// Client-side region-key data access. Fetches the two static JSON files served
// from public/ and builds the lookups the region-level UI needs:
//   - idToKey():   feature id -> region key (to stamp each map feature + resolve owner)
//   - keyToName(): region key -> human name (to show region names in the dossier)
// Each is fetched once and cached (Promise singleton) so WorldMap and NationPanel
// share a single fetch. Same-origin relative URLs (proxy/Traefik safe) — never a
// hardcoded origin.
type GeoLike = {
  features?: Array<{ id?: string | number; properties?: { name?: string } }>;
};

let idKeyPromise: Promise<Record<string, string>> | null = null;
let keyNamePromise: Promise<Record<string, string>> | null = null;

async function fetchJson<T>(url: string): Promise<T> {
  const r = await fetch(url);
  if (!r.ok) throw new Error(`${url} → HTTP ${r.status}`);
  return (await r.json()) as T;
}

/** feature id (from regions.geo.json) -> region key (from region-keys.json). */
export function idToKey(): Promise<Record<string, string>> {
  idKeyPromise ??= fetchJson<Record<string, string>>('/region-keys.json');
  return idKeyPromise;
}

/**
 * region key -> human region name. Built by joining region-keys.json (id -> key)
 * with regions.geo.json (id -> properties.name). A key with no known name is
 * simply absent (callers fall back to the raw key).
 */
export function keyToName(): Promise<Record<string, string>> {
  keyNamePromise ??= Promise.all([idToKey(), fetchJson<GeoLike>('/regions.geo.json')]).then(
    ([keys, geo]) => {
      const out: Record<string, string> = {};
      for (const f of geo.features ?? []) {
        const id = f.id == null ? '' : String(f.id);
        const key = keys[id];
        const name = f.properties?.name;
        if (key && typeof name === 'string') out[key] = name;
      }
      return out;
    },
  );
  return keyNamePromise;
}