const config = {
  'source.email@gmail.com': {
    prefix: 'Source',
    color: CalendarApp.EventColor.PALE_RED,
    visibility: 'private'
  },
  'this.calendar@company.com': {
    target: 'Other Calendar Name',
    attendees: false,
    prefix: 'Company',
    filterColor: CalendarApp.EventColor.YELLOW,
  },
};

function getCalendars() {
  let pageToken;
  let calendars = [];

  do {
    const list = Calendar.CalendarList.list({pageToken, showHidden: true});
    pageToken = list.pageToken;
    calendars = calendars.concat(list.items);
  } while (pageToken);

  calendars.sort(compareUsing(sortableId));
  return calendars;
}

function compareUsing(fn) {
  return ((a, b) => fn(a).localeCompare(fn(b)));
}

function sortableId(calendar) {
  return calendar.id.split('@').reverse().join('@');
}

function colorName(id) {
  return Object.keys(CalendarApp.EventColor).find(c => CalendarApp.EventColor[c] == id);
}

function onCalendarChanged(trigger) {
  copyEvents(trigger.calendarId);
}

function copyAll() {
  Object.keys(config).forEach(copyEvents);
}

// avoid multiple scripts running at the same time
function getLock(retries = 3) {
  try {
    const lock = LockService.getScriptLock();
    lock.tryLock(1000);
    return lock;
  } catch (err) {
    if (retries) {
      return getLock(retries - 1);
    }

    throw new Error('Could not obtain script lock');
  }
}

function copyEvents(sourceId) {
  const lock = getLock();
  if (!lock.hasLock()) {
    console.log('Process already running');
    return;
  }

  if (!config[sourceId]) {
    console.log('No config for ' + sourceId);
    return;
  }

  console.log('Copying events from ' + sourceId);
  const summaryPrefix = (config[sourceId].prefix ? `[${config[sourceId].prefix}] ` : '');
  const syncDays = config[sourceId].syncDays || 14;

  // set target calendar
  let targetId = Session.getActiveUser().getEmail();
  if (config[sourceId].target) {
    targetId = getCalendars().find(c => c.summary == config[sourceId].target).id;
  }

  // start and end dates
  let startDate = new Date();
  let endDate   = new Date();
  endDate.setDate(startDate.getDate() + syncDays);

  // delete and recreate in case things move
  cleanup(targetId, syncDays, summaryPrefix);

  // get all events
  const listResponse = Calendar.Events.list(sourceId, {
    timeMin: startDate.toISOString(),
    timeMax: endDate.toISOString(),
    singleEvents: true,
  });

  const targetEventIds = Calendar.Events.list(targetId, {
    timeMin: startDate.toISOString(),
    timeMax: endDate.toISOString(),
    singleEvents: true
  }).items.map(e => e.id);

  // loop over and copy
  listResponse.items.forEach(function (event) {
    const summary = (event.summary || 'busy');
    console.log('Copying event: ' + summary + ' (' + (event.start.dateTime || event.start.date) + ')');

    if (targetEventIds.includes(event.id)) {
      console.log('Event is already shared with the target calendar, ignoring');
      return;
    }

    if (config[sourceId].filterColor && config[sourceId].filterColor != event.colorId) {
      console.log('Ignoring event with colour: ' + (colorName(event.colorId) || 'default'));
      return;
    }

    try {
      Calendar.Events.insert({
        start:          event.start,
        end:            event.end,
        summary:        summaryPrefix + summary,
        description:    event.description,
        conferenceData: event.conferenceData,
        colorId:        config[sourceId].color || 0,
        visibility:     config[sourceId].visibility || event.visibility || 'default',
        attendees:      event.attendees ? [{
          email:          targetId,
          responseStatus: event.attendees.find(a => a.self).responseStatus
        }] : []
      }, targetId, {
        conferenceDataVersion: 1
      });
    } catch (err) {
      console.log(err.toString());
    }

    Utilities.sleep(500);
  });

  lock.releaseLock();
  console.log('All events copied');
}

function cleanup(targetId, daysAhead, titlePrefix) {
  let calendar = CalendarApp.getCalendarById(targetId);
  
  let startDate = new Date();
  let endDate   = new Date();
  endDate.setDate(startDate.getDate() + daysAhead);

  calendar.getEvents(startDate, endDate).forEach(function (event) {
    if (event.getTitle().startsWith(titlePrefix)) {
      console.log('Deleting event: ' + event.getTitle() + ' (' + event.getStartTime().toLocaleString('en-GB') + ')');
      try {
        event.deleteEvent();
      } catch (e) {
        console.log('Failed to delete: ' + e);
      }
      
      Utilities.sleep(500);
    }
  });
}