<?php

namespace Sabre\VObject\ITip;

use Sabre\VObject\Component\VCalendar;
use Sabre\VObject\DateTimeParser;
use Sabre\VObject\Reader;
use Sabre\VObject\Recur\EventIterator;

/**
 * The ITip\Broker class is a utility class that helps with processing
 * so-called iTip messages.
 *
 * iTip is defined in rfc5546, stands for iCalendar Transport-Independent
 * Interoperability Protocol, and describes the underlying mechanism for
 * using iCalendar for scheduling for for example through email (also known as
 * IMip) and CalDAV Scheduling.
 *
 * This class helps by:
 *
 * 1. Creating individual invites based on an iCalendar event for each
 *    attendee.
 * 2. Generating invite updates based on an iCalendar update. This may result
 *    in new invites, updates and cancellations for attendees, if that list
 *    changed.
 * 3. On the receiving end, it can create a local iCalendar event based on
 *    a received invite.
 * 4. It can also process an invite update on a local event, ensuring that any
 *    overridden properties from attendees are retained.
 * 5. It can create a accepted or declined iTip reply based on an invite.
 * 6. It can process a reply from an invite and update an events attendee
 *     status based on a reply.
 *
 * @copyright Copyright (C) fruux GmbH (https://fruux.com/)
 * @author Evert Pot (http://evertpot.com/)
 * @license http://sabre.io/license/ Modified BSD License
 */
class Broker
{
    /**
     * This setting determines whether the rules for the SCHEDULE-AGENT
     * parameter should be followed.
     *
     * This is a parameter defined on ATTENDEE properties, introduced by RFC
     * 6638. This parameter allows a caldav client to tell the server 'Don't do
     * any scheduling operations'.
     *
     * If this setting is turned on, any attendees with SCHEDULE-AGENT set to
     * CLIENT will be ignored. This is the desired behavior for a CalDAV
     * server, but if you're writing an iTip application that doesn't deal with
     * CalDAV, you may want to ignore this parameter.
     *
     * @var bool
     */
    public $scheduleAgentServerRules = true;

    /**
     * The broker will try during 'parseEvent' figure out whether the change
     * was significant.
     *
     * It uses a few different ways to do this. One of these ways is seeing if
     * certain properties changed values. This list of specified here.
     *
     * This list is taken from:
     * * http://tools.ietf.org/html/rfc5546#section-2.1.4
     *
     * @var string[]
     */
    public $significantChangeProperties = [
        'DTSTART',
        'DTEND',
        'DURATION',
        'DUE',
        'RRULE',
        'RDATE',
        'EXDATE',
        'STATUS',
    ];

    /**
     * This method is used to process an incoming itip message.
     *
     * Examples:
     *
     * 1. A user is an attendee to an event. The organizer sends an updated
     * meeting using a new iTip message with METHOD:REQUEST. This function
     * will process the message and update the attendee's event accordingly.
     *
     * 2. The organizer cancelled the event using METHOD:CANCEL. We will update
     * the users event to state STATUS:CANCELLED.
     *
     * 3. An attendee sent a reply to an invite using METHOD:REPLY. We can
     * update the organizers event to update the ATTENDEE with its correct
     * PARTSTAT.
     *
     * The $existingObject is updated in-place. If there is no existing object
     * (because it's a new invite for example) a new object will be created.
     *
     * If an existing object does not exist, and the method was CANCEL or
     * REPLY, the message effectively gets ignored, and no 'existingObject'
     * will be created.
     *
     * The updated $existingObject is also returned from this function.
     *
     * If the iTip message was not supported, we will always return false.
     *
     * @param VCalendar $existingObject
     *
     * @return VCalendar|null
     */
    public function processMessage(Message $itipMessage, ?VCalendar $existingObject = null)
    {
        // We only support events at the moment.
        if ('VEVENT' !== $itipMessage->component) {
            return false;
        }

        switch ($itipMessage->method) {
            case 'REQUEST':
                return $this->processMessageRequest($itipMessage, $existingObject);

