sharper.ly/apps/web/app/components/ui/LanguageSwitcher/locale-path.ts

/* ============================================================================
   LOCALE + PATH HELPERS
   ----------------------------------------------------------------------------
   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.

   These are pure functions on purpose - they run identically during SSR and in
   the browser, and they are the single source of truth for "what is the other
   language's URL for the page I am on".
   ========================================================================= */

export const LOCALES = ["en", "ar"] as const;

export type Locale = (typeof LOCALES)[number];

export type Direction = "ltr" | "rtl";

export const DEFAULT_LOCALE: Locale = "en";

/** Locale -> the value that belongs in <html dir>. */
const DIRECTION_BY_LOCALE: Record<Locale, Direction> = {
  en: "ltr",
  ar: "rtl",
};

/** Locale -> its endonym, used as the accessible name of the switcher link. */
export const LOCALE_ENDONYM: Record<Locale, string> = {
  en: "English",
  ar: "العربية",
};

/** Locale -> the two-letter badge the switcher shows. */
export const LOCALE_SHORT_LABEL: Record<Locale, string> = {
  en: "EN",
  ar: "AR",
};

export function isLocale(value: unknown): value is Locale {
  return (
    typeof value === "string" && (LOCALES as readonly string[]).includes(value)
  );
}

export function getDirection(locale: Locale): Direction {
  return DIRECTION_BY_LOCALE[locale];
}

/** The language a reader on `locale` can switch to. Two locales, so it flips. */
export function getCounterpartLocale(locale: Locale): Locale {
  return locale === "en" ? "ar" : "en";
}

/**
 * The locale prefix of a pathname, or null when there is none.
 *   "/ar/contact" -> "ar"      "/about" -> null      "/" -> null
 */
export function getLocaleFromPathname(pathname: string): Locale | null {
  const first = pathname.replace(/^\/+/, "").split("/")[0]?.toLowerCase();
  return isLocale(first) ? first : null;
}

/**
 * The pathname with its locale prefix removed, always leading-slashed.
 *   "/ar/news/2026" -> "/news/2026"     "/en" -> "/"      "/news" -> "/news"
 */
export function stripLocale(pathname: string): string {
  const locale = getLocaleFromPathname(pathname);
  const withoutLeadingSlashes = pathname.replace(/^\/+/, "");
  const rest = locale
    ? withoutLeadingSlashes.slice(locale.length)
    : withoutLeadingSlashes;

  if (rest === "" || rest === "/") return "/";
  return rest.startsWith("/") ? rest : `/${rest}`;
}

/**
 * The same page under `locale`. Safe to call with an already-prefixed path, an
 * unprefixed path, or a bare "/" - the locale prefix is normalised either way.
 *   ("/en/contact", "ar") -> "/ar/contact"
 *   ("/contact",    "ar") -> "/ar/contact"
 *   ("/",           "en") -> "/en"
 */
export function localizePath(pathname: string, locale: Locale): string {
  const rest = stripLocale(pathname);
  return rest === "/" ? `/${locale}` : `/${locale}${rest}`;
}

export interface CounterpartPathOptions {
  /** Query string to carry over, with or without the leading "?". */
  search?: string;
  /** Fragment to carry over, with or without the leading "#". */
  hash?: string;
}

/**
 * The counterpart-language URL for the current location: same page, other
 * locale, query string and fragment preserved.
 */
export function getCounterpartPath(
  pathname: string,
  locale: Locale,
  options: CounterpartPathOptions = {},
): string {
  return buildLocalizedHref(
    pathname,
    getCounterpartLocale(locale),
    options,
  );
}

/** localizePath() plus the preserved search/hash. */
export function buildLocalizedHref(
  pathname: string,
  locale: Locale,
  { search = "", hash = "" }: CounterpartPathOptions = {},
): string {
  const normalisedSearch =
    search && search !== "?" ? (search.startsWith("?") ? search : `?${search}`) : "";
  const normalisedHash =
    hash && hash !== "#" ? (hash.startsWith("#") ? hash : `#${hash}`) : "";

  return `${localizePath(pathname, locale)}${normalisedSearch}${normalisedHash}`;
}

/**
 * True when `href` addresses the page currently at `pathname`. Trailing
 * slashes and an absent locale prefix on either side are tolerated so a link
 * built as "/en/about" still matches a location of "/en/about/".
 */
export function isCurrentPath(href: string, pathname: string): boolean {
  return normalizeForCompare(href) === normalizeForCompare(pathname);
}

function normalizeForCompare(value: string): string {
  const pathOnly = value.split(/[?#]/)[0] ?? value;
  const withoutTrailingSlash = pathOnly.replace(/\/+$/, "");
  return withoutTrailingSlash === "" ? "/" : withoutTrailingSlash.toLowerCase();
}