path: root/public/bluebey-studio/src/caption.js

import {
  captionFontStack,
  hasGlyphs,
  layoutText,
  textToPathData,
} from './textOutlines.js';

/**
 * The speech bubble that sits next to the character.
 *
 * The same caption is drawn three times: into the 2D canvas that is overlaid on
 * the 3D view, into the canvas that gets composited into an exported PNG, and
 * into the vector SVG export. Drawing a shape three times by hand is how a
 * preview starts to disagree with the file it produced, so the outline is built
 * exactly once, here, as a short list of path commands. `drawCaption` replays
 * that list through a 2D context and `captionToSvg` prints the same list as SVG
 * path data, which is what keeps the two renderers together.
 *
 * The module never measures or wraps text itself: `textOutlines.js` owns line
 * breaking and glyph outlines. This file only decides how large the bubble has
 * to be for the block it is handed, padding included, and guarantees that block
 * fits inside.
 *
 * It is pure in the same sense as `trace.js`: no DOM, no globals and no
 * asynchronous work, so the tests can drive it in Node with a stub context.
 */

/** Defaults mirroring the `caption` slice of `presets.js`. */
const DEFAULT_FONT_SIZE = 34;
const DEFAULT_LINE_HEIGHT = 1.42;
const DEFAULT_PADDING = 18;
const DEFAULT_RADIUS = 24;
const DEFAULT_BORDER_WIDTH = 4;
const DEFAULT_MAX_WIDTH = 0.36;
const DEFAULT_TEXT_COLOR = '#3f2b52';
const DEFAULT_BUBBLE_COLOR = '#ffffff';
const DEFAULT_BORDER_COLOR = '#55386e';

/** Advance widths used before the vendored font has loaded, as a fraction of em. */
const FALLBACK_WIDE_EM = 1;
const FALLBACK_NARROW_EM = 0.55;

/**
 * Vertical metrics of the fallback stack, as a fraction of the font size.
 *
 * A typical Japanese family sits close to these numbers, so the baseline of the
 * pre-load preview barely moves when the real font arrives.
 */
const FALLBACK_ASCENT = 0.88;
const FALLBACK_DESCENT = 0.12;

/** Control-point distance that turns a corner into a quarter circle. */
const KAPPA = 0.5522847498307936;

/** Code-point ranges that deserve a full em in the fallback measurement. */
const WIDE_RANGES = [
  [0x1100, 0x11ff], [0x2e80, 0x30ff], [0x3130, 0x318f], [0x3400, 0x4dbf],
  [0x4e00, 0x9fff], [0xac00, 0xd7ff], [0xf900, 0xfaff], [0xff00, 0xffef],
];

/**
 * Punctuation that may hang past the bottom of a vertical column.
 *
 * Japanese line breaking (禁則処理) forbids starting a line with a closing
 * mark, so when one of these would land at the top of a new column it is kept
 * on the previous column and drawn past that column's last cell instead.
 */
const HANGING_PUNCTUATION = new Set(['、', '。', ',', '.', '，', '．']);

/** Small kana, which sit toward the upper-right of their cell when stacked. */
const SMALL_KANA = 'ぁぃぅぇぉっゃゅょゎァィゥェォッャュョヮ';

/**
 * Characters that may not open a column (行頭禁則).
 *
 * A closing mark such as `、` simply hangs off the previous column; `ー`, small
 * kana and closing brackets are kept company by handing the preceding
 * character over to the next column as well, so a column never starts with
 * something that has nothing to attach to.
 */
const COLUMN_START_FORBIDDEN = new Set([
  '」', '』', '）', '〕', '】', '〉', '》', '”', '’', '・', 'ー',
  ...SMALL_KANA,
  ...HANGING_PUNCTUATION,
]);

/** Characters that may not close a column (行末禁則). */
const COLUMN_END_FORBIDDEN = new Set(['「', '『', '（', '〔', '【', '〈', '《', '“', '‘']);

/**
 * Per-character offset for vertical (縦書き) text, in em.
 *
 * A column draws one glyph per cell, so a few characters need a nudge to read
 * the way a Japanese typesetter would place them: the prolonged sound mark
 * becomes an upright tick, small kana tuck into the upper-right, and the
 * brackets lean toward the ends of the span they enclose. Everything else
 * stays dead-centre. Sharing this table is what keeps the canvas and the SVG
 * export in step.
 */
const VERTICAL_OFFSETS = new Map();
for (const ch of SMALL_KANA) VERTICAL_OFFSETS.set(ch, { dx: 0.12, dy: -0.12 });
// 縦書きの句読点は、横書きの左下ではなく、字枠の右上に置く。
for (const ch of HANGING_PUNCTUATION) VERTICAL_OFFSETS.set(ch, { dx: 0.68, dy: -0.55 });

/**
 * Characters drawn turned a quarter turn when stacked, so they read the way
 * 縦書き sets them: the prolonged sound mark `ー` becomes an upright bar, and the
 * brackets `「」『』［］（）` turn so they open down the column.
 */
const VERTICAL_ROTATED = new Set(['ー', '～', '〜', '―']);
const VERTICAL_BRACKETS = new Set([
  '「', '」', '『', '』', '（', '）', '［', '］', '【', '】', '〔', '〕', '〈', '〉', '《', '》',
]);

/**
 * How far a rotated glyph is nudged down inside its cell, in em. The rotated `ー`
 * is centred otherwise, which leaves it hugging the character above it; the
 * brackets stay centred.
 */
const VERTICAL_ROTATED_DROP = 0.14;

/**
 * The downward nudge (em) for a glyph that is turned a quarter turn in 縦書き, or
 * `null` when the glyph is drawn the ordinary way.
 */
function verticalRotation(ch) {
  if (VERTICAL_ROTATED.has(ch)) return VERTICAL_ROTATED_DROP;
  if (VERTICAL_BRACKETS.has(ch)) return 0;
  return null;
}

/** Offset used for the characters that need no special placement. */
const NO_OFFSET = Object.freeze({ dx: 0, dy: 0 });

/**
 * The `ctx.font` string for a caption.
 *
 * Canvas wants size, weight and family in one string, and the size has to carry
 * the export scale because the same caption is drawn at 1x for the preview and
 * at 2x or more into a PNG. `loaded` is the app's answer to "is the vendored
 * subset ready?": until it is, the vendored family is dropped from the stack so
 * the preview does not ask for a font that is still downloading.
 *
 * @param {object} caption  the `caption` slice of the state
 * @param {number} [scale=1]
 * @param {boolean} [loaded=false]
 * @returns {string}
 */
export function fontSpec(caption, scale = 1, loaded = false) {
  const state = caption ?? {};
  const size = resolveFontSize(state, positiveScale(scale));
  const weight = state.bold ? 'bold ' : '';
  return `${weight}${formatNumber(size, 3)}px ${fontFamilyStack(state, loaded)}`;
}