            case 'CANCEL':
                return $this->processMessageCancel($itipMessage, $existingObject);

            case 'REPLY':
                return $this->processMessageReply($itipMessage, $existingObject);

            default:
                // Unsupported iTip message
                return;
        }

        return $existingObject;
    }

    /**
     * This function parses a VCALENDAR object and figure out if any messages
     * need to be sent.
     *
     * A VCALENDAR object will be created from the perspective of either an
     * attendee, or an organizer. You must pass a string identifying the
     * current user, so we can figure out who in the list of attendees or the
     * organizer we are sending this message on behalf of.
     *
     * It's possible to specify the current user as an array, in case the user
     * has more than one identifying href (such as multiple emails).
     *
     * It $oldCalendar is specified, it is assumed that the operation is
     * updating an existing event, which means that we need to look at the
     * differences between events, and potentially send old attendees
     * cancellations, and current attendees updates.
     *
     * If $calendar is null, but $oldCalendar is specified, we treat the
     * operation as if the user has deleted an event. If the user was an
     * organizer, this means that we need to send cancellation notices to
     * people. If the user was an attendee, we need to make sure that the
     * organizer gets the 'declined' message.
     *
     * @param VCalendar|string      $calendar
     * @param string|array          $userHref
     * @param VCalendar|string|null $oldCalendar
     *
     * @return array
     */
    public function parseEvent($calendar, $userHref, $oldCalendar = null)
    {
        if ($oldCalendar) {
            if (is_string($oldCalendar)) {
                $oldCalendar = Reader::read($oldCalendar);
            }
            if (!isset($oldCalendar->VEVENT)) {
                // We only support events at the moment
                return [];
            }

            $oldEventInfo = $this->parseEventInfo($oldCalendar);
        } else {
            $oldEventInfo = [
                'organizer' => null,
                'significantChangeHash' => '',
                'attendees' => [],
            ];
        }

        $userHref = (array) $userHref;

        if (!is_null($calendar)) {
            if (is_string($calendar)) {
                $calendar = Reader::read($calendar);
            }
            if (!isset($calendar->VEVENT)) {
                // We only support events at the moment
                return [];
            }
            $eventInfo = $this->parseEventInfo($calendar);
            if (!$eventInfo['attendees'] && !$oldEventInfo['attendees']) {
                // If there were no attendees on either side of the equation,
                // we don't need to do anything.
                return [];
            }
            if (!$eventInfo['organizer'] && !$oldEventInfo['organizer']) {
                // There was no organizer before or after the change.
                return [];
            }

            $baseCalendar = $calendar;

            // If the new object didn't have an organizer, the organizer
            // changed the object from a scheduling object to a non-scheduling
            // object. We just copy the info from the old object.
            if (!$eventInfo['organizer'] && $oldEventInfo['organizer']) {
                $eventInfo['organizer'] = $oldEventInfo['organizer'];
                $eventInfo['organizerName'] = $oldEventInfo['organizerName'];
            }
        } else {
            // The calendar object got deleted, we need to process this as a
            // cancellation / decline.
            if (!$oldCalendar) {
                // No old and no new calendar, there's no thing to do.
                return [];
            }

            $eventInfo = $oldEventInfo;

            if (in_array($eventInfo['organizer'], $userHref)) {
                // This is an organizer deleting the event.
                $eventInfo['attendees'] = [];
                // Increasing the sequence, but only if the organizer deleted
                // the event.
                ++$eventInfo['sequence'];
            } else {
                // This is an attendee deleting the event.
                foreach ($eventInfo['attendees'] as $key => $attendee) {
                    if (in_array($attendee['href'], $userHref)) {
                        $eventInfo['attendees'][$key]['instances'] = ['master' => ['id' => 'master', 'partstat' => 'DECLINED'],
                        ];
                    }
                }
            }
            $baseCalendar = $oldCalendar;
        }

