"use strict";

const { promisify } = require('util');
const { resolveCname } = require('dns');
const resolveCnameAsync = promisify(resolveCname);

let resultCache = null;
const deferClearCache = () => setTimeout(() => { resultCache = null }, 1000);

const resolve = async () => {
  if (resultCache) {
    return resultCache;
  }

  const result = await resolveCnameAsync('blahblah.host.com');
  if (result[0].includes('us-east-1')) {
    resultCache = ['us-east-1', 'east-bucket'];
  } else {
    resultCache = ['us-west-2', 'west-bucket'];           
  }
  deferClearCache();
  return resultCache;
};

exports.handler = async event => {
  const request = event.Records[0].cf.request;
  const [region, bucket] = await resolve();
  const domainName = `${bucket}.s3.${region}.amazonaws.com`;
  request.origin = {
    s3: {
      domainName,
      region,
      authMethod: 'none',
      path: '',
    }
  };
  request.headers['host'] = [{ key: 'host', value: domainName }];
  return request;
};