/**
 * Resolve a caption to pixel geometry at the output size.
 *
 * `x`/`y` are fractions of the image and land on the bubble's top-left corner;
 * `maxWidth` is a fraction of the image width and caps the text column. The box
 * is always the bubble itself, padding included and the tail excluded, and it
 * always fits inside the image: padding gives way first, then the position is
 * clamped (leaving room for the tail), and only a block larger than the image
 * is clipped. Every returned number is finite, even for an empty caption or a
 * zero-sized image.
 *
 * `font` is an opentype font from `textOutlines.js`. Without it the text is
 * wrapped by a rough character count, so the preview still shows a bubble while
 * the font is loading.
 *
 * @param {object} caption
 * @param {object} [options]
 * @param {number} [options.width=0]
 * @param {number} [options.height=0]
 * @param {number} [options.scale=1]
 * @param {import('opentype.js').Font|null} [options.font=null]
 * @returns {{
 *   box: {x: number, y: number, w: number, h: number},
 *   lines: Array<{text: string, width: number}>,
 *   fontSize: number, lineHeight: number, padding: number, scale: number,
 *   usedOutlines: boolean,
 * }}
 */
export function layoutCaption(caption, { width, height, scale = 1, font = null } = {}) {
  const state = caption ?? {};
  const resolvedScale = positiveScale(scale);
  const outWidth = toNonNegative(width, 0);
  const outHeight = toNonNegative(height, 0);

  const fontSize = resolveFontSize(state, resolvedScale);
  const lineHeight = toPositive(state.lineHeight, DEFAULT_LINE_HEIGHT);
  const requested = toNonNegative(state.padding, DEFAULT_PADDING) * resolvedScale;
  const text = typeof state.text === 'string' ? state.text : '';

  const vertical = state.vertical === true;
  const block = vertical
    ? layoutVertical(text, {
      fontSize,
      lineHeight,
      maxHeight: resolveColumn(state, outHeight, requested),
    })
    : layoutBlock(font, text, {
      fontSize,
      maxWidth: resolveColumn(state, outWidth, requested),
      lineHeight,
      align: alignOf(state),
    });

  // Padding gives way before the bubble does, so a caption that cannot fit with
  // its usual breathing room still fits inside the image.
  const padding = Math.max(
    0,
    Math.min(requested, (outWidth - block.width) / 2, (outHeight - block.height) / 2),
  );
  const w = Math.min(block.width + 2 * padding, outWidth);
  const h = Math.min(block.height + 2 * padding, outHeight);

  // The tail sticks out of the box, so the box has to keep that much clear of
  // the edge it points at.
  const tail = tailOf(state);
  const tailLength = tail === 'none' ? 0 : Math.max(0, Math.min(fontSize * 0.9, h * 0.5));
  const roomX = Math.max(0, outWidth - w);
  const roomY = Math.max(0, outHeight - h);

  // Which way the tail leaves the box, as a direction where each axis is -1,
  // 0 or 1. A diagonal tail needs room on *both* axes, so the clearance is
  // driven by the direction rather than by which name the tail has.
  const dir = tailDirection(tail);
  let lowX = 0;
  let highX = roomX;
  if (dir.x < 0) lowX = Math.min(tailLength, roomX);
  else if (dir.x > 0) highX = Math.max(0, roomX - tailLength);

  let lowY = 0;
  let highY = roomY;
  if (dir.y < 0) lowY = Math.min(tailLength, roomY);
  else if (dir.y > 0) highY = Math.max(0, roomY - tailLength);

  const box = {
    x: clamp(toFinite(state.x, 0) * outWidth, lowX, highX),
    y: clamp(toFinite(state.y, 0) * outHeight, lowY, highY),
    w,
    h,
  };

  return {
    box,
    lines: block.lines,
    fontSize,
    lineHeight,
    padding,
    scale: resolvedScale,
    vertical,
    usedOutlines: Boolean(font),
  };
}

/**
 * Lay the bubble down in a 2D context, ready to be filled and stroked.
 *
 * The tail is part of the same path as the bubble, so the border runs along it
 * and joins the outline cleanly instead of meeting it at a seam. The caller
 * owns the colours; this only traces the shape.
 *
 * @param {CanvasRenderingContext2D} ctx
 * @param {{x: number, y: number, w: number, h: number}} box  bubble box, tail excluded
 * @param {object} caption
 * @param {number} [scale=1]
 * @returns {boolean}  `true` when there was a shape to trace
 */
export function bubblePath(ctx, box, caption, scale = 1) {
  const commands = bubbleCommands(box, caption ?? {}, positiveScale(scale));
  applyCommands(ctx, commands);
  return commands.length > 0;
}

/**
 * Draw a caption: bubble first, then the text line by line.
 *
 * The 2D canvas is overlaid exactly on the 3D view for the preview and is
 * composited into the PNG on export, which is why the very same function runs
 * in both places: what the user sees is what the file contains.
 *
 * The bubble box comes back even when the caption is switched off, so the app
 * can show a placeholder and hit-test a drag without a second layout.
 *
 * @param {CanvasRenderingContext2D} ctx
 * @param {object} caption
 * @param {object} [options]
 * @param {number} [options.width=0]
 * @param {number} [options.height=0]
 * @param {number} [options.scale=1]
 * @param {import('opentype.js').Font|null} [options.font=null]
 * @returns {{x: number, y: number, w: number, h: number}}
 */
export function drawCaption(ctx, caption, { width, height, scale = 1, font = null } = {}) {
  const state = caption ?? {};
  const layout = layoutCaption(state, { width, height, scale, font });
  const { box } = layout;
  if (!state.enabled || !ctx) return box;

  ctx.save();

  const commands = bubbleCommands(box, state, layout.scale);
  if (commands.length > 0) {
    applyCommands(ctx, commands);
    ctx.fillStyle = colorOf(state.bubbleColor, DEFAULT_BUBBLE_COLOR);
    ctx.fill();

    const borderWidth = toNonNegative(state.borderWidth, DEFAULT_BORDER_WIDTH) * layout.scale;
    if (borderWidth > 0) {
      ctx.strokeStyle = colorOf(state.borderColor, DEFAULT_BORDER_COLOR);
      ctx.lineWidth = borderWidth;
      ctx.lineJoin = 'round';
      ctx.lineCap = 'round';
      // A whisper is the rounded bubble with a broken outline.
      if (bubbleOf(state) === 'whisper' && typeof ctx.setLineDash === 'function') {
        ctx.setLineDash([borderWidth * 3, borderWidth * 2]);
      }
      ctx.stroke();
    }
  }

