"use client";

/**
 * Browser-side locale store + React bindings.
 *
 * English is the default and the source of truth; Czech is a full translation.
 * Only *interface* copy lives in the dictionaries — your notes and the generated
 * decks always keep their own language (that is a core feature, not a setting).
 *
 * The dictionaries themselves live in ./i18n-dict so the server can translate
 * its API messages into the same language.
 */

import { useSyncExternalStore } from "react";

import {
  DEFAULT_LOCALE,
  LOCALE_HEADER,
  LOCALES,
  normaliseLocale,
  translate,
  translatePlural,
  type Locale,
  type Vars,
} from "./i18n-dict";

export { LOCALES, DEFAULT_LOCALE, LOCALE_HEADER, translate, translatePlural };
export type { Locale };

const STORAGE_KEY = "freezmo.locale.v1";

let cached: Locale | null = null;
const listeners = new Set<() => void>();

function read(): Locale {
  if (typeof window === "undefined") return DEFAULT_LOCALE;
  try {
    return normaliseLocale(window.localStorage.getItem(STORAGE_KEY));
  } catch {
    return DEFAULT_LOCALE;
  }
}

function emit(): void {
  listeners.forEach((listener) => listener());
}

function onStorage(event: StorageEvent): void {
  if (event.key === STORAGE_KEY) {
    cached = read();
    emit();
  }
}

export const localeStore = {
  subscribe(listener: () => void): () => void {
    if (listeners.size === 0) window.addEventListener("storage", onStorage);
    listeners.add(listener);
    return () => {
      listeners.delete(listener);
      if (listeners.size === 0) window.removeEventListener("storage", onStorage);
    };
  },
  getSnapshot(): Locale {
    if (cached === null) cached = read();
    return cached;
  },
  getServerSnapshot(): Locale {
    return DEFAULT_LOCALE;
  },
};

export function getLocale(): Locale {
  return localeStore.getSnapshot();
}

export function setLocale(locale: Locale): void {
  cached = normaliseLocale(locale);
  try {
    window.localStorage.setItem(STORAGE_KEY, cached);
  } catch {
    /* private mode */
  }
  try {
    document.documentElement.lang = cached;
  } catch {
    /* SSR */
  }
  emit();
}

/** React hook: re-renders when the locale changes. */
export function useLocale(): Locale {
  return useSyncExternalStore(localeStore.subscribe, localeStore.getSnapshot, localeStore.getServerSnapshot);
}

/** Translate a key for the current locale. */
export function t(key: string, vars?: Vars): string {
  return translate(localeStore.getSnapshot(), key, vars);
}

/** Plural-aware translate for the current locale. */
export function plural(n: number, base: string, vars?: Vars): string {
  return translatePlural(localeStore.getSnapshot(), n, base, vars);
}

/** Header handed to our own API routes so they answer in the chosen language. */
export function localeHeader(): Record<string, string> {
  return { [LOCALE_HEADER]: localeStore.getSnapshot() };
}

/**
 * Convenience hook for components.
 *
 * `t` / `plural` are bound to the locale React is currently rendering with
 * (the server snapshot during hydration, then the stored choice), so the first
 * client render always matches the server HTML. Use the module-level `t` only
 * in event handlers and non-React helpers.
 */
export function useI18n() {
  const locale = useLocale();
  return {
    locale,
    setLocale,
    t: (key: string, vars?: Vars) => translate(locale, key, vars),
    plural: (n: number, base: string, vars?: Vars) => translatePlural(locale, n, base, vars),
  };
}