视频在屏幕和EPUB里都能播放的实验报告单

// ═══ Postext Cookbook · Nº 132 · A lab sheet whose clips play on screen and in the EPUB ════
// https://postext.dev/en/cookbook/pendulum-lab-video-players
// Code: MIT · Text: original (CC BY 4.0) · Clips: rendered in code (CC BY 4.0)
// Fonts: Source Serif 4, Red Hat Display, Red Hat Mono (SIL OFL 1.1) · Needs postext ≥ 1.16.1
import {
  buildDocument, renderPageToCanvas, renderToHtml, applyHtmlViewerOverrides,
  clearMeasurementCache, registerResourceImage, defaultResourceTypes, resourceVideoLink,
} from 'https://esm.sh/postext';
import { renderToPdf, decompressWoff2 } from 'https://esm.sh/postext-pdf';
import { renderToEpub, readEpub } from 'https://esm.sh/postext-epub';

const LANG = 'en'; // @lang: the language of the sample document ('en' | 'es')
const RECIPE = 'pendulum-lab-video-players';

// ─── 1 · Design ─────────────────────────────────────────────────────────────
const palette = { // the clips' own colours: a cream ground, the rust bob, warm black ink
  ink: '#22201b', // text, the play mark, the QR modules, table heads
  rust: '#a8432b', // the accent: the band, labels, list numbers (the bob's red, darker)
  cream: '#f6f1e4', rule: '#cbc2ae', // box fills, a shade deeper than the clips; hairlines
  muted: '#68625a', paper: '#ffffff' }; // muted: running heads, notes, prompts
const col = (id) => ({ hex: palette[id], model: 'hex', paletteId: id });
const colorPalette = Object.entries({ ...palette, 'main-color': palette.rust }) // defaults: rust
  .map(([id, hex]) => ({ id, name: id, value: { hex, model: 'hex' } }));
const [TEXT, DISPLAY, MONO] = ['Source Serif 4', 'Red Hat Display', 'Red Hat Mono'];
const [TOP, INNER, OUTER, BAND, LEAD] = [22, 20, 17, 84, 13.2]; // mm, A4; LEAD in pt
const px = (value) => ({ value, unit: 'px' }); // the screen edition's sizes
const at = (to, edge, x, y, size) => ({ anchor: { to, edge }, offset: { x, y }, size });
const label = (size, color = 'rust') => ({ fontFamily: MONO, fontSize: pt(size), fontWeight: 600,
  letterSpacing: pt(size * 0.16), textTransform: 'uppercase', color: col(color) });

// #region answer: two clips of the book's own: a poster and a QR code, or a player
const clip = (id, file, poster, caption, altText) => ({
  id, typeId: 'video', kind: 'video', createdAt: 0, updatedAt: 0, caption: t(caption),
  note: t({ en: '8.00 s, 25 frames per second · Postext Cookbook, CC BY 4.0',
    es: '8,00 s, 25 fotogramas por segundo · Postext Cookbook, CC BY 4.0' }),
  altText: t(altText), placement: { position: 'bottom', span: 'column' },
  video: { source: 'file', fileId: file, format: 'mp4', width: 640, height: 360, duration: 8,
    url: asset(file), // the production address: printed as the QR code, linked from the PDF
    poster: { fileId: poster, format: 'jpeg', width: 640, height: 360 }, // the first frame
  },
});
const videoStyle = {
  playMark: { shape: 'rounded', position: 'top-left', size: mm(7), inset: mm(3),
    background: col('ink'), color: col('paper') },
  qr: { position: 'bottom-right', size: mm(19), inset: mm(2.5), // 111 characters: 45 modules
    color: col('ink'), background: col('paper'), radius: mm(1) },
  // The HTML5 player of a file honours every option. download: false writes
  // controlslist="nodownload": it hides the button, and the file stays at its address.
  player: { download: false, loop: true, muted: true, preload: 'auto',
    pictureInPicture: false, remotePlayback: false },
};
const clips = [
  clip('small', 'pendulum-small.mp4', 'pendulum-small-poster.jpg',
    { en: 'Released from rest at 10°.', es: 'Soltado desde el reposo a 10°.' },
    { en: 'A rust bob on a dark string hangs a few degrees off the vertical, under a short bar.',
      es: 'Una lenteja roja cuelga de un hilo oscuro a pocos grados de la vertical.' }),
  clip('large', 'pendulum-large.mp4', 'pendulum-large-poster.jpg',
    { en: 'Released from rest at 80°.', es: 'Soltado desde el reposo a 80°.' },
    { en: 'The same pendulum pulled out almost level with its pivot, a pale arc below it.',
      es: 'El mismo péndulo, casi a la altura de su eje, sobre un arco pálido.' }),
];
// #endregion

