const DEFAULT_NOMINATIM_BASE = 'https://nominatim.openstreetmap.org';
const MIN_REQUEST_INTERVAL_MS = 1000;

const ALLOWED_HOSTS = new Set(['nominatim.openstreetmap.org']);

let lastRequestAt = 0;

function assertAllowedHost(url) {
  const host = new URL(url).hostname.toLowerCase();
  if (!ALLOWED_HOSTS.has(host)) {
    throw new Error(`[nominatim] Host not allowlisted: ${host}`);
  }
}

function getUserAgent() {
  return process.env.NOMINATIM_USER_AGENT ?? 'DeckHearth/1.0';
}

async function throttle() {
  const now = Date.now();
  const elapsed = now - lastRequestAt;
  if (elapsed < MIN_REQUEST_INTERVAL_MS) {
    await new Promise((resolve) => setTimeout(resolve, MIN_REQUEST_INTERVAL_MS - elapsed));
  }
  lastRequestAt = Date.now();
}

/**
 * @param {string} zip Five-digit US zip code.
 * @param {{ fetchImpl?: typeof fetch }} [options]
 * @returns {Promise<{ lat: number, lng: number, displayName: string } | null>}
 */
export async function geocodeUsZip(zip, { fetchImpl = fetch } = {}) {
  const base = process.env.NOMINATIM_BASE_URL ?? DEFAULT_NOMINATIM_BASE;
  const url = new URL('/search', base);
  url.searchParams.set('postalcode', zip);
  url.searchParams.set('country', 'US');
  url.searchParams.set('format', 'json');
  url.searchParams.set('limit', '1');

  assertAllowedHost(url.toString());
  await throttle();

  const response = await fetchImpl(url.toString(), {
    headers: { 'User-Agent': getUserAgent() },
  });

  if (!response.ok) {
    throw new Error(`[nominatim] HTTP ${response.status}`);
  }

  const results = await response.json();
  if (!Array.isArray(results) || results.length === 0) {
    return null;
  }

  const hit = results[0];
  const lat = Number(hit.lat);
  const lng = Number(hit.lon);
  if (!Number.isFinite(lat) || !Number.isFinite(lng)) {
    return null;
  }

  return {
    lat,
    lng,
    displayName: hit.display_name ?? `${zip}, US`,
  };
}