/**
 * Generates an ICS file containing free/busy information from your Google Calendar
 * and saves it to Google Drive.
 */
function generateICS() {
  // Configuration
  var calendarId = 'primary'; // Calendar ID to read events from
  var fileName = 'busy.ics'; // Name of the ICS file to be saved in Google Drive
  var eventTitle = 'Busy'; // Title for the events in the ICS file
  var organizationName = 'Acme Corporation'; // Name for the PRODID property
  var pastMonths = 2; // Number of months to include past events
  var futureMonths = 6; // Number of months to include future events

  // Specify which event statuses to include
  var includedStatuses = [
    CalendarApp.GuestStatus.OWNER, // Include events you created
    CalendarApp.GuestStatus.YES,   // Include events you accepted
    // Add more statuses if needed (e.g., MAYBE, INVITED)
  ];

  // Get the date range (from current date minus pastMonths to current date plus futureMonths)
  var now = new Date();
  var startDate = addMonths(now, -pastMonths);
  var endDate = addMonths(now, futureMonths);

  // Fetch the events we want to include
  var events = getEvents(calendarId, startDate, endDate, includedStatuses);

  // Generate ICS content
  var icsContent = generateICSContent(events, eventTitle, organizationName);

  // Save ICS file to Google Drive
  saveICSToDrive(icsContent, fileName);
}

/**
 * Retrieves events from the specified calendar within the given date range
 * and filters them based on the specified statuses and conditions.
 *
 * @param {string} calendarId - The ID of the calendar.
 * @param {Date} startDate - The start date of the range.
 * @param {Date} endDate - The end date of the range.
 * @param {Array} includedStatuses - Array of CalendarApp.GuestStatus constants to include.
 * @return {Array} Array of event objects containing start and end times.
 */
function getEvents(calendarId, startDate, endDate, includedStatuses) {
  var calendar = CalendarApp.getCalendarById(calendarId);
  var events = calendar.getEvents(startDate, endDate);
  var includedEvents = [];

  events.forEach(function(event) {
    // Check the event's guest status
    var myStatus = event.getMyStatus();

    // Include the event only if its status is in the includedStatuses array
    if (includedStatuses.indexOf(myStatus) === -1) {
      return; // Skip this event
    }

    // Skip all-day events
    if (event.isAllDayEvent()) {
      return;
    }

    // Include event
    includedEvents.push({
      start: event.getStartTime(),
      end: event.getEndTime()
    });
  });

  return includedEvents;
}

/**
 * Generates ICS file content from the events.
 *
 * @param {Array} events - Array of event objects containing start and end times.
 * @param {string} eventTitle - The title to use for the events in the ICS file.
 * @param {string} organizationName - The organization name for the PRODID property.
 * @return {string} The ICS file content as a string.
 */
function generateICSContent(events, eventTitle, organizationName) {
  var uidDomain = CalendarApp.getDefaultCalendar().getId();

  var icsLines = [
    'BEGIN:VCALENDAR',
    'VERSION:2.0',
    'PRODID:-//' + organizationName + '//FreeBusy Calendar//EN',
    'CALSCALE:GREGORIAN',
    'METHOD:PUBLISH'
  ];

  events.forEach(function(ev, index) {
    var eventUID = 'event-' + index + '@' + uidDomain;
    var dtStamp = formatDate(new Date());
    var dtStart = formatDate(ev.start);
    var dtEnd = formatDate(ev.end);

    icsLines.push('BEGIN:VEVENT');
    icsLines.push('UID:' + eventUID);
    icsLines.push('DTSTAMP:' + dtStamp);
    icsLines.push('DTSTART:' + dtStart);
    icsLines.push('DTEND:' + dtEnd);
    icsLines.push('SUMMARY:' + eventTitle);
    icsLines.push('CLASS:PRIVATE');
    icsLines.push('END:VEVENT');
  });

  icsLines.push('END:VCALENDAR');

  return icsLines.join('\r\n');
}

/**
 * Saves the ICS content to a file in Google Drive.
 *
 * @param {string} icsContent - The ICS file content.
 * @param {string} fileName - The name of the file to save.
 */
function saveICSToDrive(icsContent, fileName) {
  // Search for existing file
  var files = DriveApp.getFilesByName(fileName);
  var file;

  if (files.hasNext()) {
    file = files.next();
    file.setContent(icsContent);
  } else {
    file = DriveApp.createFile(fileName, icsContent, MimeType.PLAIN_TEXT);
  }

  // Set sharing permissions to "Anyone with the link can view"
  file.setSharing(DriveApp.Access.ANYONE_WITH_LINK, DriveApp.Permission.VIEW);

  Logger.log('ICS file has been saved and is accessible at: ' + file.getDownloadUrl());
}

/**
 * Formats a Date object into a string suitable for ICS files (UTC time).
 *
 * @param {Date} date - The date to format.
 * @return {string} The formatted date string in 'yyyyMMddTHHmmssZ' format.
 */
function formatDate(date) {
  return Utilities.formatDate(date, 'UTC', 'yyyyMMdd\'T\'HHmmss\'Z\'');
}

/**
 * Adds a specified number of months to a date.
 *
 * @param {Date} date - The original date.
 * @param {number} months - Number of months to add (can be negative).
 * @return {Date} The new date.
 */
function addMonths(date, months) {
  var d = new Date(date);
  var day = d.getDate();
  d.setMonth(d.getMonth() + months);

  // Handle end-of-month issues
  if (d.getDate() < day) {
    d.setDate(0);
  }

  return d;
}