# Barcode

Themeable, accessible **1D barcode** — own zero-dependency encoder, framework
4-layer rigor, the anatomy no reference library ships.

> **Design proposal approved 2026-07-28, shipped 2026-07-29.** This README is
> both the design record and the component doc. It is the sibling of
> [`qr-code/README.md`](../qr-code/README.md): same architecture, same doctrine,
> different physics.

## Baseline

No Air baseline — new component (2026-07-28). Nothing in the legacy layers
rendered linear barcodes; the encoder is also new (`$libs/barcode`, zero-dep).
The **internal** baseline is `QrCode` (2026-06-12): eidos-native display
primitive, `scope: ['eidos']`, own encoder in `$libs`, `DownloadTrigger` that
composes `<Button>`. Every divergence from it is argued in `## Decisiones`.

## Comparativa — why a new one (vs the references)

No headless library ships a barcode: ark-ui, bits-ui, Radix and React Aria have
a QR code (or nothing) and stop there. The field belongs to imperative encoders.

| Capability                             | JsBarcode                | bwip-js           | react-barcode   | ark-ui / bits-ui / Radix | **Barcode (ours)**                       |
| -------------------------------------- | ------------------------ | ----------------- | --------------- | ------------------------ | ---------------------------------------- |
| Parts / anatomy                        | ❌ imperative            | ❌ imperative     | ❌ single comp  | ❌ **no barcode at all** | ✅ Provider·Pattern·Text·DownloadTrigger |
| Encoder                                | own dep (~40 kB)         | own dep (~500 kB) | wraps JsBarcode | —                        | **own, zero-dep (`$libs/barcode`)**      |
| Retail symbologies (EAN/UPC)           | ✅                       | ✅                | ✅              | —                        | ✅ EAN-13/8 · UPC-A/E                    |
| Logistics (Code128 A/B/C auto, ITF-14) | ✅                       | ✅                | ✅              | —                        | ✅ + bearer bars                         |
| Check digit computed                   | ✅                       | ✅                | ✅              | —                        | ✅ (returned, not hidden)                |
| Quiet zone per spec                    | ⚠️ single `margin` in px | ✅                | ⚠️ px           | —                        | ✅ **in modules, per symbology**         |
| Guard bars extend under HRI            | ✅ EAN only              | ✅                | ✅              | —                        | ✅ (the `guards` mask)                   |
| Themeable colour (tokens)              | ⚠️ free strings          | ⚠️ free strings   | ⚠️ props        | —                        | ✅ **via tokens + `color`**              |
| Invalid input is a state, not a throw  | ❌ throws / logs         | ❌ throws         | ❌ throws       | —                        | ✅ `data-invalid` + `onInvalid`          |
| `role=img` + label a11y                | ❌                       | ❌                | ⚠️ partial      | —                        | ✅                                       |
| Download PNG/SVG                       | ❌ manual                | ❌ manual         | ❌ manual       | —                        | ✅ **DownloadTrigger = `<Button>`**      |
| Framework morfo/recipe rigor           | n/a                      | n/a               | n/a             | n/a                      | ✅                                       |

**Takeaway:** JsBarcode has the right symbology coverage and the wrong shape
(imperative, mutates a DOM node you hand it, no a11y, no anatomy); bwip-js is
encyclopaedic and enormous; react-barcode is a thin wrapper that inherits both
problems. Ours keeps QrCode's clean anatomy + API, owns the encoder (every
symbology here is a published ISO standard — no npm dep), and adds themeable
colour, real a11y, a first-class invalid state and PNG/SVG export.