// #region periods: Table 2 is worked out here, not typed: T = T0 / AGM(1, cos ½θ0)
// The arithmetic-geometric mean gives the elliptic integral K of the exact period in a few steps.
const T0 = 2 * Math.PI * Math.sqrt(1 / 9.81); // s: L = 1 m, g = 9.81 m/s²
const agm = (a, b) => (Math.abs(a - b) < 1e-15 ? a : agm((a + b) / 2, Math.sqrt(a * b)));
const period = (degrees) => T0 / agm(1, Math.cos((degrees * Math.PI) / 360));
const num = (x, digits) => x.toFixed(digits).replace('.', t({ en: '.', es: ',' }));
const cell = (content, align = 'right') => ({ content, align, verticalAlign: 'middle' });
const table = (id, caption, head, rows, columnWidths) => ({ id, typeId: 'table', kind: 'table',
  createdAt: 0, updatedAt: 0, caption: t(caption), placement: { position: 'here' },
  table: { model: { headerRowCount: 1, columnWidths, rows: [head.map((h, i) =>
    ({ ...cell(h, i ? 'right' : 'left'), isHeader: true })), ...rows] } } });
const periods = table('periods', { en: 'Exact periods of a pendulum 1.00 m long.',
  es: 'Periodos exactos de un péndulo de 1,00 m.' },
['Amplitude|Amplitud', 'T (s)', 'T/T~0~', 'Longer by|Aumento'].map((h) =>
  t({ en: h.split('|')[0], es: h.split('|').at(-1) })),
[5, 10, 20, 30, 45, 60, 80, 90].map((d) => [cell(`${d}°`, 'left'), cell(num(period(d), 3)),
  cell(num(period(d) / T0, 3)), cell(`${num((period(d) / T0 - 1) * 100, 2)} %`)]),
[1.2, 1, 1, 1.1]);
const blank = cell(' \n '); // two empty lines: room to write
const data = table('data', { en: 'Your measurements.', es: 'Tus medidas.' },
  t({ en: ['Clip', 'Swings in 8 s', 'T (s)'], es: ['Vídeo', 'Oscilaciones en 8 s', 'T (s)'] }),
  [10, 80].map((d, i) => [cell(`${t({ en: 'Video', es: 'Vídeo' })} ${i + 1} · ${d}°`, 'left'),
    blank, blank]), [1.2, 1.5, 0.8]);
// #endregion

// #region opener: a rust band with the sheet's number, its title and the fields to fill in
const words = (id, content, family, size, placement, extra) => ({ kind: 'text', id, content,
  fontFamily: family, fontSize: pt(size), color: col('paper'), align: 'left',
  overflow: 'wrap', placement, ...extra }); // titles wrap (gotcha: overflow-ellipsis-default)
const names = t({ en: ['Name', 'Group', 'Date'], es: ['Nombre', 'Grupo', 'Fecha'] });
const fields = [[0, 86], [90, 30], [124, 49]].flatMap(([x, width], i) => [ // mm, from the margin
  words(`label-${i}`, names[i], MONO, 7, at('page', 'top-left', mm(INNER + x), mm(BAND - 16)),
    label(7, 'paper')),
  { kind: 'box', id: `field-${i}`, style: { backgroundColor: col('paper'), borderRadius: mm(1) },
    placement: at('page', 'top-left', mm(INNER + x), mm(BAND - 12.5),
      { width: mm(width), height: mm(6.5) }) }]);
const opener = { enabled: true, minHeight: mm(BAND - TOP + 6), slot: { elements: [
  { kind: 'box', id: 'band', style: { backgroundColor: col('rust') },
    placement: at('bleed', 'top-left', mm(0), mm(0), { width: 'fill', height: mm(BAND) }) },
  words('kicker', '{attr.kicker}', MONO, 8.5, at('page', 'top-left', mm(INNER), mm(14)),
    label(8.5, 'paper')),
  words('title', '{titleText}', DISPLAY, 46, at('#kicker', 'below', mm(0), mm(2.5),
    { width: mm(150) }), { fontWeight: 800, lineHeight: 0.98 }),
  words('lead', '{attr.lead}', TEXT, 10, at('#title', 'below', mm(0), mm(3.5),
    { width: mm(158) }), { lineHeight: 1.4 }),
  ...fields] } }; // a label over a white field for each, to fill in by hand
// #endregion

const head = (id, content, edge, x, extra) => ({ kind: 'text', id, content, parity: 'even',
  pages: 'body', ...label(7, 'muted'), fontWeight: 500, placement: at('page', edge, mm(x),
    mm(12)), ...extra });
const header = { elements: [
  head('folio', '{pageNumber}', 'top-left', OUTER, { color: col('rust'), fontWeight: 700 }),
  head('title', '{title} · {attr.kicker}', 'top-left', OUTER + 7)] };
const footer = { elements: [head('drop-folio', '{title} · {pageNumber}', 'bottom-right', 0, {
  parity: 'all', pages: 'opener', placement: at('page', 'bottom-right', mm(-OUTER), mm(-11)) })] };
const box = (id, extra) => ({ id, background: col('cream'), borderRadius: mm(1.5),
  padding: { top: mm(3), right: mm(3.5), bottom: mm(3), left: mm(3.5) },
  titleStyle: { ...label(7.5), gap: mm(1.2) }, body: { fontSize: pt(9.2), lineHeight: pt(LEAD),
    textAlign: 'left', firstLineIndent: pt(0) }, ...extra });