        if (in_array($eventInfo['organizer'], $userHref)) {
            return $this->parseEventForOrganizer($baseCalendar, $eventInfo, $oldEventInfo);
        } elseif ($oldCalendar) {
            // We need to figure out if the user is an attendee, but we're only
            // doing so if there's an oldCalendar, because we only want to
            // process updates, not creation of new events.
            foreach ($eventInfo['attendees'] as $attendee) {
                if (in_array($attendee['href'], $userHref)) {
                    return $this->parseEventForAttendee($baseCalendar, $eventInfo, $oldEventInfo, $attendee['href']);
                }
            }
        }

        return [];
    }

    /**
     * Processes incoming REQUEST messages.
     *
     * This is message from an organizer, and is either a new event
     * invite, or an update to an existing one.
     *
     * @param VCalendar $existingObject
     *
     * @return VCalendar|null
     */
    protected function processMessageRequest(Message $itipMessage, ?VCalendar $existingObject = null)
    {
        if (!$existingObject) {
            // This is a new invite, and we're just going to copy over
            // all the components from the invite.
            $existingObject = new VCalendar();
            foreach ($itipMessage->message->getComponents() as $component) {
                $existingObject->add(clone $component);
            }
        } else {
            // We need to update an existing object with all the new
            // information. We can just remove all existing components
            // and create new ones.
            foreach ($existingObject->getComponents() as $component) {
                $existingObject->remove($component);
            }
            foreach ($itipMessage->message->getComponents() as $component) {
                $existingObject->add(clone $component);
            }
        }

        return $existingObject;
    }

    /**
     * Processes incoming CANCEL messages.
     *
     * This is a message from an organizer, and means that either an
     * attendee got removed from an event, or an event got cancelled
     * altogether.
     *
     * @param VCalendar $existingObject
     *
     * @return VCalendar|null
     */
    protected function processMessageCancel(Message $itipMessage, ?VCalendar $existingObject = null)
    {
        if (!$existingObject) {
            // The event didn't exist in the first place, so we're just
            // ignoring this message.
        } else {
            foreach ($existingObject->VEVENT as $vevent) {
                $vevent->STATUS = 'CANCELLED';
                $vevent->SEQUENCE = $itipMessage->sequence;
            }
        }

        return $existingObject;
    }

    /**
     * Processes incoming REPLY messages.
     *
     * The message is a reply. This is for example an attendee telling
     * an organizer he accepted the invite, or declined it.
     *
     * @param VCalendar $existingObject
     *
     * @return VCalendar|null
     */
    protected function processMessageReply(Message $itipMessage, ?VCalendar $existingObject = null)
    {
        // A reply can only be processed based on an existing object.
        // If the object is not available, the reply is ignored.
        if (!$existingObject) {
            return;
        }
        $instances = [];
        $requestStatus = '2.0';

        // Finding all the instances the attendee replied to.
        foreach ($itipMessage->message->VEVENT as $vevent) {
            // Use the Unix timestamp returned by getTimestamp as a unique identifier for the recurrence.
            // The Unix timestamp will be the same for an event, even if the reply from the attendee
            // used a different format/timezone to express the event date-time.
            $recurId = isset($vevent->{'RECURRENCE-ID'}) ? $vevent->{'RECURRENCE-ID'}->getDateTime()->getTimestamp() : 'master';
            $attendee = $vevent->ATTENDEE;
            $instances[$recurId] = $attendee['PARTSTAT']->getValue();
            if (isset($vevent->{'REQUEST-STATUS'})) {
                $requestStatus = $vevent->{'REQUEST-STATUS'}->getValue();
                list($requestStatus) = explode(';', $requestStatus);
            }
        }