## Encoder — `$libs/barcode` (own, zero-dep)

```ts
encode(value, { symbology?, quietZone? }) → BarcodeResult
```

A from-scratch linear-barcode encoder living in `$libs` alongside the other pure
helpers. Each symbology is a **published open standard** — ISO/IEC 15417
(Code 128), ISO/IEC 15420 (EAN/UPC), ISO/IEC 16388 (Code 39), ISO/IEC 16390
(ITF) — so the tables are transcribed from the specification and the layout is
written from scratch. JsBarcode / bwip-js are **correctness references only,
never imported** — the same doctrine `$libs/qr` used with Nayuki's QR reference.
**Zero dependencies added to `package.json`**, runtime or dev.

```ts
export type Symbology = 'code128' | 'ean13' | 'ean8' | 'upca' | 'upce' | 'code39' | 'itf' | 'itf14';

export interface BarcodeResult {
	/** Module lattice, left→right. `true` = dark (bar). Excludes the quiet zone. */
	modules: boolean[];
	/** Symbol width in modules (= `modules.length`). */
	size: number;
	/** Quiet zone in modules per side — the symbology's spec minimum. */
	quietZone: { start: number; end: number };
	/**
	 * Module ranges whose bars extend below the baseline into the text band
	 * (EAN/UPC guard patterns). The renderer must honour them: they are what
	 * lets a scanner find the symbol's edges.
	 */
	guards: { start: number; end: number }[];
	/**
	 * HRI groups anchored in module space (`start` may be < 0 — the quiet zone).
	 * EAN/UPC emit ONE GROUP PER DIGIT, each over the 7 modules that encode it;
	 * the other symbologies emit a single group over the whole symbol.
	 */
	text: { text: string; start: number; end: number }[];
	/** The value as encoded (input + computed check digit). */
	value: string;
	symbology: Symbology;
	/** The check digit computed, when the symbology carries one. */
	checkDigit?: string;
	/** ITF-14 only: the symbol must be framed by a bearer bar. */
	bearerBar?: boolean;
}

export type BarcodeErrorReason =
	| 'empty'
	| 'invalid-characters'
	| 'invalid-length'
	| 'invalid-check-digit';
export class BarcodeError extends Error {
	readonly reason: BarcodeErrorReason;
}
```

`guards` is the **exact analogue of `reserved` in `$libs/qr`**: the function
modules the renderer must treat differently. There they stay square so the
scanner locks on; here they extend below the baseline for the same reason.

### v1 symbologies (Retail + Logistics)

| Symbology | Standard      | Input                          | Check digit                            | Notes                                                               |
| --------- | ------------- | ------------------------------ | -------------------------------------- | ------------------------------------------------------------------- |
| `code128` | ISO/IEC 15417 | ASCII 0–127                    | mod-103, **internal** (not in the HRI) | auto A/B/C subset switching; subset C for digit runs ≥ 4            |
| `ean13`   | ISO/IEC 15420 | 12 digits                      | mod-10, appended                       | L/G parity per first digit; first digit sits **outside** the symbol |
| `ean8`    | ISO/IEC 15420 | 7 digits                       | mod-10, appended                       | no parity table                                                     |
| `upca`    | ISO/IEC 15420 | 11 digits                      | mod-10, appended                       | EAN-13 with a leading 0; outer digits outside the symbol            |
| `upce`    | ISO/IEC 15420 | 6 digits (+ number system 0/1) | mod-10 over the expanded UPC-A         | parity table selected by the check digit                            |
| `code39`  | ISO/IEC 16388 | 43-char alphabet               | mod-43 **optional**                    | `*` start/stop; self-checking                                       |
| `itf`     | ISO/IEC 16390 | digits, **even count**         | mod-10 optional                        | interleaved pairs                                                   |
| `itf14`   | ISO/IEC 16390 | 13 digits                      | mod-10 **mandatory**                   | + bearer bars                                                       |

## ISBN — a profile, not a symbology

**An ISBN barcode IS an EAN-13.** ISO 2108 assigns books the Bookland prefixes
`978` / `979` inside the same GTIN-13 space as everything else, which is why one
scanner reads a novel and a can of beans. So ISBN is not a ninth `symbology`
value: declaring one would make `data-symbology` claim a standard that does not
exist. It is an **input profile** over `ean13` — the shape every serious
implementation ships.

```svelte
<Barcode symbology="ean13" value="978-84-339-2042-3" />
<!-- ISBN-13, hyphenated -->
<Barcode symbology="ean13" value="0306406152" />
<!-- ISBN-10 → 9780306406157 -->
<Barcode symbology="ean13" value="080442957X" />
<!-- base-11 X check character -->
```

What the profile does:

- **Tolerates separators** (`-`, whitespace) in every NUMERIC symbology — an
  ISBN travels hyphenated, a GTIN travels spaced. Code 128 and Code 39 are
  deliberately excluded: there a hyphen is encodable data, and stripping it
  would silently encode a different string.
- **Reads a 10-character `ean13` value as an ISBN-10** and converts it: prefix
  `978`, keep the 9 significant digits, recompute the EAN mod-10 check.
- **Validates the ISBN-10's own check digit first** (weights 10…1, mod 11, where
  `X` = 10). This is the part that matters: without it a mistyped ISBN-10 becomes
  a _perfectly valid_ EAN-13 pointing at a different book — the costliest failure
  mode in this domain. A bad check digit fails with `invalid-check-digit`.

