const dns = require('dns');
const util = require('util');
const LRU = require('lru-cache');

const cache = new LRU({ maxAge: 1000 * 60, stale: true });
const resolveCname = util.promisify(dns.resolveCname);

module.exports = async (domain) => {
  if (/\.amazonaws\.com$/.test(domain)) {
    return domain;
  }

  if (cache.has(domain)) {
    return cache.get(domain);
  }

  const [ resolvedName ] = await resolveCname(domain);

  if (resolvedName) {
    cache.set(domain, resolvedName);
    return resolvedName;
  } else {
    throw Error(`Could not resolve CNAME for ${domain}`);
  }
};