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

/**
 * Parse an ICS calendar and return upcoming event occurrences.
 *
 * @param {string} icsText The contents of the .ics file
 * @param {Date} now Events must start after this date
 * @param {number} maxEvents Maximum number of events to return
 * @param {number} maxDays Maximum number of days into the future
 * @returns {{start: Date, name: string, url: string}[]}
 */
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 = [];

  for (const component of calendar.getAllSubcomponents("vevent")) {
    const event = new ICAL.Event(component);

    if (!event.startDate) {
      continue;
    }

    if (event.isRecurring()) {
      const iterator = event.iterator();

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

        if (!occurrence) {
          break;
        }

        const start = occurrence.toJSDate();

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

        if (start > now) {
          events.push({
            start,
            name: event.summary ?? "",
            url: event.url ?? "",
          });
        }
      }
    } else {
      const start = event.startDate.toJSDate();

      if (start > now && start <= end) {
        events.push({
          start,
          name: event.summary ?? "",
          url: event.url ?? "",
        });
      }
    }
  }

  // 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");

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

        colBegin.innerText = `${formattedStart.replace(" ", ", ")} Uhr`;
        row.appendChild(colBegin);

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

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

        colName.appendChild(a);
        row.appendChild(colName);

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