What it deliberately does NOT do:

- **Hyphenate.** The hyphen positions are not computable from the number: they
  depend on the International ISBN Agency's prefix-range tables, which are
  versioned data with an expiry date. Vendoring them would put a maintenance
  clock inside a zero-dependency library. If the `ISBN 978-84-339-2042-3` line
  above the bars is ever added, the hyphenated string comes from the consumer's
  catalogue — where it is already stored that way.
- **Print the upper ISBN line** (the book-cover convention) or the **EAN-5 price
  add-on** — both registered as deferred gaps below.

## Scannability (the doctrine)

The QR's scannability rules restated for a linear symbol — same principle, own
physics:

- **The X-dimension is the anchor.** `moduleWidth` is the rendered width of the
  narrowest bar, in px. It is what governs whether a scanner can resolve the
  symbol (GS1 nominal X = 0.33 mm; retail range 0.264–0.66 mm). Everything else
  derives from it. Rendering below the nominal X degrades decoding — the `<svg>`
  carries `max-width: 100%` so it never overflows a layout, with that caveat
  documented rather than silently accepted.
- **The bar height is free.** A scanner reads a horizontal slice; vertical size
  is redundancy, not information. That is why `height` is an independent prop
  and not a second dimension of a single `size` token.
- **Quiet zone in modules, not pixels — and per symbology.** EAN-13 wants 11 X
  left / 7 X right; Code 128 and ITF want 10 X. The encoder returns the spec
  value; `quietZone` overrides it. There is deliberately no pixel margin prop:
  for fixed spacing in a layout, pad the container in CSS using the barcode's
  `background`.
- **Guard bars stay long.** EAN/UPC guards extend through the HRI band down to
  the digits' baseline (the `guards` mask), so the digit groups read _between_
  them. Flattening them for looks is the linear-barcode equivalent of rounding a
  QR's finder patterns — it breaks edge detection.
- **The HRI aligns digit-under-digit.** Every EAN/UPC digit is centred over the
  7 modules that encode it (the encoder emits one text group per cell), and the
  face is the system's mono anchor — the standard's HRI type is OCR-B, a
  fixed-width design. A proportional face with a compact centred string
  misaligns the digits from their bars and loses the family's silhouette.
- **ITF-14 carries its bearer bar.** The frame sits OUTSIDE the quiet zone (it
  must never eat into it), so the box grows by its thickness on every side.
- **Fixed-tone default colours** (dark-on-light). A barcode cannot invert in
  dark mode and stay scannable, so the default fg/bg are theme-independent;
  theming is opt-in via `color` / `background` (you own the contrast).
- **Export bakes resolved styles.** A serialized SVG loses the document's CSS
  custom properties **and** its inherited font. `DownloadTrigger` resolves the
  fills and the HRI's `font-family` / `fill` into the clone before writing
  PNG / SVG — the same reason QrCode bakes its fills.
- **The HRI travels in both exports.** The text is a real SVG `<text>`, not a
  `<foreignObject>` — so unlike QrCode's logo (which PNG export drops, a
  Chromium rasterization limit) it survives PNG too.

## Anatomy (parts)

```
<Barcode value="5901234123457" symbology="ean13">   ← Provider (svg, role="img", aria-label)
  └ Pattern                                          ← the <path>: bars + guards (decorative)
  └ Text                                             ← the HRI <text> (decorative, optional)
<Barcode.DownloadTrigger>                            ← <Button> → PNG / SVG (optional)
```

morfo `scope: ['eidos']`. **No soma** — a barcode is pure display, like
Avatar / Image / QrCode: `value` → modules in a `$derived`, rendered to SVG.
Download is imperative through the eidos context.