  if (layout.lines.length > 0) drawText(ctx, layout, state, font);

  ctx.restore();
  return box;
}

/**
 * The caption as an SVG fragment, plus whether the text is outlined.
 *
 * Text becomes glyph outlines whenever the font covers it, because a `<text>`
 * element is redrawn with whatever font the viewer happens to have and a
 * caption that reflows is worse than one that cannot be selected. When a glyph
 * is missing the whole caption falls back to `<text>` and `usedOutlines` is
 * `false`, which is the caller's signal to warn that the file now depends on
 * the viewer's fonts.
 *
 * The bubble and its tail are one `<path>`, traced from the same command list
 * the canvas uses. `enabled` is not consulted: the export path only runs for an
 * enabled caption, and drawing whatever is handed over keeps this usable for a
 * thumbnail.
 *
 * @param {object} caption
 * @param {object} [options]
 * @param {number} [options.width=0]
 * @param {number} [options.height=0]
 * @param {number} [options.scale=1]
 * @param {import('opentype.js').Font|null} [options.font=null]
 * @param {number} [options.round=2]  decimal places in the output
 * @returns {{svg: string, usedOutlines: boolean}}
 */
export function captionToSvg(caption, { width, height, scale = 1, font = null, round = 2 } = {}) {
  const state = caption ?? {};
  const places = Math.max(0, Math.min(8, Math.floor(Number.isFinite(round) ? round : 2)));
  const layout = layoutCaption(state, { width, height, scale, font });
  const { box, lines, fontSize, lineHeight, padding } = layout;
  const outWidth = toNonNegative(width, 0);
  const outHeight = toNonNegative(height, 0);
  const text = typeof state.text === 'string' ? state.text : '';

  let usedOutlines = Boolean(font) && text.trim() !== '' && hasGlyphs(font, text);
  // Vertical text falls back to `<text>`: the outline path lays glyphs out
  // horizontally, and a column of glyphs reads fine as live text.
  if (layout.vertical) usedOutlines = false;

  const parts = [
    '<svg xmlns="http://www.w3.org/2000/svg"'
      + ` width="${formatNumber(outWidth, places)}"`
      + ` height="${formatNumber(outHeight, places)}"`
      + ` viewBox="0 0 ${formatNumber(outWidth, places)} ${formatNumber(outHeight, places)}">`,
  ];

  const bubble = commandsToPathData(bubbleCommands(box, state, layout.scale), places);
  if (bubble) {
    const fill = escapeAttribute(colorOf(state.bubbleColor, DEFAULT_BUBBLE_COLOR));
    const borderWidth = toNonNegative(state.borderWidth, DEFAULT_BORDER_WIDTH) * layout.scale;
    const stroke = borderWidth > 0
      ? ` stroke="${escapeAttribute(colorOf(state.borderColor, DEFAULT_BORDER_COLOR))}"`
        + ` stroke-width="${formatNumber(borderWidth, places)}" stroke-linejoin="round"`
        + (bubbleOf(state) === 'whisper'
          ? ` stroke-dasharray="${formatNumber(borderWidth * 3, places)} ${formatNumber(borderWidth * 2, places)}"`
          : '')
      : '';
    parts.push(`<path d="${bubble}" fill="${fill}"${stroke}/>`);
  }

  if (lines.length > 0) {
    const textColor = colorOf(state.textColor, DEFAULT_TEXT_COLOR);
    let outlined = '';
    if (usedOutlines) {
      // `textToPathData` re-applies the alignment against the widest line, so
      // the block it needs is the one `layoutText` measured.
      outlined = textToPathData(font, {
        lines,
        width: blockWidthOf(lines),
        height: lines.length * fontSize * lineHeight,
        lineHeight,
        fontSize,
        align: alignOf(state),
      }, { x: box.x + padding, y: box.y + padding, round: places });
      usedOutlines = outlined !== '';
    }
    if (usedOutlines) {
      parts.push(`<path d="${outlined}" fill="${escapeAttribute(textColor)}"/>`);
    } else {
      parts.push(liveText(state, layout, font, textColor, places));
    }
  }

  parts.push('</svg>');
  return { svg: parts.join('\n') + '\n', usedOutlines };
}

// --- text --------------------------------------------------------------------

/** Draw every line at its own baseline, shifted sideways by the alignment. */
function drawText(ctx, layout, state, font) {
  const { box, lines, fontSize, lineHeight, padding, scale } = layout;
  const metrics = baselineMetrics(font, fontSize);
  const step = fontSize * lineHeight;
  const halfLeading = (step - (metrics.ascent + metrics.descent)) / 2;
  const blockWidth = blockWidthOf(lines);
  const left = box.x + padding;
  const top = box.y + padding;
  const align = alignOf(state);

  ctx.font = fontSpec(state, scale, Boolean(font));
  ctx.fillStyle = colorOf(state.textColor, DEFAULT_TEXT_COLOR);
  ctx.textAlign = 'left';
  ctx.textBaseline = 'alphabetic';

  if (layout.vertical) {
    // Columns run right to left; each glyph sits in its own cell. `ー` and its
    // like are turned a quarter turn so they read as vertical strokes.
    for (let i = 0; i < lines.length; i++) {
      const chars = [...lines[i].text];
      const x = left + (lines.length - 1 - i) * step;
      for (let j = 0; j < chars.length; j++) {
        const ch = chars[j];
        const drop = verticalRotation(ch);
        if (drop !== null) {
          ctx.save();
          ctx.translate(x + fontSize / 2, top + j * fontSize + fontSize / 2 + drop * fontSize);
          ctx.rotate(Math.PI / 2);
          ctx.textAlign = 'center';
          ctx.textBaseline = 'middle';
          ctx.fillText(ch, 0, 0);
          ctx.restore();
          continue;
        }
        const offset = verticalOffsetOf(ch);
        const y = top + j * fontSize + metrics.ascent + offset.dy * fontSize;
        ctx.fillText(ch, x + offset.dx * fontSize, y);
      }
    }
    return;
  }

  for (let i = 0; i < lines.length; i++) {
    const line = lines[i];
    const x = left + alignOffset(align, blockWidth, line.width);
    const y = top + i * step + halfLeading + metrics.ascent;
    ctx.fillText(line.text, x, y);
  }
}

