gerbeur/api/mail/theme-tokens.ts

/**
 * Lets the emails draw with the site's own values. It reads a style's design
 * tokens out of the stylesheets the client loads, `src/tokens.css` and then
 * `src/themes/<style>.css`, so `styles.ts` names a token wherever it needs a
 * color, a gradient, a font or a radius, and a change to a theme reaches its
 * emails with nothing copied to keep in step.
 *
 * Mail clients know no custom properties, so a token comes back resolved for
 * one scheme: its `var()`s followed, its `light-dark()` picked, its
 * `color-mix()` computed. Only the rules that set tokens on the root element
 * are read (`:root`, the style's `[data-style]`, and either of those with the
 * scheme's `[data-color-scheme]`); a theme states its forced schemes that way
 * beside each `prefers-color-scheme` block, so the media blocks are skipped.
 *
 * @module
 */

import { BASE_STYLE, type StyleName } from "../../shared/theme.ts";
import { hexOf, parseCssColor, type Rgba } from "../lib/css-color.ts";

/** A scheme a token is resolved for. "auto" is drawn from both. */
export type Scheme = "light" | "dark";

/**
 * Returned by {@link readTokens}: the tokens of one style in one scheme, read
 * as the kind of value each is.
 */
export interface ThemeTokens {
  /** Returns the resolved value of `token`, or throws when it isn't set. */
  value(token: string): string;
  /**
   * Returns the color of `token` as `#rrggbb`. A translucent one is laid over
   * `over` (another token) first, as it shows on the site; without `over` it
   * stays `rgba()`, as a shadow's must.
   */
  color(token: string, over?: string): string;
  /** Returns the colors of the gradient `token`, in order, without positions. */
  stops(token: string): string[];
  /**
   * Returns the font stack of `token`, single-quoted for a `style` attribute.
   * A family the site bundles as "X Variable" is named "X", as a mail client
   * or Google Fonts knows it.
   */
  font(token: string): string;
  /** Returns the length of `token` in pixels. */
  px(token: string): number;
}

const SOURCES = ["../../src/tokens.css", "../../src/themes/{style}.css"];

/** Returns `css` without its comments. */
function stripComments(css: string): string {
  return css.replace(/\/\*[\s\S]*?\*\//g, "");
}

/** Returns the top-level rules of `css` as their selector and body. */
function topRules(css: string): { selector: string; body: string }[] {
  const rules = [];
  let i = 0;
  while (i < css.length) {
    const open = css.indexOf("{", i);
    if (open < 0) break;
    let depth = 1;
    let j = open + 1;
    while (depth && j < css.length) {
      if (css[j] === "{") depth++;
      else if (css[j] === "}") depth--;
      j++;
    }
    rules.push({
      selector: css.slice(i, open).trim(),
      body: css.slice(open + 1, j - 1),
    });
    i = j;
  }
  return rules;
}

/**
 * Splits `text` at each `sep` outside parentheses and quotes, trimming the
 * parts: the arguments of a CSS function, or the declarations of a rule.
 */
function splitTop(text: string, sep: string): string[] {
  const parts: string[] = [];
  let depth = 0;
  let quote = "";
  let start = 0;
  for (let i = 0; i < text.length; i++) {
    const c = text[i];
    if (quote) {
      if (c === quote) quote = "";
    } else if (c === '"' || c === "'") quote = c;
    else if (c === "(") depth++;
    else if (c === ")") depth--;
    else if (c === sep && depth === 0) {
      parts.push(text.slice(start, i).trim());
      start = i + 1;
    }
  }
  parts.push(text.slice(start).trim());
  return parts.filter((part) => part !== "");
}

/**
 * Replaces each call of the CSS function `name` in `value`, innermost last
 * first, with what `replace` returns for its arguments.
 */
function replaceCalls(
  value: string,
  name: string,
  replace: (args: string[]) => string,
): string {
  for (;;) {
    const at = value.lastIndexOf(`${name}(`);
    if (at < 0) return value;
    let depth = 0;
    let end = at + name.length;
    do {
      if (value[end] === "(") depth++;
      else if (value[end] === ")") depth--;
      end++;
    } while (depth && end < value.length);
    const args = splitTop(value.slice(at + name.length + 1, end - 1), ",");
    value = value.slice(0, at) + replace(args) + value.slice(end);
  }
}

/** Parses a CSS color in the forms the themes write, or throws. */
function parseColor(text: string): Rgba {
  const color = parseCssColor(text);
  if (!color) throw new Error(`Unreadable color: ${text}`);
  return color;
}

/** Writes `color` as `#rrggbb`, or as `rgba()` when it is translucent. */
function formatColor(color: Rgba): string {
  if (color.a >= 1) return hexOf(color);
  const [r, g, b] = [color.r, color.g, color.b].map(Math.round);
  return `rgba(${r},${g},${b},${+color.a.toFixed(3)})`;
}

