/**
 * Build the demo's `I18nInstance`.
 *
 * The instance is consumed by `<ReactUiProvider>` and any
 * `@suaveplan/react-ui` primitive that calls `useTranslation()` (e.g.
 * `<Button>` for icon-only-button fallback, `<Input>` for placeholder
 * resolution).
 *
 * Locale strategy: the URL carries the language subtag only (`/en/home`,
 * `/zh/home`, etc.). The chassis bundles in `@suaveplan/react-ui` are
 * keyed by full BCP47 — `en-GB`, `zh-CN`, `ar-AE`, `cy-GB`, `ru-RU`. We
 * therefore canonicalise each demo base locale to its closest BCP47
 * variant and use THAT as the i18n locale. The demo's typed `Messages`
 * catalog continues to be consumed via `useLocale()` keyed by base, so
 * the demo never depends on react-i18n's `t(key)` for its own copy.
 *
 * The chassis bundles themselves are auto-registered by
 * `<ReactUiProvider>` (its internal `ReactUiBundleLoader` effect imports
 * the matching JSON and merges it into the catalog on mount / language
 * change), so this module deliberately does NOT preload them.
 */
import { createI18n, type I18nInstance } from "@suaveplan/i18n/plugin";
import type { BaseLocale } from "./locales.js";

export type DemoBcp47 = "en-GB" | "zh-CN" | "ar-AE" | "cy-GB" | "ru-RU";

/** Map URL base locale -> canonical BCP47 variant fed to the i18n core. */
export const BASE_TO_BCP47: Readonly<Record<BaseLocale, DemoBcp47>> = {
  en: "en-GB",
  zh: "zh-CN",
  ar: "ar-AE",
  cy: "cy-GB",
  ru: "ru-RU",
};

/**
 * Singleton i18n instance for the demo. Created lazily so the i18n core
 * (which is module-side-effect free) does not run during static module
 * graph evaluation in tests or SSR snapshots.
 */
let cached: I18nInstance | undefined;

export function getDemoI18n(): I18nInstance {
  if (cached !== undefined) return cached;
  // Use the default `string` generic so the cached singleton matches
  // `<ReactUiProvider>`'s `i18n: I18nInstance` (without the
  // narrowed locale generic).
  const instance: I18nInstance = createI18n({
    defaultLocale: BASE_TO_BCP47.en as string,
    fallbackLocale: BASE_TO_BCP47.en as string,
  });
  cached = instance;
  return instance;
}

/**
 * Drive the i18n instance to the BCP47 variant matching the URL base.
 * Safe to call from a `useEffect` — `changeLanguage(...)` short-circuits
 * when the locale already matches.
 */
export async function syncDemoLocale(base: BaseLocale): Promise<void> {
  const i18n = getDemoI18n();
  const target = BASE_TO_BCP47[base];
  if (i18n.locale !== target) {
    await i18n.changeLanguage(target);
  }
}