import type { LocaleCode } from '../types/ExplorerConfig';

/** The locales this component actually has a catalogue for. */
const SUPPORTED: readonly LocaleCode[] = ['en', 'tr'];

/**
 * The language filex falls back to when the host does not say.
 *
 * ⚠ It used to be `tr`, in eleven places. filex is an English-first
 * open-source product, so `<filex-explorer api-base="…">` with no `locale`
 * rendered a Turkish UI to whoever installed it — and the web component
 * defaults `locale` to `''`, so "no locale" is the normal case for an embed.
 * The admin SPA never showed it because it always passes one.
 */
const FALLBACK: LocaleCode = 'en';

/**
 * Resolve the locale to render in.
 *
 * Order, and the reasoning for it:
 *
 *   1. **What the host asked for.** An explicit `locale` always wins. A host
 *      that names a language has made a decision, and guessing over it would
 *      make the prop advisory.
 *   2. **What the browser asks for**, via `navigator.languages` /
 *      `navigator.language` — the same signal `Accept-Language` carries, and
 *      the reason a Turkish reader gets Turkish without the embedder doing
 *      anything.
 *   3. **`en`.**
 *
 * ⚠ Step 2 makes the same embed render differently for two people. That is
 * deliberate — it is what "follow the browser" means — but it also means a
 * screenshot, a test or a bug report about this component must state its
 * locale, because the component alone no longer determines it. Pass an
 * explicit `locale` anywhere the answer has to be stable.
 *
 * Matching is on the primary subtag, so `tr-TR` and `en-GB` resolve; anything
 * with no catalogue falls through rather than half-rendering.
 */
export function resolveLocale(explicit?: LocaleCode | '' | null): LocaleCode {
  if (explicit && SUPPORTED.includes(explicit)) return explicit;
  return detectLocale();
}

/**
 * The browser's preference, or `en`.
 *
 * ⚠ Guarded for a non-browser runtime: this package is imported by SSR and by
 * unit tests under Node, where `navigator` is absent — and an exception here
 * would take down the whole explorer over a language choice.
 */
export function detectLocale(): LocaleCode {
  try {
    const nav = typeof navigator === 'undefined' ? undefined : navigator;
    if (!nav) return FALLBACK;
    const wanted = nav.languages && nav.languages.length ? nav.languages : [nav.language];
    for (const tag of wanted) {
      if (!tag) continue;
      const primary = String(tag).toLowerCase().split('-')[0] as LocaleCode;
      if (SUPPORTED.includes(primary)) return primary;
    }
  } catch {
    /* a hostile or exotic runtime is not a reason to fail to render */
  }
  return FALLBACK;
}