import ICAL from "./vendor/ical.js";

// The calendar is published next to the site and is not built from this
// repository, see the README.
const icsUrl = "/calendars/all.ics";

/**
 * 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)
 */
export 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;
}

/**
 * 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} to End of the window
 * @returns {Date} The recurrence time to stop at
 */
function iterationEnd(event, to) {
  let last = to.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, to.getTime() + movedBy);
    }
  }

  return new Date(last);
}

/**
 * Whether an occurrence touches a window.
 *
 * @param {ICAL.Time} startDate Start of the occurrence
 * @param {ICAL.Time} endDate End of the occurrence
 * @param {Date} from Start of the window
 * @param {Date} to End of the window
 * @returns {boolean} True when the two overlap
 */
function touches(startDate, endDate, from, to) {
  return startDate.toJSDate() <= to && endDate.toJSDate() > from;
}

/**
 * Load and parse the calendar of the club.
 *
 * @returns {Promise<ICAL.Component>} The calendar
 */
export async function loadCalendar() {
  const response = await fetch(icsUrl);

  // Without this an error page would be handed to the parser below, which then
  // fails with a confusing complaint about the calendar syntax.
  if (!response.ok) {
    throw new Error(`${icsUrl}: ${response.status} ${response.statusText}`);
  }

  return new ICAL.Component(ICAL.parse(await response.text()));
}

/**
 * Every occurrence of the calendar that touches the given window.
 *
 * A recurring event is expanded, and an occurrence that was modified on its own
 * is reported with the time, the name and the URL of that modification.
 *
 * @param {ICAL.Component} calendar The parsed calendar
 * @param {Date} from An occurrence has to still be running at this time
 * @param {Date} to An occurrence has to have started by this time
 * @yields {{event: ICAL.Event, startDate: ICAL.Time, endDate: ICAL.Time}}
 */
export function* occurrencesBetween(calendar, from, to) {
  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, handling them here as well would report 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()) {
      if (touches(event.startDate, event.endDate, from, to)) {
        yield { event, startDate: event.startDate, endDate: event.endDate };
      }

      continue;
    }

    const iterator = event.iterator();
    const iterateUntil = iterationEnd(event, to);

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

      // Recurrences are chronological, so we are done once one starts after the
      // window, and after the occurrences a modification can still move back
      // into it.
      if (!occurrence || 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 modification may have moved the occurrence out of the window, so
      // judge it by the time it really takes place at.
      if (touches(details.startDate, details.endDate, from, to)) {
        yield {
          event: details.item,
          startDate: details.startDate,
          endDate: details.endDate,
        };
      }
    }
  }
}