## API (root)

| Prop          | Type                                   | Default                      |                                          |
| ------------- | -------------------------------------- | ---------------------------- | ---------------------------------------- |
| `value`       | `string`                               | `''`                         | data to encode                           |
| `symbology`   | `Symbology`                            | `'code128'`                  | which standard                           |
| `height`      | `number`                               | `60`                         | bar height in px                         |
| `moduleWidth` | `number`                               | `2`                          | px per narrow module — the X-dimension   |
| `quietZone`   | `number`                               | the symbology's spec minimum | in **modules**, never px                 |
| `color`       | `string`                               | fixed dark token             | bar colour (you own the contrast)        |
| `background`  | `string`                               | fixed light token            | quiet-zone + symbol background           |
| `showText`    | `boolean`                              | `true`                       | render the human-readable interpretation |
| `text`        | `string`                               | the encoded value            | override the HRI                         |
| `alt`         | `string`                               | `value`                      | accessible name for the Provider         |
| `onInvalid`   | `(reason: BarcodeErrorReason) => void` | —                            | fires once per distinct reason           |

Two behaviours worth knowing: `onInvalid` is **deduped by reason** (`encoded` is
a fresh object per keystroke, so an un-deduped effect would spam the handler
while a value is typed), and with nothing encoded the placeholder frame carries
`aria-hidden` — a `role="img"` with an empty name would announce an unlabelled
image.

`DownloadTrigger` props: `format: 'png' | 'svg'` (default `'png'`), `filename`
(default `'barcode'`) + all `<Button>` props (`variant`, `size`, `color`, …).
It renders the framework `<Button>` and calls the root's style-baking export via
context.

Compound: `<Barcode>` + `.DownloadTrigger`. The morfo's Pattern / Text parts are
render internals, not subcomponents.

### Geometry

```
mw      = moduleWidth                                  // px per module (the X-dimension)
barH    = height                                       // px
hri     = showText ? max(8, round(barH * 0.2)) : 0     // text band
desc    = guards.length ? round(hri * 0.55) : 0        // guard descender into the band
W       = (quiet.start + size + quiet.end) * mw
H       = barH + hri
viewBox = "0 0 W H"                                    // px, uniform scale → no glyph distortion
```

The `<text>`'s `font-size` travels as a **computed SVG attribute** (proportional
to the band), not as a CSS literal — the same pattern, and the same reason, as
QrCode's inline overlay padding: the viewBox owns geometry, the recipe owns
`font-family` / `letter-spacing` / `fill`.

## Decisiones

- **Own zero-dep encoder** (`$libs/barcode`) — every v1 symbology is a published
  ISO standard; JsBarcode / bwip-js are correctness references only. No npm
  dependency, runtime or dev.
- **No soma** — pure display (like Avatar / Image / QrCode): `value` → modules
  in a `$derived`; download is imperative through the eidos context.
- **Absolute px geometry instead of QrCode's `size` token.** A QR is square, so
  its box collapses into one token. A barcode has two physically independent
  axes: the X-dimension governs scannability, the bar height is free redundancy.
  Collapsing them into one t-shirt token would make the X-dimension a function
  of the height — exactly the property that must not float. The precedent is the
  `chart` family (`chart/gauge.svelte`: numeric `size`, px viewBox, `<text>`
  styled by the recipe), which is the catalog's home for SVG-drawn primitives.
- **HRI inside the SVG** (`<text>`, not an HTML sibling) — a sibling would be
  lost on export, and it is what lets guard bars interleave with the digits.
- **Invalid input is a state, not an exception.** Unlike a QR (any UTF-8 string
  encodes), EAN-13 needs 12/13 digits, ITF an even count, Code 39 a 43-char
  alphabet — a half-typed value is a _normal_ state. `encode` throws a typed
  `BarcodeError`; the wrapper catches it, stamps `data-invalid` and calls
  `onInvalid(reason)` (the framework's canonical validation vocabulary).
  Letting it throw inside a `$derived` would take the page down.
- **`quietZone` in modules with a per-symbology default** (not QrCode's single
  `4`) — each standard fixes its own; pixel spacing belongs to the container.
- **The `*` delimiters stay out of Code 39's value.** They are structure, not
  data: ISO/IEC 16388 keeps the start/stop out of the HRI, and every scanner
  returns the bare string. `result.value` is the (uppercased) input.
- **The HRI is one text group per cell**, emitted by the encoder in module
  space. Placement belongs to whoever knows the cell geometry — the renderer
  only centres each group in its range, so it needs no per-symbology branch.
- **Fixed-tone default colours** — a barcode can't invert in dark mode and stay
  scannable; theming is opt-in via `color` / `background`.
- **Exports bake resolved styles** (`getComputedStyle` into the clone) — a
  serialized SVG loses both the document's custom properties and its font.
- **One `<path>` for the whole pattern** — bars and extended guards are subpaths
  of different heights, mirroring QrCode's single-path module matrix.