/**
 * The `<text>` fallback.
 *
 * The family comes from the font stack alone, not from `fontSpec`: size and
 * weight have attributes of their own, and a `font-family` that carried them
 * would be ignored by every viewer.
 */
function liveText(state, layout, font, fill, places) {
  const { box, lines, fontSize, lineHeight, padding } = layout;
  const blockWidth = blockWidthOf(lines);
  const align = alignOf(state);
  const left = box.x + padding;
  const anchor = align === 'center' ? 'middle' : align === 'right' ? 'end' : 'start';
  const x = align === 'center'
    ? left + blockWidth / 2
    : align === 'right'
      ? left + blockWidth
      : left;

  const metrics = baselineMetrics(font, fontSize);
  const step = fontSize * lineHeight;
  const halfLeading = (step - (metrics.ascent + metrics.descent)) / 2;
  const top = box.y + padding;

  if (layout.vertical) {
    // The stacked glyphs share one left-anchored <text>. The rotated ones (`ー`)
    // get a <text> of their own, centred on the cell and turned a quarter turn.
    const base = [
      `font-family="${escapeAttribute(fontFamilyStack(state, Boolean(font)))}"`,
      `font-size="${formatNumber(fontSize, places)}"`,
      state.bold ? 'font-weight="bold"' : '',
      `fill="${escapeAttribute(fill)}"`,
    ].filter(Boolean).join(' ');
    const tspans = [];
    const rotated = [];
    for (let i = 0; i < lines.length; i++) {
      const chars = [...lines[i].text];
      const x = left + (lines.length - 1 - i) * step;
      for (let j = 0; j < chars.length; j++) {
        const ch = chars[j];
        const drop = verticalRotation(ch);
        if (drop !== null) {
          const cx = x + fontSize / 2;
          const cy = top + j * fontSize + fontSize / 2 + drop * fontSize;
          rotated.push(`<text ${base} text-anchor="middle" dy="0.36em"`
            + ` transform="rotate(90 ${formatNumber(cx, places)} ${formatNumber(cy, places)})"`
            + ` x="${formatNumber(cx, places)}" y="${formatNumber(cy, places)}">${escapeText(ch)}</text>`);
          continue;
        }
        const offset = verticalOffsetOf(ch);
        const y = top + j * fontSize + metrics.ascent + offset.dy * fontSize;
        tspans.push(`<tspan x="${formatNumber(x + offset.dx * fontSize, places)}" y="${formatNumber(y, places)}">${escapeText(ch)}</tspan>`);
      }
    }
    const main = tspans.length ? `<text ${base} text-anchor="start">${tspans.join('')}</text>` : '';
    return `${main}${rotated.join('')}`;
  }

  const attributes = [
    `font-family="${escapeAttribute(fontFamilyStack(state, Boolean(font)))}"`,
    `font-size="${formatNumber(fontSize, places)}"`,
    state.bold ? 'font-weight="bold"' : '',
    `fill="${escapeAttribute(fill)}"`,
    `text-anchor="${anchor}"`,
  ].filter(Boolean).join(' ');

  const tspans = lines.map((line, i) => {
    const y = top + i * step + halfLeading + metrics.ascent;
    return `<tspan x="${formatNumber(x, places)}" y="${formatNumber(y, places)}">`
      + `${escapeText(line.text)}</tspan>`;
  }).join('');

  return `<text ${attributes}>${tspans}</text>`;
}

/**
 * Wrap text without a font, from a character count.
 *
 * The preview has to show something while the vendored font downloads, and this
 * is a deliberately crude stand-in: wide (CJK) characters count as one em and
 * everything else as half an em. Once `layoutText` can run its metrics replace
 * these numbers, so the rough version only ever decides a preview, never a file.
 */
function wrapByCount(text, { fontSize, maxWidth, lineHeight }) {
  const limit = Number.isFinite(maxWidth) && maxWidth > 0 ? maxWidth : Infinity;
  const lines = [];

  for (const paragraph of String(text).split(/\r\n|\r|\n/)) {
    let current = '';
    for (const ch of paragraph) {
      const candidate = current + ch;
      if (current.trimEnd() !== '' && measureFallback(candidate, fontSize) > limit) {
        lines.push(current.trimEnd());
        current = /\s/.test(ch) ? '' : ch; // the break swallows the space
      } else {
        current = candidate;
      }
    }
    lines.push(current.trimEnd());
  }

  const measured = lines.map((line) => ({ text: line, width: measureFallback(line, fontSize) }));
  let width = 0;
  for (const line of measured) width = Math.max(width, line.width);
  return { lines: measured, width, height: measured.length * fontSize * lineHeight };
}

/** Wrap with `layoutText`, or with the character count when there is no font. */
function layoutBlock(font, text, options) {
  if (text === '') return { lines: [], width: 0, height: 0 };
  if (font) {
    const layout = layoutText(font, text, options);
    return { lines: layout.lines, width: layout.width, height: layout.height };
  }
  return wrapByCount(text, options);
}

/**
 * Offset of one character inside its vertical cell, in em.
 *
 * Shared by `drawText` and `liveText` so a glyph that needs a nudge lands in
 * the same place on screen and in the export.
 */
function verticalOffsetOf(ch) {
  return VERTICAL_OFFSETS.get(ch) ?? NO_OFFSET;
}

/**
 * Vertical (縦書き) layout: each paragraph becomes one column, characters stacked
 * top to bottom, columns running right to left. A paragraph longer than the
 * available height is split into more columns.
 *
 * Closing punctuation hangs: when `、` or `。` (or `,`/`.`) would fall at the top
 * of a new column, it stays on the previous one and is drawn just below that
 * column's last cell instead. Such a hanging character does not count toward
 * the column's length, so it never makes the block a cell taller or adds a
 * column of its own. `lines[i].hang` reports how many characters on that line
 * are hanging.
 *
 * The other 禁則 cases are repaired too: ー, small kana and closing brackets are
 * kept off the top of a column by handing the preceding character down with
 * them (追い出し), and an opening bracket is never left at the bottom of one.
 */
function layoutVertical(text, { fontSize, lineHeight, maxHeight }) {
  const limit = Number.isFinite(maxHeight) && maxHeight > 0
    ? Math.max(1, Math.floor(maxHeight / fontSize))
    : Infinity;
  const columns = [];
  for (const paragraph of String(text).split(/\r\n|\r|\n/)) {
    const chars = [...paragraph];
    if (chars.length === 0) {
      columns.push({ text: '', count: 0 });
      continue;
    }
    let i = 0;
    while (i < chars.length) {
      // Fill the column cell by cell, then repair its two ends for 禁則処理.
      const cells = [];
      const hanging = [];
      while (i < chars.length && cells.length < limit) {
        cells.push(chars[i]);
        i += 1;
      }
      // 行末禁則: an opening bracket may not close a column, so hand it over to
      // the next one (it will open that column instead).
      while (cells.length > 1 && COLUMN_END_FORBIDDEN.has(cells[cells.length - 1])) {
        cells.pop();
        i -= 1;
      }
      // 行頭禁則 (追い出し): ー, small kana and closing brackets may not open a
      // column; move the preceding character down to keep them company.
      if (i < chars.length && cells.length > 1
        && COLUMN_START_FORBIDDEN.has(chars[i]) && !HANGING_PUNCTUATION.has(chars[i])) {
        cells.pop();
        i -= 1;
      }
      // 行頭禁則 (ぶら下げ): a closing mark that would open the next column
      // stays on this one, drawn just below its last cell.
      while (i < chars.length && HANGING_PUNCTUATION.has(chars[i])) {
        hanging.push(chars[i]);
        i += 1;
      }
      columns.push({ text: cells.join('') + hanging.join(''), count: cells.length });
    }
  }
  const longest = columns.reduce((max, column) => Math.max(max, column.count), 0);
  return {
    lines: columns.map((column) => ({
      text: column.text,
      width: fontSize,
      hang: [...column.text].length - column.count,
    })),
    // The block is exactly as wide as the columns that are drawn: `step` apart,
    // each cell one em wide (not one whole `step`, which would leave a leading
    // of dead space on the right and push the text off-centre).
    width: columns.length === 0
      ? 0
      : (columns.length - 1) * fontSize * lineHeight + fontSize,
    height: longest * fontSize,
    vertical: true,
  };
}