        // Now we need to loop through the original organizer event, to find
        // all the instances where we have a reply for.
        $masterObject = null;
        foreach ($existingObject->VEVENT as $vevent) {
            // Use the Unix timestamp returned by getTimestamp as a unique identifier for the recurrence.
            $recurId = isset($vevent->{'RECURRENCE-ID'}) ? $vevent->{'RECURRENCE-ID'}->getDateTime()->getTimestamp() : 'master';
            if ('master' === $recurId) {
                $masterObject = $vevent;
            }
            if (isset($instances[$recurId])) {
                $attendeeFound = false;
                if (isset($vevent->ATTENDEE)) {
                    foreach ($vevent->ATTENDEE as $attendee) {
                        if ($attendee->getValue() === $itipMessage->sender) {
                            $attendeeFound = true;
                            $attendee['PARTSTAT'] = $instances[$recurId];
                            $attendee['SCHEDULE-STATUS'] = $requestStatus;
                            // Un-setting the RSVP status, because we now know
                            // that the attendee already replied.
                            unset($attendee['RSVP']);
                            break;
                        }
                    }
                }
                if (!$attendeeFound) {
                    // Adding a new attendee. The iTip documentation calls this
                    // a party crasher.
                    $attendee = $vevent->add('ATTENDEE', $itipMessage->sender, [
                        'PARTSTAT' => $instances[$recurId],
                    ]);
                    if ($itipMessage->senderName) {
                        $attendee['CN'] = $itipMessage->senderName;
                    }
                }
                unset($instances[$recurId]);
            }
        }

        if (!$masterObject) {
            // No master object, we can't add new instances.
            return;
        }
        // If we got replies to instances that did not exist in the
        // original list, it means that new exceptions must be created.
        foreach ($instances as $recurId => $partstat) {
            $recurrenceIterator = new EventIterator($existingObject, $itipMessage->uid);
            $found = false;
            $iterations = 1000;
            do {
                $newObject = $recurrenceIterator->getEventObject();
                $recurrenceIterator->next();

                // Compare the Unix timestamp returned by getTimestamp with the previously calculated timestamp.
                // If they are the same, then this is a matching recurrence, even though its date-time may have
                // been expressed in a different format/timezone.
                if (isset($newObject->{'RECURRENCE-ID'}) && $newObject->{'RECURRENCE-ID'}->getDateTime()->getTimestamp() === $recurId) {
                    $found = true;
                }
                --$iterations;
            } while ($recurrenceIterator->valid() && !$found && $iterations);

            // Invalid recurrence id. Skipping this object.
            if (!$found) {
                continue;
            }

            unset(
                $newObject->RRULE,
                $newObject->EXDATE,
                $newObject->RDATE
            );
            $attendeeFound = false;
            if (isset($newObject->ATTENDEE)) {
                foreach ($newObject->ATTENDEE as $attendee) {
                    if ($attendee->getValue() === $itipMessage->sender) {
                        $attendeeFound = true;
                        $attendee['PARTSTAT'] = $partstat;
                        break;
                    }
                }
            }
            if (!$attendeeFound) {
                // Adding a new attendee
                $attendee = $newObject->add('ATTENDEE', $itipMessage->sender, [
                    'PARTSTAT' => $partstat,
                ]);
                if ($itipMessage->senderName) {
                    $attendee['CN'] = $itipMessage->senderName;
                }
            }
            $existingObject->add($newObject);
        }

        return $existingObject;
    }

    /**
     * This method is used in cases where an event got updated, and we
     * potentially need to send emails to attendees to let them know of updates
     * in the events.
     *
     * We will detect which attendees got added, which got removed and create
     * specific messages for these situations.
     *
     * @return array
     */
    protected function parseEventForOrganizer(VCalendar $calendar, array $eventInfo, array $oldEventInfo)
    {
        // Merging attendee lists.
        $attendees = [];
        foreach ($oldEventInfo['attendees'] as $attendee) {
            $attendees[$attendee['href']] = [
                'href' => $attendee['href'],
                'oldInstances' => $attendee['instances'],
                'newInstances' => [],
                'name' => $attendee['name'],
                'forceSend' => null,
            ];
        }
        foreach ($eventInfo['attendees'] as $attendee) {
            if (isset($attendees[$attendee['href']])) {
                $attendees[$attendee['href']]['name'] = $attendee['name'];
                $attendees[$attendee['href']]['newInstances'] = $attendee['instances'];
                $attendees[$attendee['href']]['forceSend'] = $attendee['forceSend'];
            } else {
                $attendees[$attendee['href']] = [
                    'href' => $attendee['href'],
                    'oldInstances' => [],
                    'newInstances' => $attendee['instances'],
                    'name' => $attendee['name'],
                    'forceSend' => $attendee['forceSend'],
                ];
            }
        }