const config = () => ({ // a factory: the engine caches configs by identity
  locale: t({ en: 'en-us', es: 'es' }), // exact codes (gotcha: hyphenation-locales)
  // Figure, Table, Video in the sheet's language, counted 1, 2… (gotcha: resource-types-locale)
  resourceTypes: defaultResourceTypes(LANG).map((type) => ({ ...type, numberingTemplate: '{n}',
    resetOn: 'never', shortLabel: t({ en: type.name, es: type.name.toLowerCase() }) })),
  colorPalette, videoStyle,
  page: { sizePreset: 'custom', width: mm(210), height: mm(297), dpi: 150, margins: {
    top: mm(TOP), bottom: mm(22), left: mm(INNER), right: mm(OUTER), mirror: true } },
  layout: { layoutType: 'double', gutterWidth: mm(7) }, // columns of 83 mm
  bodyText: { fontFamily: TEXT, fontSize: pt(9.4), lineHeight: pt(LEAD), color: col('ink'),
    boldColor: col('ink'), italicColor: col('ink'), referenceColor: col('rust'),
    textAlign: 'justify', firstLineIndent: mm(4), indentAfterHeading: false,
    hyphenation: { enabled: true }, optimalLineBreaking: true,
    avoidWidows: true, avoidOrphans: true, avoidRunts: true },
  headings: { fontFamily: DISPLAY, color: col('ink'), fontWeight: 700, levels: [
    // Restated: any headings object drops the H1 break (gotcha: headings-drop-h1-break).
    { level: 1, span: 'page', breakBefore: { enabled: true, parity: 'odd' },
      marginTop: pt(0), marginBottom: pt(0), advancedDesign: opener },
    { level: 2, fontSize: pt(12.5), lineHeight: pt(LEAD), marginTop: pt(LEAD),
      marginBottom: pt(LEAD / 2) },
  ] },
  orderedLists: { numberFormat: 'arabic', fontFamily: MONO, fontWeight: 700,
    color: col('rust'), marginTop: pt(0), marginBottom: pt(0) },
  calloutStyles: [
    box('aim', { stripe: { enabled: true, side: 'top', width: pt(2.5), color: col('rust') } }),
    box('method'), box('sheet'),
    { id: 'answer', background: col('paper'), borderRadius: mm(1), marginTop: mm(1.2),
      marginBottom: mm(2.5), border: { enabled: true, color: col('rule'), width: pt(0.6) },
      padding: { top: mm(1), right: mm(2.5), bottom: mm(1), left: mm(2.5) },
      body: { ...label(6.5, 'muted'), fontWeight: 500, lineHeight: pt(LEAD) } },
  ],
  tableStyle: { rules: 'horizontal', borderColor: col('rule'), borderWidth: pt(0.5),
    headerBackground: col('ink'), headerColor: col('paper'), headerFontFamily: MONO,
    headerFontSize: pt(7.5), bodyFontFamily: MONO, bodyFontSize: pt(8.5),
    bodyColor: col('ink'), cellPadding: mm(1.4) },
  captionStyle: { fontFamily: TEXT, fontSize: pt(8), color: col('ink'), labelBold: true,
    labelColor: col('rust'), gap: mm(2), note: { fontSize: pt(7), color: col('muted') } },
  paragraphStyles: [
    { id: 'formula', fontSize: pt(11), textAlign: 'center', firstLineIndent: pt(0),
      marginTop: pt(LEAD / 2), marginBottom: pt(LEAD / 2) },
    { id: 'colophon', fontFamily: MONO, fontSize: pt(6.5), lineHeight: pt(9.5),
      color: col('muted'), textAlign: 'left', firstLineIndent: pt(0), marginTop: pt(LEAD) },
  ],
  header, footer, htmlViewer: { overrides: screen }, // canvas and PDF ignore the overrides
});

// #region screen: the screen edition: one column at the pane's width, the clips as players
const [PANE_W, PANE_H] = [400, 640]; // px
const screen = {
  page: { width: px(PANE_W), height: px(PANE_H), dpi: 122, // so the 9.4 pt text is 16 px
    margins: { top: px(28), bottom: px(28), left: px(22), right: px(22), mirror: false } },
  layout: { layoutType: 'single' }, bodyText: { textAlign: 'left' }, // ragged in a narrow pane
  headings: { levels: [{ level: 1, span: 'column', breakBefore: { enabled: false },
    fontSize: px(28), lineHeight: px(32), fontWeight: 800, marginBottom: px(16),
    advancedDesign: { enabled: false } }] }, // the title alone: the band and fields are print's
  header: { elements: [] }, footer: { elements: [] } };
// #endregion

// ─── 2 · Content ────────────────────────────────────────────────────────────
const markdown = String.raw`---
Markdown样例 · 86行 · content.en.md
title: "Bench Physics"
subtitle: "A lab workbook for ages 16 to 18"
---

# The period of a pendulum {kicker="Unit 4 · Oscillations · Lab sheet 4.2" lead="Two pendulums of the same length, released from 10° and from 80°. Time both on video and find out whether the period depends on the size of the swing."}

:::callout{type="aim" title="Aim"}
To measure the period of a pendulum 1.00 m long from two video clips, and to find out how far the small-angle formula can be trusted.

**You need** the two clips, a phone or a screen to play them, and a calculator.
:::

## The two clips

:ref{id="small"} and :ref{id="large"} were rendered from a numerical solution of the pendulum’s equation of motion, θ″ = −(*g*/*L*) sin θ, worked out in steps of one millisecond for a pendulum 1.00 m long. Each clip lasts 8.00 s at 25 frames per second, so one frame is 0.04 s. There is no air resistance in the model, so the swings never die away.

