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

import { parse } from "date-fns";
import matter from "gray-matter";
import { MDXRemoteSerializeResult } from "next-mdx-remote";
import { serialize } from "next-mdx-remote/serialize";

import {
  Term,
  TERMS,
  isTerm,
  DATE_FORMAT,
  getLocalDateFromEST,
  TermYear,
  getTermYear,
  getCurrentTermYear,
} from "@/utils";

import type { Props } from "../pages/events/[year]/[term]";

const EVENTS_PATH = path.join("content", "events");

export async function getEventYears(): Promise<number[]> {
  return (await fs.readdir(EVENTS_PATH, { withFileTypes: true }))
    .filter((dirent) => dirent.isDirectory())
    .map((dirent) => parseInt(dirent.name))
    .sort();
}

export async function getEventTermsByYear(year: number): Promise<Term[]> {
  return (
    await fs.readdir(path.join(EVENTS_PATH, year.toString()), {
      withFileTypes: true,
    })
  )
    .filter((dirent) => dirent.isDirectory() && isTerm(dirent.name))
    .map((dirent) => dirent.name as Term)
    .sort((a, b) => TERMS.indexOf(a) - TERMS.indexOf(b));
}

interface RawMetadata {
  name: string;
  poster?: string;
  short: string;
  startDate: string;
  endDate?: string;
  online?: boolean;
  location: string;
  registerLink?: string;
}

interface Metadata {
  name: string;
  poster?: string;
  short: string;
  startDate: string;
  endDate?: string;
  online: boolean;
  location: string;
  permaLink: string;
  registerLink?: string;
  year: number;
  term: string;
  slug: string;
}

export interface Event {
  content: MDXRemoteSerializeResult<Record<string, unknown>>;
  metadata: Metadata;
}

export async function getEventBySlug(
  year: number,
  term: Term,
  slug: string
): Promise<Event> {
  const file = await fs.readFile(
    path.join(EVENTS_PATH, year.toString(), term, `${slug}.md`),
    "utf-8"
  );
  const { content, data } = matter(file);
  const raw = data as RawMetadata;

  return {
    content: await serialize(content),
    metadata: {
      ...raw,
      online: raw.online ?? false,
      startDate: getLocalDateFromEST(
        parse(raw.startDate, DATE_FORMAT, new Date())
      ).toString(),
      // permaLink is based on the directory structure in /pages
      permaLink: `/events/${year}/${term}/${slug}`,
      year: year,
      term: term,
      slug: slug,
    } as Metadata,
  };
}

export async function getEventsByTerm(
  year: number,
  term: Term
): Promise<string[]> {
  try {
    return (await fs.readdir(path.join(EVENTS_PATH, year.toString(), term)))
      .filter((name) => name.endsWith(".md"))
      .map((name) => name.slice(0, -".md".length));
  } catch {
    return [];
  }
}

export async function getUpcomingEvents(): Promise<Event[]> {
  const terms: TermYear[] = [];

  // Get events for the next two terms
  for (const termYear of getTermYear()) {
    if (terms.length >= 2) {
      break;
    }

    terms.push(termYear);
  }

  const events: Event[] = (
    await Promise.all(
      terms.map(async ({ year, term }) => {
        try {
          const eventsInTerm = await getEventsByTerm(year, term);
          return await Promise.all(
            eventsInTerm.map((slug) => getEventBySlug(year, term, slug))
          );
        } catch (error) {
          return [];
        }
      })
    )
  ).flat();

  return events
    .filter(
      (ev) =>
        // use endDate if possible, else use startDate
        new Date(ev.metadata.endDate ?? ev.metadata.startDate).getTime() >=
        Date.now()
    )
    .sort((a, b) => {
      return (
        new Date(a.metadata.startDate).getTime() -
        new Date(b.metadata.startDate).getTime()
      );
    });
}

export async function getAllEvents(): Promise<Event[]> {
  const events: Event[] = [];

  for (const year of await getEventYears()) {
    for (const term of await getEventTermsByYear(year)) {
      for (const slug of await getEventsByTerm(year, term)) {
        events.push(await getEventBySlug(year, term, slug));
      }
    }
  }

  return events;
}

export async function getEventsPageProps({
  term,
  year,
}: TermYear): Promise<Props> {
  const eventNames = await getEventsByTerm(year, term);

  const events: Event[] = (
    await Promise.all(
      eventNames.map((file: string) => getEventBySlug(year, term, file))
    )
  ).sort(
    (a, b) =>
      new Date(a.metadata.startDate).getTime() -
      new Date(b.metadata.startDate).getTime()
  );

  const currentDate = Date.now();

  const pastEvents = events
    .filter(
      (event) =>
        // fallback to startDate if endDate is not set
        new Date(event.metadata.endDate ?? event.metadata.startDate).getTime() <
        currentDate
    )
    .reverse();

  const futureEvents = events.filter(
    // We display events that are currently going on as upcoming so they still show up homepage and other pages on the top
    (event) =>
      new Date(event.metadata.endDate ?? event.metadata.startDate).getTime() >=
      currentDate
  );

  const eventYears = await getEventYears();

  const minYear = eventYears[0];
  const pastTerms: TermYear[] = [];

  for (const current of getTermYear(
    { year, term },
    { goBackwards: true, skipCurrent: true }
  )) {
    if (pastTerms.length >= 2 || current.year < minYear) {
      break;
    }

    if ((await getEventsByTerm(current.year, current.term)).length !== 0) {
      pastTerms.push(current);
    }
  }
  pastTerms.reverse();

  const maxYear = eventYears[eventYears.length - 1];
  const futureTerms: TermYear[] = [];

  for (const current of getTermYear(
    { year, term },
    { goBackwards: false, skipCurrent: true }
  )) {
    if (futureTerms.length >= 2 || maxYear < current.year) {
      break;
    }

    if ((await getEventsByTerm(current.year, current.term)).length !== 0) {
      futureTerms.push(current);
    }
  }

  const current = getCurrentTermYear();

  return {
    year: year,
    term: term,
    pastEvents: pastEvents,
    futureEvents: futureEvents,
    isCurrentTerm: term === current.term && year === current.year,
    pastTerms: pastTerms,
    futureTerms: futureTerms,
  };
}