import ICAL from "https://unpkg.com/ical.js/dist/ical.min.js";

/**
 * Read the URL of an event.
 *
 * ICAL.Event does not expose the URL property, so read it from the component.
 *
 * The value ends up in the href of a link, and the calendar is exported from a
 * CalDAV server, so whoever may write to it decides what that value is. A
 * "javascript:" URL there would run on our page as soon as a visitor clicks
 * the event, so hand on nothing but http and https.
 *
 * @param {ICAL.Event} event The event to read the URL of
 * @returns {string} The URL, empty when the event has none or it is not http(s)
 */
function eventUrl(event) {
  const url = event.component.getFirstPropertyValue("url") ?? "";

  if (!url) {
    return "";
  }

  try {
    // A relative URL is resolved against the page and keeps its scheme.
    const { protocol } = new URL(url, document.baseURI);

    return protocol === "http:" || protocol === "https:" ? url : "";
  } catch {
    // Not a URL at all.
    return "";
  }
}

/**
 * Group the occurrences that were modified on their own by the UID of the event
 * they belong to.
 *
 * Unless it is told which exceptions belong to an event, ICAL.Event relates
 * every VEVENT with a RECURRENCE-ID in the file to every recurring event, and
 * it keys them by the recurrence id alone. Two series that meet at the same
 * time would therefore take over each other's modifications.
 *
 * @param {ICAL.Component[]} components The VEVENTs of the calendar
 * @returns {Map<string, ICAL.Component[]>} The exceptions per UID
 */
function exceptionsByUid(components) {
  const exceptions = new Map();

  for (const component of components) {
    if (!component.hasProperty("recurrence-id")) {
      continue;
    }

    const uid = component.getFirstPropertyValue("uid");
    const ofEvent = exceptions.get(uid);

    if (ofEvent) {
      ofEvent.push(component);
    } else {
      exceptions.set(uid, [component]);
    }
  }

  return exceptions;
}

/**
 * When an occurrence starts, as a point in time.
 *
 * A date has no time and no zone, its digits are the day itself. toJSDate()
 * reads them as midnight in the zone of the browser, which moves an all day
 * event by the offset that zone has to Berlin and, far enough east or west,
 * onto the day before or after. Keep the digits and read them as UTC instead,
 * the table prints an all day event in UTC as well.
 *
 * @param {ICAL.Time} time Start of the occurrence
 * @returns {Date} The point in time to sort and print by
 */
function startOf(time) {
  if (time.isDate) {
    return new Date(Date.UTC(time.year, time.month - 1, time.day));
  }

  return time.toJSDate();
}

/**
 * How far the recurrences of an event have to be iterated.
 *
 * The iteration walks the unmodified recurrence times, so an occurrence that
 * was moved to an earlier time is only reached through the time it originally
 * had, which can lie past the end of the window. Keep going for as long as the
 * largest move towards the past can still carry an occurrence into it.
 *
 * @param {ICAL.Event} event The event whose recurrences are iterated
 * @param {Date} end End of the window
 * @returns {Date} The recurrence time to stop at
 */
function iterationEnd(event, end) {
  let last = end.getTime();

  for (const exception of Object.values(event.exceptions)) {
    const movedBy = exception.recurrenceId.toJSDate().getTime()
      - exception.startDate.toJSDate().getTime();

    if (movedBy > 0) {
      last = Math.max(last, end.getTime() + movedBy);
    }
  }

  return new Date(last);
}

/**
 * Parse an ICS calendar and return upcoming event occurrences.
 *
 * @param {string} icsText The contents of the .ics file
 * @param {Date} now Events must still be running at this date
 * @param {number} maxEvents Maximum number of events to return
 * @param {number} maxDays Maximum number of days into the future
 * @returns {{start: Date, allDay: boolean, name: string, url: string}[]} url is empty when the event has no URL
 */
function getUpcomingEvents(icsText, now, maxEvents, maxDays) {
  const jcal = ICAL.parse(icsText);
  const calendar = new ICAL.Component(jcal);

  const end = new Date(now.getTime());
  end.setDate(end.getDate() + maxDays);

  const events = [];

  const components = calendar.getAllSubcomponents("vevent");
  const exceptions = exceptionsByUid(components);

  for (const component of components) {
    // Occurrences modified via RECURRENCE-ID are reached through the event they
    // belong to, listing them here as well would show them twice.
    if (component.hasProperty("recurrence-id")) {
      continue;
    }

    const event = new ICAL.Event(component, {
      exceptions: exceptions.get(component.getFirstPropertyValue("uid")) ?? [],
    });

    if (!event.startDate) {
      continue;
    }

    if (event.isRecurring()) {
      const iterator = event.iterator();
      const iterateUntil = iterationEnd(event, end);

      while (true) {
        const occurrence = iterator.next();

        if (!occurrence) {
          break;
        }

        // Recurrences are chronological, so we're done
        // once we pass the end of our search window.
        if (occurrence.toJSDate() > iterateUntil) {
          break;
        }

        // Details resolve time, name and URL of an occurrence that was
        // modified via RECURRENCE-ID.
        const details = event.getOccurrenceDetails(occurrence);

        // A running event stays listed until it is over, so filter on its end.
        // A modification may have moved the occurrence out of the window, so
        // judge it by the time it really takes place at.
        if (details.startDate.toJSDate() <= end && details.endDate.toJSDate() > now) {
          events.push({
            start: startOf(details.startDate),
            allDay: details.startDate.isDate,
            name: details.item.summary ?? "",
            url: eventUrl(details.item),
          });
        }
      }
    } else if (event.startDate.toJSDate() <= end && event.endDate.toJSDate() > now) {
      events.push({
        start: startOf(event.startDate),
        allDay: event.startDate.isDate,
        name: event.summary ?? "",
        url: eventUrl(event),
      });
    }
  }

  // We have occurrences from multiple events, so sort them
  // before applying the maximum event count.
  events.sort((a, b) => a.start - b.start);

  return events.slice(0, maxEvents);
}

document.addEventListener("DOMContentLoaded", () => {
  const ics = "/calendars/all.ics";
  const max_days = 20;
  const max_items = 5;

  const now = new Date();

  const table = document.getElementById("upcoming");
  fetch(ics)
    .then(response => response.text())
    .then(icsText => {
      getUpcomingEvents(icsText, now, max_items, max_days).forEach(event => {
        const row = document.createElement("tr");

        const colBegin = document.createElement("td");

        // The events take place in Berlin, so name their time in Berlin time
        // instead of in the time zone the visitor happens to be in. An all day
        // event has no time of day and carries its date in UTC, see startOf().
        const whenFormat = event.allDay
          ? { timeZone: "UTC" }
          : { timeZone: "Europe/Berlin", hour: "2-digit", minute: "2-digit" };

        const formattedStart = event.start.toLocaleString("de-DE", {
          weekday: "long",
          day: "2-digit",
          month: "2-digit",
          ...whenFormat,
        });

        colBegin.innerText = event.allDay ? formattedStart : `${formattedStart} Uhr`;
        row.appendChild(colBegin);

        const colName = document.createElement("td");

        if (event.url) {
          const a = document.createElement("a");
          a.href = event.url;
          a.text = event.name;

          colName.appendChild(a);
        } else {
          colName.innerText = event.name;
        }

        row.appendChild(colName);

        table.appendChild(row);
      });
    });
});