        $messages = [];

        foreach ($attendees as $attendee) {
            // An organizer can also be an attendee. We should not generate any
            // messages for those.
            if ($attendee['href'] === $eventInfo['organizer']) {
                continue;
            }

            $message = new Message();
            $message->uid = $eventInfo['uid'];
            $message->component = 'VEVENT';
            $message->sequence = $eventInfo['sequence'];
            $message->sender = $eventInfo['organizer'];
            $message->senderName = $eventInfo['organizerName'];
            $message->recipient = $attendee['href'];
            $message->recipientName = $attendee['name'];

            // Creating the new iCalendar body.
            $icalMsg = new VCalendar();

            foreach ($calendar->select('VTIMEZONE') as $timezone) {
                $icalMsg->add(clone $timezone);
            }

            if (!$attendee['newInstances'] || 'CANCELLED' === $eventInfo['status']) {
                // If there are no instances the attendee is a part of, it means
                // the attendee was removed and we need to send them a CANCEL message.
                // Also If the meeting STATUS property was changed to CANCELLED
                // we need to send the attendee a CANCEL message.
                $message->method = 'CANCEL';

                $icalMsg->METHOD = $message->method;

                $event = $icalMsg->add('VEVENT', [
                    'UID' => $message->uid,
                    'SEQUENCE' => $message->sequence,
                    'DTSTAMP' => gmdate('Ymd\\THis\\Z'),
                ]);
                if (isset($calendar->VEVENT->SUMMARY)) {
                    $event->add('SUMMARY', $calendar->VEVENT->SUMMARY->getValue());
                }
                $event->add(clone $calendar->VEVENT->DTSTART);
                if (isset($calendar->VEVENT->DTEND)) {
                    $event->add(clone $calendar->VEVENT->DTEND);
                } elseif (isset($calendar->VEVENT->DURATION)) {
                    $event->add(clone $calendar->VEVENT->DURATION);
                }
                $org = $event->add('ORGANIZER', $eventInfo['organizer']);
                if ($eventInfo['organizerName']) {
                    $org['CN'] = $eventInfo['organizerName'];
                }
                $event->add('ATTENDEE', $attendee['href'], [
                    'CN' => $attendee['name'],
                ]);
                $message->significantChange = true;
            } else {
                // The attendee gets the updated event body
                $message->method = 'REQUEST';

                $icalMsg->METHOD = $message->method;

                // We need to find out that this change is significant. If it's
                // not, systems may opt to not send messages.
                //
                // We do this based on the 'significantChangeHash' which is
                // some value that changes if there's a certain set of
                // properties changed in the event, or simply if there's a
                // difference in instances that the attendee is invited to.

                $oldAttendeeInstances = array_keys($attendee['oldInstances']);
                $newAttendeeInstances = array_keys($attendee['newInstances']);

                $message->significantChange =
                    'REQUEST' === $attendee['forceSend'] ||
                    count($oldAttendeeInstances) != count($newAttendeeInstances) ||
                    count(array_diff($oldAttendeeInstances, $newAttendeeInstances)) > 0 ||
                    $oldEventInfo['significantChangeHash'] !== $eventInfo['significantChangeHash'];

                foreach ($attendee['newInstances'] as $instanceId => $instanceInfo) {
                    $currentEvent = clone $eventInfo['instances'][$instanceId];
                    if ('master' === $instanceId) {
                        // We need to find a list of events that the attendee
                        // is not a part of to add to the list of exceptions.
                        $exceptions = [];
                        foreach ($eventInfo['instances'] as $instanceId => $vevent) {
                            if (!isset($attendee['newInstances'][$instanceId])) {
                                $exceptions[] = $instanceId;
                            }
                        }