## v1 scope (proposed)

8 symbologies (Code 128 A/B/C auto · EAN-13 · EAN-8 · UPC-A · UPC-E · Code 39 ·
ITF · ITF-14) · computed check digits · spec quiet zones · guard bars · HRI with
override · themeable colour · invalid state · download PNG + SVG · `role=img` +
label.

## Gaps

- **Codabar · MSI · Pharmacode** — the niche tail of JsBarcode's set —
  disposition: **diferir** (v2).
- **GS1-128 (FNC1 + Application Identifier parsing)** — shipped by bwip-js —
  disposition: **diferir** (v2); it is a Code 128 profile, not a new encoder.
- **EAN-2 / EAN-5 add-ons** — shipped by JsBarcode; the EAN-5 is the price
  supplement printed beside a book's ISBN — disposition: **diferir** (v2).
- **The `ISBN 978-…` line above the bars** — the book-cover convention of the
  industry guidelines. The band is easy; the hyphenation is not ours (see
  `## ISBN`), so the consumer would pass the already-hyphenated string —
  disposition: **diferir** (v2), pairs with the EAN-5 add-on.
- **2D symbologies (DataMatrix, PDF417, Aztec)** — shipped by bwip-js —
  disposition: **diferir**; each is a whole different encoder (own
  Reed–Solomon, matrix layout), so they belong to their own component, not to
  this one's scope.
- **`textPosition: 'top'`** — shipped by JsBarcode — disposition: **diferir** (v2).
- **Custom webfont in PNG export** — a rasterized SVG does not load webfonts —
  disposition: **descartar** as a defect; the HRI token resolves to a generic
  (`ui-monospace, monospace`) family precisely so PNG export stays faithful.
  Documented, not worked around.

## Passive justification

Zero events by design: a barcode is pure display — `value` in, SVG out; no
interaction surface of its own. The one interactive part, `DownloadTrigger`, IS
a composed `<Button>` (its `contact` firma and states live there). The morfo may
later add an optional `commit-download` cue (`scope` would gain `sema`), the same
recorded exception path QrCode declares.

## Layers

- **`$libs/barcode`** — own encoder + tests.
- **morfo** `barcode.ts` — Provider / Pattern / Text / DownloadTrigger contract.
- **eidos** `barcode/` — wrapper + recipe (`--barcode-*` tokens) + SVG render
  from the module lattice.
- **langs** `langs/components/barcode.ts` — the `download` catalog entry.
- **demo** `web/routes/uix/components/barcode/` + nav entry (Media group).

## Recipe tokens (proposed)

| Token                                                    | Role                                                               |
| -------------------------------------------------------- | ------------------------------------------------------------------ |
| `--barcode-fg` / `--barcode-bg`                          | bar colour / quiet-zone (fixed-tone, annotated `/* literal: … */`) |
| `--barcode-radius`                                       | frame radius                                                       |
| `--barcode-text-font-family`                             | HRI family — anchored to `var(--style-label-font-family)`          |
| `--barcode-text-letter-spacing` / `--barcode-text-fg` | HRI treatment                                                      |
| `--barcode-invalid-border` / `--barcode-invalid-color`   | the invalid placeholder                                            |

No geometry tokens: geometry is computed inline (see Geometry), and a public
token with no real consumer fails `recipe-css-contract.test.ts`.