The still printed for each clip is its first frame, and the code in its corner opens the clip itself. Scan it with a phone, or play the clips in the screen edition of this workbook. Both clips loop, start with the sound off and have a speed menu: slow them to half speed when you count.

## Background

A simple pendulum is a small heavy bob on a light string that does not stretch. Pulled aside and let go, it swings to and fro through its lowest point. The **period**, *T*, is the time it takes to go out and come back to where it started.

Galileo noticed that a pendulum keeps the same period as its swings die away, and clockmakers relied on that for three hundred years. For small swings the period depends only on the length *L* and on the gravitational field strength *g*:

:::paragraphs{style="formula"}
*T*~0~ = 2π (*L*/*g*)^1/2^
:::

With *L* = 1.00 m and *g* = 9.81 m s^−2^, the formula gives *T*~0~ = 2.006 s. Its derivation replaces sin θ by θ in radians, which is within 1 % of the truth only up to about 14°. Past that, the pull back towards the middle grows more slowly than the angle, the bob lingers near the turning points and every swing takes a little longer. With the amplitude θ~0~, the angle the bob is released from, and *K*, the complete elliptic integral of the first kind, the exact period is

:::paragraphs{style="formula"}
*T* = 4 (*L*/*g*)^1/2^ *K*(sin ½θ~0~)
:::

## Method

:::callout{type="method"}
1. Play the first clip from the start. The bob is let go from rest in the first frame.
2. Count the complete swings, out and back to the release point, until the clip ends at 8.00 s. Estimate the last one to a quarter of a swing.
3. Divide 8.00 s by the number of swings to find the period.
4. Repeat for the second clip and fill in the table below.
:::

## Results

::resource{id="data"}

The values in :ref{id="periods"} come from the exact formula, worked out to three decimals. Compare your periods with the rows for 10° and 80°.

::resource{id="periods"}

## Questions

:::callout{type="sheet"}
1. Which clip has the longer period, and by what percentage?

:::callout{type="answer"}
Answer
:::space{lines=3}
:::

2. The bob in the second clip travels a much longer arc. Why does its period not grow in the same proportion?

:::callout{type="answer"}
Answer
:::space{lines=4}
:::

3. The first correction to the small-angle formula gives *T* = *T*~0~ (1 + θ~0~^2^/16), with θ~0~ in radians. Work it out at 80° and compare it with the table. Is one term enough?

:::callout{type="answer"}
Answer
:::space{lines=4}
:::

4. A long-case clock swings 4° either side of the vertical. Would its maker need the exact formula?

:::callout{type="answer"}
Answer
:::space{lines=3}
:::
:::

:::paragraphs{style="colophon"}
Bench Physics, lab sheet 4.2 · Set in Source Serif 4, Red Hat Display and Red Hat Mono (SIL Open Font License) · Text and clips: Postext Cookbook, CC BY 4.0.
:::
`; // content.<lang>.md, inlined by the Cookbook
const resources = [...clips, data, periods];

// ─── 3 · Fonts ──────────────────────────────────────────────────────────────
const FONTS = { // every face the pages use, loaded before the build (gotcha: fonts-first)
  'Source Serif 4': ['400', '400i', '700', '700i'], 'Red Hat Display': ['700', '800'],
  'Red Hat Mono': ['400', '500', '600', '700'] };

// #region greek: θ and π come from the text face's greek file, which the kit does not load
// Fontsource cuts each face by script, and the PDF keeps its latin file (gotcha: latin-subset).
const GREEK = 'U+0370-03FF'; // Greek and Coptic
const file = (family, subset, [weight, style]) => 'https://cdn.jsdelivr.net/npm/@fontsource/'
  + `${fontsourceId(family)}@5/files/${fontsourceId(family)}-${subset}-${weight}-${style}.woff2`;
const faceOf = (spec) => [parseInt(spec, 10), spec.endsWith('i') ? 'italic' : 'normal'];
const bytesOf = async (url) => new Uint8Array(await (await fetch(url)).arrayBuffer());
const greek = FONTS[TEXT].map(faceOf).map(([weight, style]) => ({ family: TEXT, weight, style,
  url: file(TEXT, 'greek', [weight, style]) }));
const loadGreek = () => Promise.all(greek.map(async ({ weight, style, url }) => document.fonts.add(
  await new FontFace(TEXT, `url(${url})`, { weight: `${weight}`, style, unicodeRange: GREEK })
    .load())));
// The PDF takes both files of a text face; it asks for every weight, and Source Serif 4 ships
// 200 to 900 (gotcha: pdf-provider-all-styles). A character comes from the first file with it.
const pdfFont = async (family, weight, style) => {
  const latin = await fontsourceProvider(family, weight, style);
  return family !== TEXT ? latin : [latin, await decompressWoff2(await bytesOf(file(TEXT,
    'greek', [Math.max(weight, 200), style])))];
};
// #endregion

// ─── 4 · Build & show ───────────────────────────────────────────────────────
await Promise.all([loadFonts(FONTS, markdown), loadGreek(),
  ...clips.map(({ video }) => loadImage(video.poster.fileId, asset(video.poster.fileId)))]);
const doc = await buildWithFonts(() => buildDocument({ markdown, resources }, config()), markdown);
showPages(doc, { title: t({ en: 'The period of a pendulum', es: 'El periodo de un péndulo' }) });
offerPdf(() => renderToPdf(doc, { fontProvider: pdfFont, resourceBytes: imageBytes }),
  `${RECIPE}.pdf`);