                        // If there were exceptions, we need to add it to an
                        // existing EXDATE property, if it exists.
                        if ($exceptions) {
                            if (isset($currentEvent->EXDATE)) {
                                $currentEvent->EXDATE->setParts(array_merge(
                                    $currentEvent->EXDATE->getParts(),
                                    $exceptions
                                ));
                            } else {
                                $currentEvent->EXDATE = $exceptions;
                            }
                        }

                        // Cleaning up any scheduling information that
                        // shouldn't be sent along.
                        unset($currentEvent->ORGANIZER['SCHEDULE-FORCE-SEND']);
                        unset($currentEvent->ORGANIZER['SCHEDULE-STATUS']);

                        foreach ($currentEvent->ATTENDEE as $attendee) {
                            unset($attendee['SCHEDULE-FORCE-SEND']);
                            unset($attendee['SCHEDULE-STATUS']);

                            // We're adding PARTSTAT=NEEDS-ACTION to ensure that
                            // iOS shows an "Inbox Item"
                            if (!isset($attendee['PARTSTAT'])) {
                                $attendee['PARTSTAT'] = 'NEEDS-ACTION';
                            }
                        }
                    }

                    $currentEvent->DTSTAMP = gmdate('Ymd\\THis\\Z');
                    $icalMsg->add($currentEvent);
                }
            }

            $message->message = $icalMsg;
            $messages[] = $message;
        }

        return $messages;
    }

    /**
     * Parse an event update for an attendee.
     *
     * This function figures out if we need to send a reply to an organizer.
     *
     * @param string $attendee
     *
     * @return Message[]
     */
    protected function parseEventForAttendee(VCalendar $calendar, array $eventInfo, array $oldEventInfo, $attendee)
    {
        if ($this->scheduleAgentServerRules && 'CLIENT' === $eventInfo['organizerScheduleAgent']) {
            return [];
        }

        // Don't bother generating messages for events that have already been
        // cancelled.
        if ('CANCELLED' === $eventInfo['status']) {
            return [];
        }

        $oldInstances = !empty($oldEventInfo['attendees'][$attendee]['instances']) ?
            $oldEventInfo['attendees'][$attendee]['instances'] :
            [];

        $instances = [];
        foreach ($oldInstances as $instance) {
            $instances[$instance['id']] = [
                'id' => $instance['id'],
                'oldstatus' => $instance['partstat'],
                'newstatus' => null,
            ];
        }
        foreach ($eventInfo['attendees'][$attendee]['instances'] as $instance) {
            if (isset($instances[$instance['id']])) {
                $instances[$instance['id']]['newstatus'] = $instance['partstat'];
            } else {
                $instances[$instance['id']] = [
                    'id' => $instance['id'],
                    'oldstatus' => null,
                    'newstatus' => $instance['partstat'],
                ];
            }
        }

        // We need to also look for differences in EXDATE. If there are new
        // items in EXDATE, it means that an attendee deleted instances of an
        // event, which means we need to send DECLINED specifically for those
        // instances.
        // We only need to do that though, if the master event is not declined.
        if (isset($instances['master']) && 'DECLINED' !== $instances['master']['newstatus']) {
            foreach ($eventInfo['exdate'] as $exDate) {
                if (!in_array($exDate, $oldEventInfo['exdate'])) {
                    if (isset($instances[$exDate])) {
                        $instances[$exDate]['newstatus'] = 'DECLINED';
                    } else {
                        $instances[$exDate] = [
                            'id' => $exDate,
                            'oldstatus' => null,
                            'newstatus' => 'DECLINED',
                        ];
                    }
                }
            }
        }

        // Gathering a few extra properties for each instance.
        foreach ($instances as $recurId => $instanceInfo) {
            if (isset($eventInfo['instances'][$recurId])) {
                $instances[$recurId]['dtstart'] = clone $eventInfo['instances'][$recurId]->DTSTART;
            } else {
                $instances[$recurId]['dtstart'] = $recurId;
            }
        }

