import { LOCALES, preferredLocale, type Locale } from "./locale";

/**
 * The public addresses of the site. German is the main language and lives at the
 * root, with German names for the legal pages; English lives under /en.
 * Internally every page is rendered under its language (/de/..., /en/...);
 * `routeRequest` maps one onto the other.
 */
export type PageId = "home" | "freelance" | "imprint" | "privacy";

type LanguageAlternates = Readonly<Record<Locale | "x-default", string>>;

export type RouteDecision =
  | {
      readonly kind: "redirect";
      readonly to: string;
      readonly permanent: boolean;
      /** The answer depends on the visitor's language, so caches must not share it. */
      readonly negotiated: boolean;
    }
  | {
      readonly kind: "rewrite";
      readonly to: string;
      readonly negotiated: boolean;
    };

export type SitemapEntry = {
  readonly url: string;
  readonly alternates: { readonly languages: LanguageAlternates };
};

type PagePaths = Readonly<Record<PageId, string>>;

const PAGES: readonly PageId[] = ["home", "freelance", "imprint", "privacy"];

/** The path of a page below its language, as the app directory names it. */
const INTERNAL: PagePaths = {
  home: "",
  freelance: "/freelance",
  imprint: "/imprint",
  privacy: "/privacy",
};

/** The path of a page below its language, as visitors see it. */
const PUBLIC: Readonly<Record<Locale, PagePaths>> = {
  de: {
    home: "",
    freelance: "/freelance",
    imprint: "/impressum",
    privacy: "/datenschutz",
  },
  en: INTERNAL,
};

const PREFIX: Readonly<Record<Locale, string>> = { de: "", en: "/en" };

const withoutTrailingSlash = (pathname: string): string =>
  pathname.replace(/\/+$/, "");

/** What follows a prefix such as "/en", or null when the path is not below it. */
const restAfter = (pathname: string, prefix: string): string | null => {
  const path = withoutTrailingSlash(pathname);
  if (path === prefix) return "";
  return path.startsWith(`${prefix}/`) ? path.slice(prefix.length) : null;
};

const pageAt = (rest: string, paths: PagePaths): PageId | null =>
  PAGES.find((page) => paths[page] === rest) ?? null;

const redirect = (to: string, permanent: boolean): RouteDecision => ({
  kind: "redirect",
  to,
  permanent,
  negotiated: !permanent,
});

export function pathFor(page: PageId, locale: Locale): string {
  return `${PREFIX[locale]}${PUBLIC[locale][page]}` || "/";
}

/** The addresses of one page in every language, for hreflang links. */
export function languageAlternates(page: PageId): LanguageAlternates {
  return {
    de: pathFor(page, "de"),
    en: pathFor(page, "en"),
    "x-default": pathFor(page, "de"),
  };
}

export function localeOfPath(pathname: string): Locale {
  return restAfter(pathname, PREFIX.en) === null ? "de" : "en";
}

/** Where the language switch leads from a given address. */
export function switchPath(pathname: string, target: Locale): string {
  const locale = localeOfPath(pathname);
  const rest = withoutTrailingSlash(pathname).slice(PREFIX[locale].length);
  return pathFor(pageAt(rest, PUBLIC[locale]) ?? "home", target);
}

/**
 * What to do with a request. Addresses without a language are German and follow
 * the visitor's preference; English addresses are shown to everyone; every page
 * has exactly one public address, so all other forms are moved to it.
 */
export function routeRequest(request: {
  readonly pathname: string;
  readonly cookie?: string | null;
  readonly acceptLanguage?: string | null;
}): RouteDecision {
  const internalGerman = restAfter(request.pathname, "/de");
  if (internalGerman !== null) {
    const page =
      pageAt(internalGerman, PUBLIC.de) ?? pageAt(internalGerman, INTERNAL);
    return redirect(page === null ? internalGerman : pathFor(page, "de"), true);
  }

  const english = restAfter(request.pathname, PREFIX.en);
  if (english !== null) {
    // A German page name under /en is moved, unless the page has the same name in both languages.
    const misnamed = pageAt(english, PUBLIC.de);
    return misnamed === null || PUBLIC.de[misnamed] === INTERNAL[misnamed]
      ? { kind: "rewrite", to: `/en${english}`, negotiated: false }
      : redirect(pathFor(misnamed, "en"), true);
  }

  const rest = withoutTrailingSlash(request.pathname);
  const page = pageAt(rest, PUBLIC.de);
  if (page === null) {
    const formerEnglish = pageAt(rest, INTERNAL);
    return formerEnglish === null
      ? { kind: "rewrite", to: `/de${rest}`, negotiated: false }
      : redirect(pathFor(formerEnglish, "en"), true);
  }
  const locale = preferredLocale(request);
  return locale === "de"
    ? { kind: "rewrite", to: `/de${INTERNAL[page]}`, negotiated: true }
    : redirect(pathFor(page, locale), false);
}

/** Sitemap entries for the given pages: one per language, each naming all of them. */
export function sitemapEntries(
  siteUrl: string,
  pages: readonly PageId[],
): readonly SitemapEntry[] {
  return pages.flatMap((page) => {
    const alternates = languageAlternates(page);
    const languages: LanguageAlternates = {
      de: `${siteUrl}${alternates.de}`,
      en: `${siteUrl}${alternates.en}`,
      "x-default": `${siteUrl}${alternates["x-default"]}`,
    };
    return LOCALES.map((locale) => ({
      url: languages[locale],
      alternates: { languages },
    }));
  });
}