// #region epub: a fixed-layout EPUB that carries both MP4 files and plays them on the page
const media = new Map(await Promise.all(clips.map(async ({ video }) =>
  [video.fileId, { bytes: await bytesOf(asset(video.fileId)), mediaType: 'video/mp4' }])));
const fonts = await Promise.all([ // every face as bytes (gotcha: epub-embeds-given-fonts)
  ...Object.entries(FONTS).flatMap(([family, specs]) => specs.map(faceOf).map(([w, s]) =>
    ({ family, weight: w, style: s, url: file(family, 'latin', [w, s]) }))),
  ...greek.map((face) => ({ ...face, unicodeRange: GREEK })), // after latin: tried first
].map(async ({ url, ...face }) => ({ ...face, format: 'woff2', bytes: await bytesOf(url) })));
const epub = await renderToEpub([doc], { layout: 'fixed', fonts,
  metadata: { title: doc.metadata.title, creators: ['Postext Cookbook'], language: LANG,
    rights: 'CC BY 4.0', modified: new Date('2026-10-05T00:00:00Z') }, // the same bytes each run
  // The clips as video/mp4 and the posters as pictures; the book packs both under media/.
  resourceBytes: (fileId) => media.get(fileId)
    ?? { bytes: imageBytes(fileId), mediaType: 'image/jpeg' },
});
// #endregion

const [read, kb] = [readEpub(epub), (bytes) => `${Math.round(bytes.length / 1024)} KB`];
const list = (lines) => `<ul>${lines.map((line) => `<li>${line}</li>`).join('')}</ul>`;
const desk = Object.assign(document.createElement('section'), { id: 'editions', innerHTML: `
<style>#editions{display:flex;flex-wrap:wrap;gap:28px;justify-content:center;align-items:start;
padding:28px 16px 0;color:#d9d5cc}#editions figure{margin:0}#editions figcaption{margin-top:16px;
color:#8b8f97}#screen{width:${PANE_W}px;height:${PANE_H}px;overflow:auto;border-radius:12px;
box-shadow:0 0 0 8px #1d1f24}#editions article{width:min(380px,92vw);padding:4px 18px;
border-radius:8px;background:#15181d;overflow-wrap:anywhere}#editions a{color:#d8a21a}</style>
<figure><div id="screen" role="region" tabindex="0"></div><figcaption>${t({ en: 'Screen'
  + ' edition', es: 'Edición de pantalla' })} · HTML · ${PANE_W} × ${PANE_H} px</figcaption>
</figure><article><h3>EPUB 3 · ${read.layout}</h3>
<p>${read.spine.length} pages · ${kb(epub)}. The book carries:</p>${list([...read.manifest
  .values()].filter((item) => item.mediaType === 'video/mp4').map(({ path }) =>
  `${path.slice(read.root.length)} · ${kb(read.files.get(path))}`))}
<p>The QR codes and the PDF links open:</p>${list(clips.map(resourceVideoLink))}
<a download="${RECIPE}.epub" href="${URL.createObjectURL(new Blob([epub],
  { type: 'application/epub+zip' }))}">Download ${RECIPE}.epub</a></article>` });
document.getElementById('pages').before(desk);

// #region pane: the screen edition as HTML, each clip in the browser's own player
// A shadow root keeps the page's styles out, so the pane shows the lines as they were measured.
const pane = desk.querySelector('#screen').attachShadow({ mode: 'open' });
pane.innerHTML = renderToHtml(buildDocument({ markdown, resources },
  applyHtmlViewerOverrides(config())), { mode: 'single', padding: 0, background: '#ffffff',
  resourceImageUrl: imageUrl, // the posters, until a clip plays
  resourceVideoUrl: (fileId) => asset(fileId) }); // the file each <video> plays: the asset here
const top = (el) => el.getBoundingClientRect().top; // open the pane on the first player
pane.host.scrollTop = top(pane.querySelector('video')) - top(pane.host) - 48;
// #endregion

工具包 · core, fonts, viewer, pdf, images：每道食谱都相同 · 310行
// ─── Kit ── helpers shared by every Cookbook recipe · postext.dev/cookbook ─────

// ─── Kit · core v1 ── the same in every recipe · postext.dev/cookbook ─────────
function mm(value) { return { value, unit: 'mm' }; }
function pt(value) { return { value, unit: 'pt' }; }
function em(value) { return { value, unit: 'em' }; }
/** The sample language's string: t({ en: 'Figure', es: 'Figura' }). */
function t(strings) { return strings[LANG] ?? Object.values(strings)[0]; }
/** A file in this recipe's assets folder, served from the Postext repo by jsDelivr. */
function asset(file) { return `https://cdn.jsdelivr.net/gh/drnachio/postext@main/cookbook/${RECIPE}/assets/${file}`; }

// ─── Kit · fonts v1 ── the same in every recipe · postext.dev/cookbook ────────
// Postext measures text with the faces the browser has loaded, and caches the
// widths, so every face must be ready before the first build. Faces come from
// Fontsource: the same static files the PDF embeds, so screen and PDF agree.

