/*
 * Copyright (c) 2017-2021 Digital Bazaar, Inc. All rights reserved.
 */
'use strict';

const https = require('https');
const {parseLinkHeader, buildHeaders} = require('../util');
const {LINK_HEADER_CONTEXT} = require('../constants');
const JsonLdError = require('../JsonLdError');
const RequestQueue = require('../RequestQueue');
const {prependBase} = require('../url');
const {httpClient} = require('@digitalbazaar/http-client');

/**
 * Creates a built-in node document loader.
 *
 * @param options the options to use:
 *          [secure]: require all URLs to use HTTPS. (default: false)
 *          [strictSSL]: true to require SSL certificates to be valid,
 *            false not to. (default: true)
 *          [maxRedirects]: the maximum number of redirects to permit.
 *            (default: none)
 *          [headers]: an object (map) of headers which will be passed as
 *            request headers for the requested document. Accept is not
 *            allowed. (default: none).
 *          [httpAgent]: a Node.js `http.Agent` to use with 'http' requests.
 *            (default: none)
 *          [httpsAgent]: a Node.js `https.Agent` to use with 'https' requests.
 *            (default: An agent with rejectUnauthorized to the strictSSL
 *            value)
 *
 * @return the node document loader.
 */
module.exports = ({
  secure,
  strictSSL = true,
  maxRedirects = -1,
  headers = {},
  httpAgent,
  httpsAgent
} = {strictSSL: true, maxRedirects: -1, headers: {}}) => {
  headers = buildHeaders(headers);
  // if no default user-agent header, copy headers and set one
  if(!('user-agent' in headers)) {
    headers = Object.assign({}, headers, {
      'user-agent': 'jsonld.js'
    });
  }
  const http = require('http');

  const queue = new RequestQueue();
  return queue.wrapLoader(function(url) {
    return loadDocument(url, []);
  });

  async function loadDocument(url, redirects) {
    const isHttp = url.startsWith('http:');
    const isHttps = url.startsWith('https:');
    if(!isHttp && !isHttps) {
      throw new JsonLdError(
        'URL could not be dereferenced; only "http" and "https" URLs are ' +
        'supported.',
        'jsonld.InvalidUrl', {code: 'loading document failed', url});
    }
    if(secure && !isHttps) {
      throw new JsonLdError(
        'URL could not be dereferenced; secure mode is enabled and ' +
        'the URL\'s scheme is not "https".',
        'jsonld.InvalidUrl', {code: 'loading document failed', url});
    }
    // TODO: disable cache until HTTP caching implemented
    let doc = null;//cache.get(url);
    if(doc !== null) {
      return doc;
    }

    let alternate = null;

    const {res, body} = await _fetch({
      url, headers, strictSSL, httpAgent, httpsAgent
    });
    doc = {contextUrl: null, documentUrl: url, document: body || null};

    // handle error
    const statusText = http.STATUS_CODES[res.status];
    if(res.status >= 400) {
      throw new JsonLdError(
        `URL "${url}" could not be dereferenced: ${statusText}`,
        'jsonld.InvalidUrl', {
          code: 'loading document failed',
          url,
          httpStatusCode: res.status
        });
    }
    const link = res.headers.get('link');
    let location = res.headers.get('location');
    const contentType = res.headers.get('content-type');

    // handle Link Header
    if(link && contentType !== 'application/ld+json') {
      // only 1 related link header permitted
      const linkHeaders = parseLinkHeader(link);
      const linkedContext = linkHeaders[LINK_HEADER_CONTEXT];
      if(Array.isArray(linkedContext)) {
        throw new JsonLdError(
          'URL could not be dereferenced, it has more than one associated ' +
          'HTTP Link Header.',
          'jsonld.InvalidUrl',
          {code: 'multiple context link headers', url});
      }
      if(linkedContext) {
        doc.contextUrl = linkedContext.target;
      }

      // "alternate" link header is a redirect
      alternate = linkHeaders.alternate;
      if(alternate &&
        alternate.type == 'application/ld+json' &&
        !(contentType || '')
          .match(/^application\/(\w*\+)?json$/)) {
        location = prependBase(url, alternate.target);
      }
    }

    // handle redirect
    if((alternate ||
      res.status >= 300 && res.status < 400) && location) {
      if(redirects.length === maxRedirects) {
        throw new JsonLdError(
          'URL could not be dereferenced; there were too many redirects.',
          'jsonld.TooManyRedirects', {
            code: 'loading document failed',
            url,
            httpStatusCode: res.status,
            redirects
          });
      }
      if(redirects.indexOf(url) !== -1) {
        throw new JsonLdError(
          'URL could not be dereferenced; infinite redirection was detected.',
          'jsonld.InfiniteRedirectDetected', {
            code: 'recursive context inclusion',
            url,
            httpStatusCode: res.status,
            redirects
          });
      }
      redirects.push(url);
      // location can be relative, turn into full url
      const nextUrl = new URL(location, url).href;
      return loadDocument(nextUrl, redirects);
    }

    // cache for each redirected URL
    redirects.push(url);
    // TODO: disable cache until HTTP caching implemented
    /*
    for(let i = 0; i < redirects.length; ++i) {
      cache.set(
        redirects[i],
        {contextUrl: null, documentUrl: redirects[i], document: body});
    }
    */

    return doc;
  }
};

async function _fetch({url, headers, strictSSL, httpAgent, httpsAgent}) {
  try {
    const options = {
      headers,
      redirect: 'manual',
      // ky specific to avoid redirects throwing
      throwHttpErrors: false
    };
    const isHttps = url.startsWith('https:');
    if(isHttps) {
      options.agent =
        httpsAgent || new https.Agent({rejectUnauthorized: strictSSL});
    } else {
      if(httpAgent) {
        options.agent = httpAgent;
      }
    }
    const res = await httpClient.get(url, options);
    return {res, body: res.data};
  } catch(e) {
    // HTTP errors have a response in them
    // ky considers redirects HTTP errors
    if(e.response) {
      return {res: e.response, body: null};
    }
    throw new JsonLdError(
      'URL could not be dereferenced, an error occurred.',
      'jsonld.LoadDocumentError',
      {code: 'loading document failed', url, cause: e});
  }
}