import { Feed } from "feed";
import { getAllPosts } from "./posts";
import * as config from "../config";
import { meJpg } from "../config/favicons";
import type { GetServerSideProps } from "next";

export type GetServerSideFeedProps = GetServerSideProps<Record<string, never>>;

export type BuildFeedOptions = {
  edgeCacheAge?: number; // in seconds, defaults to 43200 (12 hours)
};

// handles literally *everything* about building the server-side rss/atom feeds and writing the response.
// all the page needs to do is `return buildFeed(context, "rss")` from getServerSideProps.
export const buildFeed = async (
  context: Parameters<GetServerSideFeedProps>[0],
  type: "rss" | "atom" | "json",
  options?: BuildFeedOptions
): Promise<ReturnType<GetServerSideFeedProps>> => {
  const { res } = context;
  const baseUrl = process.env.NEXT_PUBLIC_BASE_URL || `https://${config.siteDomain}`;

  // https://github.com/jpmonette/feed#example
  const feed = new Feed({
    id: `${baseUrl}/`,
    link: `${baseUrl}/`,
    title: config.siteName,
    description: config.longDescription,
    copyright: config.licenseUrl,
    updated: new Date(process.env.RELEASE_DATE || Date.now()),
    image: `${baseUrl}${meJpg.src}`,
    feedLinks: {
      rss: `${baseUrl}/feed.xml`,
      atom: `${baseUrl}/feed.atom`,
    },
    author: {
      name: config.authorName,
      link: `${baseUrl}/`,
      email: config.authorEmail,
    },
  });

  // add posts separately using their frontmatter
  const posts = await getAllPosts();
  posts.forEach((post) => {
    feed.addItem({
      guid: post.permalink,
      link: post.permalink,
      title: post.title,
      description: post.description,
      image: post.image && `${baseUrl}${post.image}`,
      author: [
        {
          name: config.authorName,
          link: `${baseUrl}/`,
        },
      ],
      date: new Date(post.date),
    });
  });

  // cache on edge for 12 hours by default
  res.setHeader(
    "cache-control",
    `public, max-age=0, s-maxage=${options?.edgeCacheAge ?? 86400}, stale-while-revalidate`
  );

  // generates RSS by default
  if (type === "rss") {
    res.setHeader("content-type", "application/rss+xml; charset=utf-8");
    res.write(feed.rss2());
  } else if (type === "atom") {
    res.setHeader("content-type", "application/atom+xml; charset=utf-8");
    res.write(feed.atom1());
  } else if (type === "json") {
    // rare but including as an option because why not...
    // https://www.jsonfeed.org/
    res.setHeader("content-type", "application/feed+json; charset=utf-8");
    res.write(feed.json1());
  } else {
    throw new TypeError(`Invalid feed type "${type}", must be "rss", "atom", or "json".`);
  }

  res.end();

  return {
    props: {},
  };
};