sharper.ly/apps/web/app/i18n/config.ts

/* ============================================================================
   I18N CONFIG
   ----------------------------------------------------------------------------
   SHARP-ER is served with an EXPLICIT locale prefix on BOTH languages:
   /en/... and /ar/.... Bare "/" is redirected by the server on Accept-Language.
   There is no ?lang= query parameter anywhere in the product.

   The locale primitives themselves live in the LanguageSwitcher's pure
   `locale-path` module - it is imported by <Navigation />, <LanguageSwitcher />
   and <Footer />, so re-exporting from there (rather than declaring a second
   `Locale` union) keeps one type across the whole app. A relative import is
   used because this module is reachable from `app/routes.ts`, which the dev
   plugin evaluates outside the app's normal alias resolution.
   ========================================================================= */

import {
  DEFAULT_LOCALE,
  getDirection,
  getLocaleFromPathname,
  isLocale,
  LOCALES,
  LOCALE_ENDONYM,
  LOCALE_SHORT_LABEL,
  type Direction,
  type Locale,
} from "../components/ui/LanguageSwitcher/locale-path";

export type { Direction, Locale };
export {
  DEFAULT_LOCALE,
  getLocaleFromPathname,
  isLocale,
  LOCALE_ENDONYM,
  LOCALE_SHORT_LABEL,
};

/** Every locale the site is published in, in preference order. */
export const SUPPORTED_LOCALES = LOCALES;

/** `<html dir>` for a locale. Alias of the component library's helper. */
export const dirFor = getDirection;

/**
 * The BCP-47 tag handed to `Intl.*` and to `<html lang>`'s consumers.
 * Libyan Arabic content is written in MSA, and the draft copy uses Western
 * (latn) digits throughout, so number formatting stays latn - see ./format.ts.
 */
export const INTL_LOCALE: Readonly<Record<Locale, string>> = {
  en: "en-GB",
  ar: "ar-LY",
};

/** Open Graph `og:locale` values. */
export const OG_LOCALE: Readonly<Record<Locale, string>> = {
  en: "en_GB",
  ar: "ar_LY",
};

/* ---------------------------------------------------------------------------
   ACCEPT-LANGUAGE NEGOTIATION
   Only ever used for bare "/" - once the reader is on a prefixed URL the URL
   wins, so a shared /ar/... link never silently flips to English.
   ------------------------------------------------------------------------ */

/** One parsed entry of an `Accept-Language` header. */
export interface LanguageRange {
  readonly tag: string;
  readonly quality: number;
}

/**
 * Parse an `Accept-Language` header into ranges sorted by descending quality.
 * Malformed entries are dropped rather than throwing - this runs on a header
 * an arbitrary client controls.
 */
export function parseAcceptLanguage(
  header: string | null | undefined,
): LanguageRange[] {
  if (!header) return [];

  return header
    .split(",")
    .map((part, index): LanguageRange | null => {
      const [rawTag, ...parameters] = part.trim().split(";");
      const tag = rawTag?.trim().toLowerCase();
      if (!tag) return null;

      const qParameter = parameters
        .map((parameter) => parameter.trim())
        .find((parameter) => parameter.startsWith("q="));
      const parsed = qParameter ? Number.parseFloat(qParameter.slice(2)) : 1;
      const quality = Number.isFinite(parsed) ? parsed : 0;

      // Index keeps the header's own order stable inside one quality band.
      return { tag, quality: quality - index * 1e-6 };
    })
    .filter((range): range is LanguageRange => range !== null && range.quality > 0)
    .sort((a, b) => b.quality - a.quality);
}

/**
 * The best supported locale for an `Accept-Language` header.
 * "ar-LY,ar;q=0.9,en;q=0.8" -> "ar";  "fr-FR,fr;q=0.9" -> the default locale.
 */
export function negotiateLocale(
  header: string | null | undefined,
  fallback: Locale = DEFAULT_LOCALE,
): Locale {
  for (const { tag } of parseAcceptLanguage(header)) {
    if (tag === "*") return fallback;

    const base = tag.split("-")[0];
    if (isLocale(base)) return base;
  }

  return fallback;
}

/** `negotiateLocale()` applied to a Fetch `Request`'s headers. */
export function negotiateLocaleFromRequest(
  request: Request,
  fallback: Locale = DEFAULT_LOCALE,
): Locale {
  return negotiateLocale(request.headers.get("accept-language"), fallback);
}

/**
 * The locale a URL addresses. Every page URL carries its prefix, so this is
 * the authoritative resolver used by the root layout and by `useLocale()`.
 */
export function localeFromRequest(
  request: Request,
  fallback: Locale = DEFAULT_LOCALE,
): Locale {
  return getLocaleFromPathname(new URL(request.url).pathname) ?? fallback;
}