## Build plan

The route of [`docs/building-a-component.md`](../../../../../docs/building-a-component.md):

| Phase          | Deliverable                                                                                                              | Guard                                                           |
| -------------- | ------------------------------------------------------------------------------------------------------------------------ | --------------------------------------------------------------- |
| 1 · Encoder    | `src/libs/barcode/{barcode,index,barcode.test}.ts`                                                                       | `npx vitest run src/libs/barcode`                               |
| 2 · Morfo      | `morfo/components/barcode.ts` + `'text'` added to `MorfoElement` + `langs/components/barcode.ts` registered in the index | `npm run check` · `npm run translations:check`                  |
| 3 · Sema       | none — 0 events (see `## Passive justification`)                                                                         | `npm run morfo:vocabulary`                                      |
| 4 · Wrapper    | root + download-trigger + context + types + index                                                                        | `component-api-contract.test.ts`                                |
| 5 · Recipe     | `barcode` block in `lib/recipes/base.ts` + `barcode.css`                                                                 | `npm run generate:eidos-css` · `recipe-css-contract.test.ts`    |
| 6 · Demo       | `web/routes/uix/components/barcode/+page.svelte` — **v2 9-tab template** + nav entry                                     | audit `D-*` · `npm run smoke`                                   |
| 7 · README     | this file, minus the "pending implementation" note                                                                       | audit `F-*`                                                     |
| 8 · Acceptance | it passes                                                                                                                | `component:audit --only barcode` · `morfo:check` · `eidos-lint` |

### Morfo skeleton

```
Provider          svg     role='img'   aria-label ← propRef('alt')
                          data-symbology (enum of 8) · data-invalid (optional)
Pattern           path    archetype 'indicator'  aria-hidden          (bars + guards)
Text              text    archetype 'indicator'  aria-hidden  optional (HRI)
DownloadTrigger   button  archetype 'trigger'    type=button + aria-label  optional
texts: { download: '#?components.barcode.download|Download barcode' }
```

`data-symbology` documents the closed set in the contract the same way
`data-cell-shape` does for QrCode (which likewise carries no CSS rule —
`architecture/eidos.md` §"the unused column" classifies that as legitimate).
`data-invalid` **does** get a recipe rule (R-1.4).

### Verification

1. `npx vitest run src/libs/barcode` — published vectors per symbology (EAN-13
   `5901234123457`, UPC-A `036000291452`, Code 39 `*ABC*`, …), check digits,
   module counts, guard positions, quiet zones, **plus an independently written
   test decoder** (tables transcribed a second time) that re-derives the input
   from `modules` — it catches layout / interleaving errors the shared tables
   would hide.
2. `npm run check` — 0 errors in the new files.
3. `npm run generate:eidos-css` + `npx vitest run src/uix/eidos` — TSC + recipe
   contract.
4. `node --import tsx/esm scripts/eidos-lint.ts barcode` — 0 `invalid`.
5. `npm run translations:check`.
6. With `npm run dev` up: `npm run morfo:check` ·
   `SMOKE_SCOPE=/uix/components/barcode npm run smoke`.
7. `npm run component:audit -- --only barcode` — 0 errors; every remaining
   warning justified under `## Audit exceptions`.
8. Browser: open `/uix/components/barcode`, look at it, exercise light/dark, RTL
   and density from the System tab, and check both exports.

**Open item — real-world scanning.** Chrome on Windows does not expose
`BarcodeDetector` (the Shape Detection API ships only on Android / ChromeOS /
macOS), and adding a decoder package would break the zero-dependency premise.
Two ways out, user's call: **(a)** scan the demo with a phone — the real proof;
**(b)** a one-off cross-check with `npx @zxing/library` in a throwaway script
outside the repo (`npx` never touches `package.json`), with exactly the status
Nayuki had for the QR: correctness reference, never an import. Without one of
the two, scannability rests on the standards' vectors alone.

## Audit exceptions

- `R-1.5 exception:` the only interactive part (`DownloadTrigger`) composes the
  canonical `<Button>` — the focus ring lives in `button.css`; `Pattern` and
  `Text` are display indicators.
- `D-1.5` / `D-4.3` (warnings, shared with the canary): the audit greps the demo
  source for an inlined `new MutationObserver` and a literal `uix.events.emit`.
  The v2 migration moved both into the shared harness — the observer lives in
  `DemoTrace`, the ▶ play in `SemaPanel` — so **`button`, the canonical
  reference demo, reports the same two warnings**. Verified by running the audit
  on it. The regexes lag the harness; this demo uses both correctly.
- The Sema ▶ play surface is empty by construction: 0 events (see
  `## Passive justification`), so `SemaPanel` renders its justified empty state.