import { parseApiEvents } from "@/lib/events";
import { matchesRange, sortEvents } from "@/lib/filter";
import type { ApiEvent, CalendarEvent } from "@/lib/types";

/**
 * The public calendar as the landing page shows it: subscription links and
 * the next few events. Pure, apart from `fetchUpcomingPublicEvents`.
 *
 * The public feed needs no credential at all - nachklang.art reads the same
 * endpoint anonymously - so this runs on the server without a session.
 */

const PUBLIC_PATH = "/calendar/events/public";

/** https://api.nachklang.art/calendar/events/public/ical */
export const icalUrl = (apiBaseUrl: string): string => `${apiBaseUrl}${PUBLIC_PATH}/ical`;

/**
 * Any iCal URL as a `webcal://` link, which Apple Calendar (and most desktop
 * clients) open as "subscribe" rather than "download". The scheme replaces
 * http(s) whichever the API uses - calendar apps fetch it over https for an
 * https host.
 */
export const toWebcal = (ical: string): string => ical.replace(/^https?:\/\//, "webcal://");

/** Google Calendar's "add by URL" deep link for any iCal URL. It wants the webcal form, URL-encoded. */
export const toGoogleCalendar = (ical: string): string =>
  `https://calendar.google.com/calendar/r?cid=${encodeURIComponent(toWebcal(ical))}`;

export const webcalUrl = (apiBaseUrl: string): string => toWebcal(icalUrl(apiBaseUrl));

export const googleCalendarUrl = (apiBaseUrl: string): string => toGoogleCalendar(icalUrl(apiBaseUrl));

/** Upcoming (not yet over, Berlin days) public events, earliest first. */
export const selectUpcoming = (rows: ApiEvent[], now: Date, limit: number): CalendarEvent[] =>
  sortEvents(
    parseApiEvents(rows).filter((e) => matchesRange(e, "future", now)),
    "start_asc",
    now,
  ).slice(0, limit);

/**
 * Server-side fetch, cached for five minutes (Next's `revalidate`), so the
 * landing page is static HTML that refreshes itself rather than a request to
 * the API per visitor.
 *
 * Null on any failure. The subscription links are the page's real job and
 * must not depend on the API answering - that includes `next build` on a
 * machine that cannot reach it.
 */
export const fetchUpcomingPublicEvents = async (
  apiBaseUrl: string,
  now: Date,
  limit: number,
): Promise<CalendarEvent[] | null> => {
  try {
    const res = await fetch(`${apiBaseUrl}${PUBLIC_PATH}/json`, { next: { revalidate: 300 } });
    if (!res.ok) {
      return null;
    }
    return selectUpcoming((await res.json()) as ApiEvent[], now, limit);
  } catch {
    return null;
  }
};