/**
 * Island-side i18n. This is the file that ships.
 *
 * The page carries its own copy: `Base.astro` emits one `<script type="application/json"
 * data-i18n>` holding the current locale's client-facing keys, already resolved against
 * English. So a Spanish page ships Spanish strings and no others, in the HTML it was
 * going to send anyway. No second request, no catalog chunk per language, no waterfall
 * before an island can draw a label, and nothing in a shared JS chunk that has to be
 * one language for everybody.
 *
 * The alternative, a dynamic `import('./es.json')` keyed on locale, also avoids
 * shipping three languages, but it costs a round trip on the critical path of every
 * island and it leaves all three catalogs sitting in `dist/_astro/`. This does not.
 */
import { DEFAULT_LOCALE, type Locale } from './config.js';
import { createTranslator, type Catalog, type Translator } from './translate.js';

interface Payload {
  locale: Locale;
  catalog: Catalog;
}

/**
 * Read the payload out of the current document.
 *
 * Read every time rather than cached at module scope, because the view transition
 * router swaps the document under this module without reloading it. An island that
 * cached its translator on first load would keep drawing Spanish after the reader
 * clicked through to a Dutch page. Islands call `useI18n()` inside their `onReady`
 * setup, which runs again on every arrival, so this reads the page that is actually on
 * screen.
 */
function readPayload(): Payload {
  const tag = document.querySelector<HTMLScriptElement>('script[data-i18n]');
  if (!tag) return { locale: DEFAULT_LOCALE, catalog: {} };

  try {
    const parsed = JSON.parse(tag.textContent ?? '{}') as Partial<Payload>;
    return {
      locale: parsed.locale ?? DEFAULT_LOCALE,
      catalog: parsed.catalog ?? {},
    };
  } catch {
    // A payload that will not parse is a build bug, not a runtime condition. Falling
    // back to the key humaniser beats throwing inside every island on the page.
    return { locale: DEFAULT_LOCALE, catalog: {} };
  }
}

/**
 * One translator per document, rebuilt when the router swaps in a new one.
 *
 * Keyed on the script element itself: a new page brings a new element, and identity
 * comparison is cheaper and more honest than re-parsing the JSON on every `t()`.
 */
let cachedTag: Element | null = null;
let cached: Translator | null = null;

export function useI18n(): Translator {
  const tag = document.querySelector('script[data-i18n]');
  if (cached && tag === cachedTag) return cached;

  const { locale, catalog } = readPayload();
  // The payload is already resolved against English at build time, so the fallback
  // catalog here is the same object: there is nothing further to fall back to.
  cached = createTranslator(locale, catalog, catalog);
  cachedTag = tag;
  return cached;
}

/** The locale this page is in, for `Intl` calls that do not go through `t()`. */
export function currentLocale(): Locale {
  return useI18n().locale;
}