import fs from "fs/promises";
import path from "path";

import matter from "gray-matter";
import { serialize } from "next-mdx-remote/serialize";

const BASE_PATH = "content";
const ORDER_FILE = "ORDER.json";

export async function getSectionNamesForPage(page: string) {
  const sectionDir = path.join(BASE_PATH, page);
  return (await fs.readdir(sectionDir))
    .filter((name) => name.endsWith(".md"))
    .map((name) => name.slice(0, -".md".length));
}

export async function getSectionsForPage(page: string) {
  const names = await getSectionNamesForPage(page);
  const order = await getSectionOrder(page);

  if (!isEquivalent(new Set(names), new Set(order))) {
    throw new Error(
      `${ORDER_FILE} must contain all sections defined for page ${page}`
    );
  }

  const sections = names
    .sort((a, b) => order.indexOf(a) - order.indexOf(b))
    .map(async (name) => ({
      name,
      data: await getSectionForPage(page, name),
    }));

  return await Promise.all(sections);
}

async function getSectionForPage(page: string, section: string) {
  const raw = await fs.readFile(
    path.join(BASE_PATH, page, `${section}.md`),
    "utf-8"
  );
  const { content, data } = matter(raw);

  return {
    content: await serialize(content),
    title: data.title as string,
  };
}

async function getSectionOrder(page: string) {
  const raw = await fs.readFile(
    path.join(BASE_PATH, page, ORDER_FILE),
    "utf-8"
  );

  return JSON.parse(raw) as string[];
}

function isEquivalent<T>(setA: Set<T>, setB: Set<T>): boolean {
  const aContainsB = [...setB].every((b) => setA.has(b));
  const bContainsA = [...setA].every((a) => setB.has(a));

  return aContainsB && bContainsA;
}