/** faces = { 'Family Name': ['400', '400i', '700'] }. `text` is the sample:
 *  letters beyond Latin-1 (č, ł, ő…) also load the latin-ext files. With
 *  `optional`, a face Fontsource does not ship is skipped instead of failing.
 *  Resolves to the number of faces added. */
async function loadFonts(faces, text = '', { optional = false } = {}) {
  kitStatus('Loading fonts…');
  const ranges = {
    latin: 'U+0000-00FF,U+0131,U+0152-0153,U+02BB-02BC,U+02C6,U+02DA,U+02DC,U+0304,U+0308,U+0329,'
      + 'U+2000-206F,U+20AC,U+2122,U+2191,U+2193,U+2212,U+2215,U+FEFF,U+FFFD',
    'latin-ext': 'U+0100-02BA,U+02BD-02C5,U+02C7-02CC,U+02CE-02D7,U+02DD-02FF,U+0304,U+0308,U+0329,'
      + 'U+1D00-1DBF,U+1E00-1E9F,U+1EF2-1EFF,U+2020,U+20A0-20AB,U+20AD-20C0,U+2113,U+2C60-2C7F,U+A720-A7FF',
  };
  const subsets = /[Ā-˿Ḁ-ỿ]/.test(text) ? ['latin', 'latin-ext'] : ['latin'];
  const jobs = [];
  let added = 0;
  for (const [family, specs] of Object.entries(faces)) {
    const id = fontsourceId(family);
    const meta = optional ? await fontsourceMeta(family) : null;
    for (const spec of new Set(specs)) {
      const weight = parseInt(spec, 10);
      const style = spec.endsWith('i') ? 'italic' : 'normal';
      if (hasFace(family, weight, style)) continue;
      if (optional && !(meta?.weights.includes(weight) && meta.styles.includes(style))) continue;
      for (const subset of subsets) {
        const url = `https://cdn.jsdelivr.net/npm/@fontsource/${id}@5/files/${id}-${subset}-${weight}-${style}.woff2`;
        const face = new FontFace(family, `url(${url}) format('woff2')`,
          { weight: String(weight), style, unicodeRange: ranges[subset] });
        jobs.push(face.load().then((ready) => { document.fonts.add(ready); added++; }, () => {
          if (subset === 'latin' && !optional) throw new Error(`Fontsource has no ${family} ${weight} ${style}`);
        }));
      }
    }
  }
  await Promise.all(jobs).catch((error) => { kitFail(error); throw error; });
  return added;
}

/** Runs `build` (a buildDocument or buildBundle call) and checks the faces
 *  the pages use. A regular face missing from FONTS is loaded with a warning;
 *  bold and italic variants are loaded when the family ships them. Then the
 *  measurement caches are cleared and the build runs again. */
async function buildWithFonts(build, text = '') {
  const tried = new Set();
  for (let round = 0; round < 3; round++) {
    kitStatus('Laying out…');
    await new Promise(requestAnimationFrame);          // let the status paint first
    const result = await Promise.resolve().then(build).catch((error) => { kitFail(error); throw error; });
    const wanted = { base: {}, variants: {} };
    for (const { font, base } of [result].flat().flatMap(fontStringsOf)) {
      const { family, weight, style } = parseFont(font);
      const key = `${family}|${weight}|${style}`;
      if (tried.has(key) || hasFace(family, weight, style)) continue;
      tried.add(key);
      (wanted[base ? 'base' : 'variants'][family] ??= []).push(`${weight}${style === 'italic' ? 'i' : ''}`);
    }
    if (Object.keys(wanted.base).length) {
      console.warn(`[cookbook] FONTS does not list ${JSON.stringify(wanted.base)}: loading them.`);
    }
    const added = await loadFonts(wanted.base, text) + await loadFonts(wanted.variants, text, { optional: true });
    if (added === 0) return result;
    clearMeasurementCache();
  }
  throw new Error('The fonts did not settle after three builds.');
}

/** Every font string of the layout. `base` marks a block's own face; its
 *  bold, italic and bold-italic variants are listed whether or not used. */
function fontStringsOf(doc) {
  const found = new Map();
  const walk = (node) => {
    if (!node || typeof node !== 'object') return;
    if (Array.isArray(node)) { node.forEach(walk); return; }
    for (const [key, value] of Object.entries(node)) {
      if (typeof value === 'string' && /fontString$/i.test(key)) {
        found.set(value, found.get(value) || key === 'fontString');
      } else if (value && typeof value === 'object') walk(value);
    }
  };
  walk(doc.pages);
  walk(doc.blocks);
  return [...found].map(([font, base]) => ({ font, base }));
}

/** '700 37.5px Open Sans' / 'italic 400 13px "Source Serif 4"' → { family, weight, style }.
 *  A string with no weight ('95.8px Young Serif', from a design text) is 400. */
function parseFont(font) {
  const m = /^(?:(italic|oblique)\s+)?(?:small-caps\s+)?(?:(\d+|bold|normal)\s+)?[\d.]+px\s+(.+)$/.exec(font.trim());
  if (!m) throw new Error(`Unexpected font string: ${font}`);
  const weight = m[2] === 'bold' ? 700 : !m[2] || m[2] === 'normal' ? 400 : Number(m[2]);
  return { family: m[3].replace(/^["']|["']$/g, ''), weight, style: m[1] ? 'italic' : 'normal' };
}