/** Text column in pixels: a fraction of the image, never wider than the image. */
function resolveColumn(state, outWidth, padding) {
  const fraction = toNonNegative(state.maxWidth, DEFAULT_MAX_WIDTH);
  if (!(fraction > 0)) return 0; // 0 means "do not wrap", as in `layoutText`
  return Math.min(fraction * outWidth, Math.max(0, outWidth - 2 * padding));
}

/** Ascent above and descent below the baseline, in pixels. */
function baselineMetrics(font, fontSize) {
  const unitsPerEm = font?.unitsPerEm;
  if (Number.isFinite(font?.ascender) && Number.isFinite(font?.descender) && unitsPerEm > 0) {
    return {
      ascent: (font.ascender * fontSize) / unitsPerEm,
      descent: (-font.descender * fontSize) / unitsPerEm,
    };
  }
  return { ascent: fontSize * FALLBACK_ASCENT, descent: fontSize * FALLBACK_DESCENT };
}

/** Widest line of the block, which is the width the alignment works against. */
function blockWidthOf(lines) {
  let width = 0;
  for (const line of lines) width = Math.max(width, toFinite(line?.width, 0));
  return width;
}

/** Sideways shift of one line inside the block. */
function alignOffset(align, blockWidth, lineWidth) {
  const slack = Math.max(0, blockWidth - lineWidth);
  if (align === 'center') return slack / 2;
  if (align === 'right') return slack;
  return 0;
}

/** Width of a string by character count, in pixels. */
function measureFallback(text, fontSize) {
  let width = 0;
  for (const ch of String(text ?? '')) {
    width += isWideChar(ch) ? fontSize * FALLBACK_WIDE_EM : fontSize * FALLBACK_NARROW_EM;
  }
  return width;
}

/** Characters that usually take a full em in a Japanese font. */
function isWideChar(ch) {
  const code = ch.codePointAt(0);
  for (const [start, end] of WIDE_RANGES) {
    if (code >= start && code <= end) return true;
  }
  return false;
}

// --- geometry ----------------------------------------------------------------

/**
 * The bubble outline as path commands.
 *
 * Both renderers go through here, which is the point: the canvas preview, the
 * PNG and the SVG are three views of one geometry rather than three
 * implementations of it.
 *
 * @returns {Array<object>}  empty when there is nothing to draw
 */
function bubbleCommands(box, state, scale) {
  const shape = bubbleOf(state);
  const rect = {
    x: toFinite(box?.x, 0),
    y: toFinite(box?.y, 0),
    w: toNonNegative(box?.w, 0),
    h: toNonNegative(box?.h, 0),
  };
  if (shape === 'none' || !(rect.w > 0) || !(rect.h > 0)) return [];

  if (shape === 'think') return thinkCommands(rect, state, scale);

  const radius = toNonNegative(state.radius, DEFAULT_RADIUS) * scale;
  const edges = shape === 'shout'
    ? shoutEdges(rect, scale)
    : shape === 'rect'
      ? rectEdges(rect)
      : roundEdges(rect, radius);

  return edgesToCommands(withTail(edges, rect, state, scale));
}

/** Clockwise rectangle, starting at the top-left corner. */
function rectEdges(box) {
  const { x, y, w, h } = box;
  const tl = { x, y };
  const tr = { x: x + w, y };
  const br = { x: x + w, y: y + h };
  const bl = { x, y: y + h };
  return [
    lineEdge(tl, tr),
    lineEdge(tr, br),
    lineEdge(br, bl),
    lineEdge(bl, tl),
  ];
}

/**
 * Rounded rectangle as straight sides plus quarter-circle corners.
 *
 * Keeping the sides straight (rather than approximating the whole outline with
 * a polyline) is what lets the tail attach to a real straight edge; the radius
 * is clamped so the corners can never cross each other.
 */
function roundEdges(box, radius) {
  const { x, y, w, h } = box;
  const r = clamp(radius, 0, Math.min(w, h) / 2);
  if (!(r > 0.5)) return rectEdges(box);

  const k = r * KAPPA;
  const p = (px, py) => ({ x: px, y: py });
  const corner = (from, c1, c2, to) => cubicEdge(p(...from), p(...c1), p(...c2), p(...to));
  return [
    lineEdge(p(x + r, y), p(x + w - r, y)),
    corner([x + w - r, y], [x + w - r + k, y], [x + w, y + r - k], [x + w, y + r]),
    lineEdge(p(x + w, y + r), p(x + w, y + h - r)),
    corner([x + w, y + h - r], [x + w, y + h - r + k], [x + w - r + k, y + h], [x + w - r, y + h]),
    lineEdge(p(x + w - r, y + h), p(x + r, y + h)),
    corner([x + r, y + h], [x + r - k, y + h], [x, y + h - r + k], [x, y + h - r]),
    lineEdge(p(x, y + h - r), p(x, y + r)),
    corner([x, y + r], [x, y + r - k], [x + r - k, y], [x + r, y]),
  ];
}