/** Returns `top` laid over `under`, both straight (not premultiplied) alpha. */
function over(top: Rgba, under: Rgba): Rgba {
  const a = top.a + under.a * (1 - top.a);
  if (a === 0) return { r: 0, g: 0, b: 0, a: 0 };
  const mix = (t: number, u: number) =>
    (t * top.a + u * under.a * (1 - top.a)) / a;
  return {
    r: mix(top.r, under.r),
    g: mix(top.g, under.g),
    b: mix(top.b, under.b),
    a,
  };
}

/**
 * Computes `color-mix(in srgb, …)` from its arguments, the two colors with
 * their optional percentages, as CSS does: premultiplied, and scaled down
 * when the percentages sum under 100%.
 */
function colorMix(args: string[]): string {
  const [space, ...colors] = args;
  if (space !== "in srgb" || colors.length !== 2) {
    throw new Error(`Unsupported color-mix: ${args.join(", ")}`);
  }
  const parsed = colors.map((arg) => {
    const pct = arg.match(/\s([\d.]+)%$/);
    return {
      color: parseColor(pct ? arg.slice(0, pct.index) : arg),
      pct: pct ? parseFloat(pct[1]) / 100 : null,
    };
  });
  const [p1, p2] = [
    parsed[0].pct ?? 1 - (parsed[1].pct ?? 0.5),
    parsed[1].pct ?? 1 - (parsed[0].pct ?? 0.5),
  ];
  const sum = p1 + p2;
  const [w1, w2] = [p1 / sum, p2 / sum];
  const [c1, c2] = [parsed[0].color, parsed[1].color];
  const a = c1.a * w1 + c2.a * w2;
  const mix = (x: number, y: number) =>
    a === 0 ? 0 : (x * c1.a * w1 + y * c2.a * w2) / a;
  return formatColor({
    r: mix(c1.r, c2.r),
    g: mix(c1.g, c2.g),
    b: mix(c1.b, c2.b),
    a: a * Math.min(sum, 1),
  });
}

/**
 * Returns the tokens of `style` in `scheme`, read from the site's stylesheets
 * at the time of the call. `styles.ts` reads each style once, at load.
 */
export function readTokens(style: StyleName, scheme: Scheme): ThemeTokens {
  const styleSel = style === BASE_STYLE
    ? `(?:\\[data-style="${style}"\\]|:not\\(\\[data-style\\]\\))`
    : `\\[data-style="${style}"\\]`;
  const rootRule = new RegExp(
    `^(?::root)?(?:${styleSel})?(?:\\[data-color-scheme="${scheme}"\\])?$`,
  );

  // Later declarations win, as in the cascade: the base tokens, then the
  // theme's, then its forced scheme's.
  const declared = new Map<string, string>();
  for (const source of SOURCES) {
    const css = stripComments(Deno.readTextFileSync(
      new URL(source.replace("{style}", style), import.meta.url),
    ));
    for (const { selector, body } of topRules(css)) {
      if (selector.startsWith("@")) continue;
      if (!selector.split(",").some((part) => rootRule.test(part.trim()))) {
        continue;
      }
      for (const declaration of splitTop(body, ";")) {
        const colon = declaration.indexOf(":");
        const name = declaration.slice(0, colon).trim();
        if (!name.startsWith("--")) continue;
        declared.set(
          name,
          declaration.slice(colon + 1).replace(/\s+/g, " ").trim(),
        );
      }
    }
  }

  const resolved = new Map<string, string>();
  const value = (token: string): string => {
    const known = resolved.get(token);
    if (known !== undefined) return known;
    const raw = declared.get(token);
    if (raw === undefined) throw new Error(`${style} sets no ${token}`);
    let result = replaceCalls(
      raw,
      "var",
      ([name, fallback]) =>
        declared.has(name) ? value(name) : fallback ?? value(name),
    );
    result = replaceCalls(
      result,
      "light-dark",
      ([light, dark]) => scheme === "light" ? light : dark,
    );
    result = replaceCalls(result, "color-mix", colorMix);
    resolved.set(token, result);
    return result;
  };

  return {
    value,
    color(token, under) {
      const color = parseColor(value(token));
      return formatColor(under ? over(color, parseColor(value(under))) : color);
    },
    stops(token) {
      const gradient = value(token).match(/^linear-gradient\((.*)\)$/)?.[1];
      if (!gradient) throw new Error(`${style} ${token} is no linear gradient`);
      return splitTop(gradient, ",")
        .filter((stop) => !/^(?:to\s|[\d.-]+(?:deg|turn|rad)$)/.test(stop))
        .map((stop) => formatColor(parseColor(splitTop(stop, " ")[0])));
    },
    font(token) {
      return splitTop(value(token), ",")
        .map((family) => family.replace(/["']/g, "").replace(/ Variable$/, ""))
        .map((family) => /^[\w-]+$/.test(family) ? family : `'${family}'`)
        .join(", ");
    },
    px(token) {
      const length = value(token).match(/^([\d.]+)(?:px)?$/)?.[1];
      if (length === undefined) {
        throw new Error(`${style} ${token} is no pixel length`);
      }
      return parseFloat(length);
    },
  };
}