/** True when a loaded FontFace covers exactly this family, weight and style
 *  (document.fonts.check() is also true for families nobody declared). */
function hasFace(family, weight, style) {
  for (const face of document.fonts) {
    if (face.status !== 'loaded' || face.style !== style) continue;
    if (face.family.replace(/^["']|["']$/g, '') !== family) continue;
    const [low, high = low] = face.weight.split(' ').map(Number);
    if (weight >= low && weight <= high) return true;
  }
  return false;
}

/** Fontsource's id for a family: 'Source Serif 4' → 'source-serif-4'. */
function fontsourceId(family) { return family.toLowerCase().replace(/\s+/g, '-'); }

/** The weights and styles a family ships ({ weights: [400, 700], styles: ['normal', 'italic'] }), or null. */
function fontsourceMeta(family) {
  fontsourceMeta.cache ??= new Map();
  const id = fontsourceId(family);
  if (!fontsourceMeta.cache.has(id)) {
    fontsourceMeta.cache.set(id, fetch(`https://api.fontsource.org/v1/fonts/${id}`)
      .then((res) => (res.ok ? res.json() : null), () => null));
  }
  return fontsourceMeta.cache.get(id);
}

// ─── Kit · viewer v1 ── the same in every recipe · postext.dev/cookbook ───────
/** Shows the pages as facing spreads on a dark desk: the first page is a
 *  recto on its own, then verso | recto pairs, as in a bound book. Pages
 *  are painted when they scroll near the screen. */
function showPages(docs, { title, width = 460 } = {}) {
  const root = viewer(title);
  const pages = [docs].flat().flatMap((doc) =>
    doc.pages.map((page) => ({ doc, page, n: (doc.pageIndexOffset ?? 0) + page.index })));
  const spreads = [];
  let verso = null;
  for (const p of pages) {
    if (p.n % 2 === 1) { if (verso) spreads.push([verso, null]); verso = p; }
    else { spreads.push([verso, p]); verso = null; }
  }
  if (verso) spreads.push([verso, null]);
  const density = Math.min(window.devicePixelRatio || 1, 2);
  showPages.painter?.disconnect();
  const painter = new IntersectionObserver((entries) => {
    for (const { isIntersecting, target } of entries) {
      if (!isIntersecting) continue;
      painter.unobserve(target);
      const { doc, page } = target.postext;
      renderPageToCanvas(page, doc, target, { scale: (width * density) / page.width });
    }
  }, { rootMargin: '800px' });
  showPages.painter = painter;
  root.replaceChildren(...spreads.map((pair) => {
    const spread = document.createElement('div');
    spread.className = 'pt-spread';
    for (const p of pair) {
      const figure = document.createElement('figure');
      if (p) {
        const label = p.page.pageLabel || String(p.n + 1);
        const canvas = document.createElement('canvas');
        canvas.postext = p;
        canvas.style.aspectRatio = `${p.page.width} / ${p.page.height}`;
        canvas.setAttribute('role', 'img');
        canvas.setAttribute('aria-label', `Page ${label}`);
        const folio = document.createElement('figcaption');
        folio.textContent = label;
        figure.append(canvas, folio);
        painter.observe(canvas);
      } else figure.className = 'pt-blank';
      spread.append(figure);
    }
    return spread;
  }));
  kitStatus(`${pages.length} ${pages.length === 1 ? 'page' : 'pages'}`);
  document.documentElement.dataset.postext = 'ready';
  return pages.length;
}

/** The desk, the bar and the error reporting, created once. */
function viewer(title) {
  if (!document.getElementById('pt-kit')) {
    document.head.insertAdjacentHTML('beforeend', `<style id="pt-kit">
      :root { color-scheme: dark; }
      body { margin: 0; background: #0e1014; color: #b9bcc4; font: 13px/1.45 system-ui, sans-serif; }
      #pt-bar { position: sticky; top: 0; z-index: 1; display: flex; flex-wrap: wrap; align-items: center;
        gap: 6px 16px; padding: 10px 16px; background: rgb(14 16 20 / .92); backdrop-filter: blur(6px);
        border-bottom: 1px solid #23262d; }
      #pt-bar strong { color: #f4f1ea; font-weight: 600; }
      #pt-actions { display: flex; gap: 12px; margin-left: auto; }
      #pt-actions a, #pt-actions button { color: #d8a21a; font: inherit; background: none; border: 0; padding: 0; cursor: pointer; }
      #pages { display: grid; justify-items: center; gap: 48px; padding: 32px 16px 72px; }
      .pt-spread { display: flex; }
      .pt-spread figure { margin: 0; width: min(460px, 44vw); }
      .pt-spread canvas { display: block; width: 100%; background: #fff;
        box-shadow: 0 1px 2px rgb(0 0 0 / .5), 0 22px 44px -16px rgb(0 0 0 / .8); }
      .pt-spread figure:first-child canvas { box-shadow: inset -14px 0 14px -14px rgb(0 0 0 / .18), 0 1px 2px rgb(0 0 0 / .5), 0 22px 44px -16px rgb(0 0 0 / .8); }
      .pt-spread figcaption { margin-top: 10px; text-align: center; font: 600 10px/1 system-ui, sans-serif;
        letter-spacing: .18em; text-transform: uppercase; color: #6c7079; }
      .pt-blank { visibility: hidden; }
      @media (max-width: 760px) {
        .pt-spread { flex-direction: column; gap: 32px; }
        .pt-spread figure { width: min(460px, 92vw); }
        .pt-blank { display: none; }
      }
    </style>`);
    document.body.insertAdjacentHTML('afterbegin',
      '<header id="pt-bar"><strong id="pt-title"></strong><span id="pt-status" role="status"></span><span id="pt-actions"></span></header>');
    document.getElementById('pt-title').textContent = document.title || 'Postext';
    addEventListener('error', (event) => kitFail(event.error ?? event.message));
    addEventListener('unhandledrejection', (event) => kitFail(event.reason));
  }
  if (title) document.getElementById('pt-title').textContent = title;
  return document.getElementById('pages')
    ?? document.body.appendChild(Object.assign(document.createElement('main'), { id: 'pages' }));
}