/**
 * Star-burst: a rectangle whose four sides zig-zag outwards.
 *
 * The tooth depth and pitch are fractions of the bubble, so a big "shout" and a
 * small one have the same number of spikes instead of the big one looking like
 * a saw.
 */
function shoutEdges(box, scale) {
  const { x, y, w, h } = box;
  const amp = Math.min(w, h) * 0.07;
  const pitch = Math.max(scale, Math.min(w, h) * 0.22);
  const corners = [{ x, y }, { x: x + w, y }, { x: x + w, y: y + h }, { x, y: y + h }];
  const normals = [{ x: 0, y: -1 }, { x: 1, y: 0 }, { x: 0, y: 1 }, { x: -1, y: 0 }];

  const points = [];
  for (let side = 0; side < 4; side++) {
    const a = corners[side];
    const b = corners[(side + 1) % 4];
    const normal = normals[side];
    const length = Math.hypot(b.x - a.x, b.y - a.y);
    const teeth = Math.max(2, Math.round(length / pitch));
    points.push(a);
    for (let i = 0; i < teeth; i++) {
      if (i > 0) points.push(lerp(a, b, i / teeth));
      const out = lerp(a, b, (i + 0.5) / teeth);
      points.push({ x: out.x + normal.x * amp, y: out.y + normal.y * amp });
    }
  }

  return points.map((from, i) => lineEdge(from, points[(i + 1) % points.length]));
}

/**
 * Cloud outline for a thought bubble, plus how far its scallops bulge out.
 *
 * The four sides of the box are replaced by a ring of outward scallops: a
 * rounded rectangle is sampled evenly by arc length and each pair of samples is
 * joined by a half-ellipse bulging away from the centre.
 */
function thinkEdges(box, scale) {
  const { x, y, w, h } = box;
  const cx = x + w / 2;
  const cy = y + h / 2;
  const ring = roundedRectRing(box, Math.min(w, h) * 0.28);
  const perimeter = ringLength(ring);
  // Fluffier: more scallops, each bulging further, so it reads as a soft cloud.
  const pitch = Math.max(scale * 6, Math.min(w, h) * 0.30);
  const count = Math.max(6, Math.round(perimeter / pitch));
  const points = sampleRing(ring, count);
  const amp = Math.min((perimeter / count) * 0.55, Math.min(w, h) * 0.26);

  const edges = [];
  let outer = amp;
  for (let i = 0; i < points.length; i++) {
    const a = points[i];
    const b = points[(i + 1) % points.length];
    const mid = { x: (a.x + b.x) / 2, y: (a.y + b.y) / 2 };
    // Bulge perpendicular to the edge, not away from the centre: on a wide box
    // the centre direction points sideways along the top and bottom edges, which
    // is what tilted those scallops. The sign is flipped so it always faces out,
    // which makes every edge scallop the same way the left and right ones do.
    const dx = b.x - a.x;
    const dy = b.y - a.y;
    const length = Math.hypot(dx, dy) || 1;
    let nx = -dy / length;
    let ny = dx / length;
    if (nx * (mid.x - cx) + ny * (mid.y - cy) < 0) {
      nx = -nx;
      ny = -ny;
    }
    // A small, *deterministic* wobble so the scallops are not all identical - the
    // even size read as too regular. Same index, same bubble, every render.
    const scallopAmp = amp * (0.72 + 0.56 * scallopJitter(i));
    if (scallopAmp > outer) outer = scallopAmp;
    edges.push(scallopEdge(a, b, { x: nx, y: ny }, scallopAmp));
  }
  return { edges, amp: outer };
}

/** A closed cloud, followed by the tail: two or three shrinking puffs. */
function thinkCommands(box, state, scale) {
  const cloud = thinkEdges(box, scale);
  const commands = edgesToCommands(cloud.edges);
  for (const puff of thoughtTrail(box, state, scale, cloud.amp)) {
    commands.push(...circleCommands(puff.x, puff.y, puff.r));
  }
  return commands;
}

/**
 * The circles that trail away from a thought bubble instead of a pointed tail.
 *
 * They start just outside the cloud (`outer`) and shrink as they go, ending
 * inside the room `layoutCaption` reserves for the tail.
 */
function thoughtTrail(box, state, scale, outer) {
  const tail = tailOf(state);
  if (tail === 'none') return [];
  const dir = tailDirection(tail);
  const diagonal = dir.x !== 0 && dir.y !== 0;
  const nx = dir.x * (diagonal ? Math.SQRT1_2 : 1);
  const ny = dir.y * (diagonal ? Math.SQRT1_2 : 1);
  const anchor = {
    x: dir.x < 0 ? box.x : dir.x > 0 ? box.x + box.w : box.x + box.w / 2,
    y: dir.y < 0 ? box.y : dir.y > 0 ? box.y + box.h : box.y + box.h / 2,
  };
  // Sized from the font, not the leftover room, so the puffs stay visible at any
  // bubble size; they start just outside the cloud (`outer`) and shrink away.
  const unit = resolveFontSize(state, scale);
  // 80% of the first cut, which read a touch large.
  const r1 = unit * 0.48;
  const r2 = unit * 0.34;
  const r3 = unit * 0.21;
  const gap = unit * 0.5;
  const d1 = outer + r1;
  const d2 = d1 + r1 + gap;
  const d3 = d2 + r2 + gap;
  return [
    { x: anchor.x + nx * d1, y: anchor.y + ny * d1, r: r1 },
    { x: anchor.x + nx * d2, y: anchor.y + ny * d2, r: r2 },
    { x: anchor.x + nx * d3, y: anchor.y + ny * d3, r: r3 },
  ];
}

/** One outward scallop: a half-ellipse from `from` to `to` bulging along `normal`. */
function scallopEdge(from, to, normal, amp) {
  const k = (4 / 3) * amp;
  return cubicEdge(
    from,
    { x: from.x + normal.x * k, y: from.y + normal.y * k },
    { x: to.x + normal.x * k, y: to.y + normal.y * k },
    to,
  );
}

/**
 * A small deterministic wobble in 0..1 for the i-th scallop of a cloud.
 *
 * It is derived from the index alone, so a bubble draws its scallops exactly
 * the same in the live preview, a screenshot, and the SVG export.
 */
function scallopJitter(i) {
  const x = Math.sin((i + 1) * 12.9898) * 43758.5453;
  return x - Math.floor(x);
}

/** A circle as `M`/`C`/`Z` commands, so it can join the bubble's one path. */
function circleCommands(cx, cy, r) {
  const k = r * KAPPA;
  return [
    { type: 'M', x: cx + r, y: cy },
    { type: 'C', x1: cx + r, y1: cy + k, x2: cx + k, y2: cy + r, x: cx, y: cy + r },
    { type: 'C', x1: cx - k, y1: cy + r, x2: cx - r, y2: cy + k, x: cx - r, y: cy },
    { type: 'C', x1: cx - r, y1: cy - k, x2: cx - k, y2: cy - r, x: cx, y: cy - r },
    { type: 'C', x1: cx + k, y1: cy - r, x2: cx + r, y2: cy - k, x: cx + r, y: cy },
    { type: 'Z' },
  ];
}

