 9th char (digit) | `037833100` |
| `iban`    | 2-letter country + BBAN             | chars 3–4 (two digits) | `GB82WEST12345698765432` |

Use it to build test card numbers, ISBNs, barcodes, securities identifiers or account numbers whose check digits actually verify.

### `checkdigit_validate`

Verify the check digit of a full identifier. The scheme is auto-detected when omitted (ISIN → CUSIP → IBAN → EAN-13 → UPC-A → EAN-8 → ISBN-10 → Luhn); pass `scheme` to force one.

```text
checkdigit_validate(value="GB82WEST12345698765432")
→ valid: true, scheme: "iban", iban.country: "GB", iban.formatted: "GB82 WEST 1234 5698 7654 32"

checkdigit_validate(value="79927398714")
→ valid: false, scheme: "luhn", checkDigit: "4", expected: "3",
  detail: "Luhn: check digit should be 3, but the value ends in 4"
```

Spaces and dashes in the input are ignored. When a value is invalid, the result carries the **expected** check digit so callers can fix it programmatically.

### `checkdigit_info`

Describe the supported schemes: identifier lengths, payload formats, check-digit positions and worked examples. Call with a `scheme` for one scheme, or without arguments for all 11.

## Supported schemes

| Scheme | Standard | Detail |
|--------|----------|--------|
| Luhn | ISO/IEC 7812 | Credit/debit cards, IMEI, loyalty cards |
| Verhoeff | — | Dihedral-group checksum; catches all single errors and adjacent transpositions |
| Damm | — | Quasigroup operation; full-value tally must be zero |
| ISBN-10 | ISO 2108 | Mod-11; check digit may be `X` (= 10) |
| ISBN-13 | ISO 2108 | EAN-13 arithmetic (Bookland) |
| EAN-8 / EAN-13 | GS1 GTIN-8 / GTIN-13 | Retail barcodes |
| UPC-A | GS1 GTIN-12 | North American retail barcodes |
| ISIN | ISO 6166 | Securities; letters expand A=10…Z=35, then Luhn |
| CUSIP | ANSI X9.6 | North American securities; `* @ #` allowed; even positions doubled |
| IBAN | ISO 13616 | Mod-97 over letter-expanded rearranged string; BBAN length table for 75+ countries |

## Development

```bash
pnpm install
pnpm build
pnpm test   # 51 tests, anchors cross-checked against published worked examples
pnpm lint
```

## License

MIT