function kitStatus(text) {
  viewer();
  document.getElementById('pt-status').textContent = text;
}

function kitFail(error) {
  document.documentElement.dataset.postext = 'error';
  kitStatus(`Error: ${error?.message ?? error}`);
}

// ─── Kit · pdf v1 ── the same in every recipe that exports a PDF ──────────────
/** postext-pdf embeds TrueType bytes. Fetch the Fontsource file the screen
 *  used, snapping to a weight the family ships and falling back to upright
 *  when it has no italic: the PDF asks for every face a block could use. */
async function fontsourceProvider(family, weight, style) {
  const id = fontsourceId(family);
  const meta = await fontsourceMeta(family);
  const weights = meta?.weights?.length ? meta.weights : [400, 700];
  const w = weights.reduce((a, b) => (Math.abs(b - weight) < Math.abs(a - weight) ? b : a));
  const s = style === 'italic' && meta && !meta.styles.includes('italic') ? 'normal' : style;
  const res = await fetch(`https://cdn.jsdelivr.net/npm/@fontsource/${id}@5/files/${id}-latin-${w}-${s}.woff2`);
  if (!res.ok) throw new Error(`Fontsource has no ${family} ${w} ${s} (${res.status})`);
  return decompressWoff2(new Uint8Array(await res.arrayBuffer()));
}

/** A "Build the PDF" button in the bar. Once built: "Open the PDF" (a new
 *  tab, since CodePen's preview frame cannot show PDFs) and a download link. */
function offerPdf(makePdf, filename) {
  viewer();
  const button = Object.assign(document.createElement('button'), { type: 'button', textContent: 'Build the PDF' });
  button.dataset.postextPdf = filename;
  button.addEventListener('click', async () => {
    button.disabled = true;
    button.textContent = 'Building the PDF…';
    try {
      const bytes = await makePdf();
      const url = URL.createObjectURL(new Blob([bytes], { type: 'application/pdf' }));
      const size = `${Math.max(1, Math.round(bytes.length / 1024))} KB`;
      button.replaceWith(
        Object.assign(document.createElement('a'), { href: url, target: '_blank', rel: 'noopener', textContent: 'Open the PDF ↗' }),
        Object.assign(document.createElement('a'), { href: url, download: filename, textContent: `Download ${filename} · ${size}` }));
    } catch (error) {
      button.disabled = false;
      button.textContent = 'Build the PDF';
      kitFail(error);
    }
  });
  document.getElementById('pt-actions').append(button);
}

// ─── Kit · images v1 ── recipes with pictures · postext.dev/cookbook ──────────
/** Registers a photo or PNG for the canvas and keeps its bytes for the PDF.
 *  fetch → ImageBitmap never taints the canvas (a plain cross-origin <img> would). */
async function loadImage(fileId, url) {
  const res = await fetch(url);
  if (!res.ok) throw new Error(`Image not found (${res.status}): ${url}`);
  const bytes = new Uint8Array(await res.arrayBuffer());
  registerResourceImage(fileId, await createImageBitmap(new Blob([bytes])));
  (loadImage.bytes ??= new Map()).set(fileId, bytes);
}

/** Registers SVG markup (drawn in code, or fetched) as a vector image. */
async function loadSvg(fileId, svg) {
  const img = new Image();
  img.src = `data:image/svg+xml;charset=utf-8,${encodeURIComponent(svg)}`;
  await img.decode();
  registerResourceImage(fileId, img);
  (loadImage.bytes ??= new Map()).set(fileId, new TextEncoder().encode(svg));
}

/** renderToPdf({ resourceBytes: imageBytes }) */
function imageBytes(fileId) { return loadImage.bytes?.get(fileId); }

/** renderToHtml({ resourceImageUrl: imageUrl }) */
function imageUrl(fileId) {
  const bytes = imageBytes(fileId);
  if (!bytes) return undefined;
  imageUrl.urls ??= new Map();
  if (!imageUrl.urls.has(fileId)) {
    const type = /\.svg$/i.test(fileId) ? 'image/svg+xml' : /\.png$/i.test(fileId) ? 'image/png' : 'image/jpeg';
    imageUrl.urls.set(fileId, URL.createObjectURL(new Blob([bytes], { type })));
  }
  return imageUrl.urls.get(fileId);
}

// ─── /Kit ───────────────────────────────────────────────────────────────────────