/** Points around a rounded rectangle, clockwise from the top-left arc. */
function roundedRectRing(box, radius) {
  const { x, y, w, h } = box;
  const r = clamp(radius, 0, Math.min(w, h) / 2);
  const points = [];
  const arc = (cx, cy, from, to) => {
    const steps = 6;
    for (let i = 1; i <= steps; i++) {
      const t = from + (to - from) * (i / steps);
      points.push({ x: cx + Math.cos(t) * r, y: cy + Math.sin(t) * r });
    }
  };

  points.push({ x: x + r, y });
  points.push({ x: x + w - r, y });
  arc(x + w - r, y + r, -Math.PI / 2, 0);
  points.push({ x: x + w, y: y + h - r });
  arc(x + w - r, y + h - r, 0, Math.PI / 2);
  points.push({ x: x + r, y: y + h });
  arc(x + r, y + h - r, Math.PI / 2, Math.PI);
  points.push({ x, y: y + r });
  arc(x + r, y + r, Math.PI, Math.PI * 1.5);
  return points;
}

/** Total length of a closed polyline's perimeter. */
function ringLength(points) {
  let total = 0;
  for (let i = 0; i < points.length; i++) {
    total += distance(points[i], points[(i + 1) % points.length]);
  }
  return total;
}

/** `count` points spaced evenly by arc length around a closed polyline. */
function sampleRing(points, count) {
  const n = points.length;
  const total = ringLength(points);
  if (!(total > 0) || !(count > 0)) return [];
  const out = [];
  let seg = 0;
  let start = 0;
  let length = n > 1 ? distance(points[0], points[1 % n]) : 0;
  for (let i = 0; i < count; i++) {
    const target = (total * i) / count;
    while (seg < n - 1 && start + length < target) {
      start += length;
      seg += 1;
      length = distance(points[seg], points[(seg + 1) % n]);
    }
    const t = length > 0 ? clamp((target - start) / length, 0, 1) : 0;
    out.push(lerp(points[seg], points[(seg + 1) % n], t));
  }
  return out;
}

/**
 * Replace part of the edge the tail points at with a spike.
 *
 * The spike is inserted *into* the outline, not appended to it: the two sides
 * of the tail and the bubble become one continuous border. The straight edge
 * nearest the tail's anchor is used, which for every shape except the shout is
 * the obvious side and for the shout is the nearest zig-zag segment.
 */
function withTail(edges, box, state, scale) {
  const tail = tailOf(state);
  if (tail === 'none' || edges.length === 0) return edges;

  const tailLength = Math.min(resolveFontSize(state, scale) * 0.9, Math.min(box.w, box.h) * 0.5);
  if (!(tailLength > 0)) return edges;

  const dir = tailDirection(tail);
  // A diagonal tail leaves from a corner; an axis-aligned one leaves from the
  // middle of the side it points at.
  const anchor = {
    x: dir.x < 0 ? box.x : dir.x > 0 ? box.x + box.w : box.x + box.w / 2,
    y: dir.y < 0 ? box.y : dir.y > 0 ? box.y + box.h : box.y + box.h / 2,
  };
  const diagonal = dir.x !== 0 && dir.y !== 0;
  const normal = {
    x: dir.x * (diagonal ? Math.SQRT1_2 : 1),
    y: dir.y * (diagonal ? Math.SQRT1_2 : 1),
  };

  const index = nearestEdge(edges, anchor, box, tail);
  if (index < 0) return edges;

  const edge = edges[index];
  const length = distance(edge.from, edge.to);
  const base = Math.min(tailLength * 0.7, length * 0.45);
  if (!(base > 0) || !(length > 0)) return edges;

  // The spike sits at the point of the edge nearest the anchor - the middle for
  // a side, the corner for a diagonal - kept far enough in that both feet land
  // on the edge itself.
  const t = clamp(projectionT(edge, anchor), base / length, 1 - base / length);
  const middle = lerp(edge.from, edge.to, t);
  // A diagonal spike advances `tailLength` on *each* axis, so it reaches as far
  // as a side tail does and reads as a proper corner.
  const reach = tailLength * (diagonal ? Math.SQRT2 : 1);
  const tip = { x: middle.x + normal.x * reach, y: middle.y + normal.y * reach };
  const leftFoot = lerp(edge.from, edge.to, t - base / length);
  const rightFoot = lerp(edge.from, edge.to, t + base / length);
  const replacement = [
    lineEdge(edge.from, leftFoot),
    lineEdge(leftFoot, tip),
    lineEdge(tip, rightFoot),
    lineEdge(rightFoot, edge.to),
  ];
  return edges.slice(0, index).concat(replacement, edges.slice(index + 1));
}

/** Index of the straight edge closest to the tail's anchor, or `-1`. */
function nearestEdge(edges, anchor, box, tail) {
  let best = -1;
  let bestDistance = Infinity;
  for (let i = 0; i < edges.length; i++) {
    const edge = edges[i];
    if (edge.kind !== 'line') continue;
    const middle = { x: (edge.from.x + edge.to.x) / 2, y: (edge.from.y + edge.to.y) / 2 };
    if (!onSide(middle, box, tail)) continue;
    const d = distance(middle, anchor);
    if (d < bestDistance) {
      bestDistance = d;
      best = i;
    }
  }
  return best;
}

/** Is `point` on the side of the box the tail points at? */
function onSide(point, box, tail) {
  const dir = tailDirection(tail);
  const midX = box.x + box.w * 0.5;
  const midY = box.y + box.h * 0.5;
  if (dir.x < 0 && point.x > midX) return false;
  if (dir.x > 0 && point.x < midX) return false;
  if (dir.y < 0 && point.y > midY) return false;
  if (dir.y > 0 && point.y < midY) return false;
  return true;
}

/** How far along `edge` (0..1) the point nearest `point` falls. */
function projectionT(edge, point) {
  const dx = edge.to.x - edge.from.x;
  const dy = edge.to.y - edge.from.y;
  const lengthSq = dx * dx + dy * dy;
  if (!(lengthSq > 0)) return 0.5;
  return ((point.x - edge.from.x) * dx + (point.y - edge.from.y) * dy) / lengthSq;
}

/** One straight edge of a closed outline. */
function lineEdge(from, to) {
  return { kind: 'line', from, to };
}

/** One cubic edge of a closed outline. */
function cubicEdge(from, c1, c2, to) {
  return { kind: 'cubic', from, c1, c2, to };
}

/** Walk the edges into `M`/`L`/`C`/`Z` commands. */
function edgesToCommands(edges) {
  if (edges.length === 0) return [];
  const commands = [{ type: 'M', x: edges[0].from.x, y: edges[0].from.y }];
  for (const edge of edges) {
    if (edge.kind === 'line') {
      commands.push({ type: 'L', x: edge.to.x, y: edge.to.y });
    } else {
      commands.push({
        type: 'C',
        x1: edge.c1.x, y1: edge.c1.y,
        x2: edge.c2.x, y2: edge.c2.y,
        x: edge.to.x, y: edge.to.y,
      });
    }
  }
  commands.push({ type: 'Z' });
  return commands;
}

/** Replay commands into a 2D context. */
function applyCommands(ctx, commands) {
  ctx.beginPath();
  for (const command of commands) {
    if (command.type === 'M') ctx.moveTo(command.x, command.y);
    else if (command.type === 'L') ctx.lineTo(command.x, command.y);
    else if (command.type === 'C') {
      ctx.bezierCurveTo(command.x1, command.y1, command.x2, command.y2, command.x, command.y);
    } else if (command.type === 'Z') {
      ctx.closePath();
    }
  }
}

/** Print commands as SVG path data. */
function commandsToPathData(commands, places) {
  const parts = [];
  for (const command of commands) {
    if (command.type === 'M') {
      parts.push(`M${formatNumber(command.x, places)} ${formatNumber(command.y, places)}`);
    } else if (command.type === 'L') {
      parts.push(`L${formatNumber(command.x, places)} ${formatNumber(command.y, places)}`);
    } else if (command.type === 'C') {
      parts.push(
        `C${formatNumber(command.x1, places)} ${formatNumber(command.y1, places)}`
          + ` ${formatNumber(command.x2, places)} ${formatNumber(command.y2, places)}`
          + ` ${formatNumber(command.x, places)} ${formatNumber(command.y, places)}`,
      );
    } else if (command.type === 'Z') {
      parts.push('Z');
    }
  }
  return parts.join(' ');
}

// --- shared helpers ----------------------------------------------------------

/** The family list for a caption, or the system-only list before the font loads. */
function fontFamilyStack(caption, loaded) {
  const stack = captionFontStack();
  if (loaded && caption?.font !== 'system') return stack;
  return dropLeadingFamily(stack);
}

/** `captionFontStack()` minus its first family, for a plain system fallback. */
function dropLeadingFamily(stack) {
  let quote = null;
  for (let i = 0; i < stack.length; i++) {
    const ch = stack[i];
    if (quote) {
      if (ch === '\\') i++;
      else if (ch === quote) quote = null;
    } else if (ch === "'" || ch === '"') {
      quote = ch;
    } else if (ch === ',') {
      const rest = stack.slice(i + 1).trim();
      return rest === '' ? 'sans-serif' : rest;
    }
  }
  return 'sans-serif';
}

/** Font size in output pixels, always positive and finite. */
function resolveFontSize(caption, scale) {
  const base = toPositive(caption?.fontSize, DEFAULT_FONT_SIZE);
  return toPositive(base * positiveScale(scale), DEFAULT_FONT_SIZE);
}

function bubbleOf(caption) {
  const value = caption?.bubble;
  if (value === 'round' || value === 'rect' || value === 'shout'
    || value === 'whisper' || value === 'think' || value === 'none') return value;
  return 'round';
}

function tailOf(caption) {
  const value = caption?.tail;
  return TAIL_VALUES.has(value) ? value : 'left';
}

const TAIL_VALUES = new Set([
  'left', 'right', 'top', 'bottom',
  'topLeft', 'topRight', 'bottomLeft', 'bottomRight',
  'none',
]);

/**
 * Which way a tail leaves the box, as a direction where each axis is -1, 0 or
 * 1 (0 for `none` or anything unknown). A diagonal tail carries a sign on both
 * axes, so `bottomLeft` is `{ x: -1, y: 1 }`.
 */
function tailDirection(tail) {
  switch (tail) {
    case 'left': return { x: -1, y: 0 };
    case 'right': return { x: 1, y: 0 };
    case 'top': return { x: 0, y: -1 };
    case 'bottom': return { x: 0, y: 1 };
    case 'topLeft': return { x: -1, y: -1 };
    case 'topRight': return { x: 1, y: -1 };
    case 'bottomLeft': return { x: -1, y: 1 };
    case 'bottomRight': return { x: 1, y: 1 };
    default: return { x: 0, y: 0 };
  }
}

function alignOf(caption) {
  const value = caption?.align;
  return value === 'center' || value === 'right' ? value : 'left';
}

function colorOf(value, fallback) {
  return typeof value === 'string' && value !== '' ? value : fallback;
}

/** Scale is a multiplier: anything that is not a positive number means 1. */
function positiveScale(value) {
  return Number.isFinite(value) && value > 0 ? value : 1;
}

function toFinite(value, fallback) {
  return Number.isFinite(value) ? value : fallback;
}

function toPositive(value, fallback) {
  return Number.isFinite(value) && value > 0 ? value : fallback;
}

function toNonNegative(value, fallback) {
  return Number.isFinite(value) && value >= 0 ? value : fallback;
}

function clamp(value, low, high) {
  if (high < low) return low;
  return Math.min(Math.max(value, low), high);
}

/** Decimal string for both `ctx.font` and SVG, never `NaN` and never `-0`. */
function formatNumber(value, places = 2) {
  if (!Number.isFinite(value)) return '0';
  const decimals = Number.isFinite(places) ? Math.max(0, Math.min(8, Math.floor(places))) : 2;
  const factor = Math.pow(10, decimals);
  const rounded = Math.round(value * factor) / factor;
  return Object.is(rounded, -0) ? '0' : String(rounded);
}

/**
 * Escape character data.
 *
 * Quotes are escaped too even though element content allows them: the caption
 * is user text, and the extra entities cost nothing next to never emitting a
 * document a stricter parser could object to.
 */
function escapeText(value) {
  return String(value ?? '')
    .replace(/&/g, '&amp;')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;')
    .replace(/'/g, '&apos;');
}

/**
 * Escape an attribute value.
 *
 * Only the double quote matters inside a double-quoted attribute, so single
 * quotes survive and a family stack such as `'Hiragino Maru Gothic ProN',
 * sans-serif` stays readable in the file.
 */
function escapeAttribute(value) {
  return String(value ?? '')
    .replace(/&/g, '&amp;')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;');
}

function distance(a, b) {
  return Math.hypot(b.x - a.x, b.y - a.y);
}

function lerp(a, b, t) {
  return { x: a.x + (b.x - a.x) * t, y: a.y + (b.y - a.y) * t };
}