        $message = new Message();
        $message->uid = $eventInfo['uid'];
        $message->method = 'REPLY';
        $message->component = 'VEVENT';
        $message->sequence = $eventInfo['sequence'];
        $message->sender = $attendee;
        $message->senderName = $eventInfo['attendees'][$attendee]['name'];
        $message->recipient = $eventInfo['organizer'];
        $message->recipientName = $eventInfo['organizerName'];

        $icalMsg = new VCalendar();
        $icalMsg->METHOD = 'REPLY';

        foreach ($calendar->select('VTIMEZONE') as $timezone) {
            $icalMsg->add(clone $timezone);
        }

        $hasReply = false;

        foreach ($instances as $instance) {
            if ($instance['oldstatus'] == $instance['newstatus'] && 'REPLY' !== $eventInfo['organizerForceSend']) {
                // Skip
                continue;
            }

            $event = $icalMsg->add('VEVENT', [
                'UID' => $message->uid,
                'SEQUENCE' => $message->sequence,
            ]);
            $summary = isset($calendar->VEVENT->SUMMARY) ? $calendar->VEVENT->SUMMARY->getValue() : '';
            // Adding properties from the correct source instance
            if (isset($eventInfo['instances'][$instance['id']])) {
                $instanceObj = $eventInfo['instances'][$instance['id']];
                $event->add(clone $instanceObj->DTSTART);
                if (isset($instanceObj->DTEND)) {
                    $event->add(clone $instanceObj->DTEND);
                } elseif (isset($instanceObj->DURATION)) {
                    $event->add(clone $instanceObj->DURATION);
                }
                if (isset($instanceObj->SUMMARY)) {
                    $event->add('SUMMARY', $instanceObj->SUMMARY->getValue());
                } elseif ($summary) {
                    $event->add('SUMMARY', $summary);
                }
            } else {
                // This branch of the code is reached, when a reply is
                // generated for an instance of a recurring event, through the
                // fact that the instance has disappeared by showing up in
                // EXDATE
                $dt = DateTimeParser::parse($instance['id'], $eventInfo['timezone']);
                // Treat is as a DATE field
                if (strlen($instance['id']) <= 8) {
                    $event->add('DTSTART', $dt, ['VALUE' => 'DATE']);
                } else {
                    $event->add('DTSTART', $dt);
                }
                if ($summary) {
                    $event->add('SUMMARY', $summary);
                }
            }
            if ('master' !== $instance['id']) {
                $dt = DateTimeParser::parse($instance['id'], $eventInfo['timezone']);
                // Treat is as a DATE field
                if (strlen($instance['id']) <= 8) {
                    $event->add('RECURRENCE-ID', $dt, ['VALUE' => 'DATE']);
                } else {
                    $event->add('RECURRENCE-ID', $dt);
                }
            }
            $organizer = $event->add('ORGANIZER', $message->recipient);
            if ($message->recipientName) {
                $organizer['CN'] = $message->recipientName;
            }
            $attendee = $event->add('ATTENDEE', $message->sender, [
                'PARTSTAT' => $instance['newstatus'],
            ]);
            if ($message->senderName) {
                $attendee['CN'] = $message->senderName;
            }
            $hasReply = true;
        }

        if ($hasReply) {
            $message->message = $icalMsg;

            return [$message];
        } else {
            return [];
        }
    }

    /**
     * Returns attendee information and information about instances of an
     * event.
     *
     * Returns an array with the following keys:
     *
     * 1. uid
     * 2. organizer
     * 3. organizerName
     * 4. organizerScheduleAgent
     * 5. organizerForceSend
     * 6. instances
     * 7. attendees
     * 8. sequence
     * 9. exdate
     * 10. timezone - strictly the timezone on which the recurrence rule is
     *                based on.
     * 11. significantChangeHash
     * 12. status
     *
     * @param VCalendar $calendar
     *
     * @return array
     */
    protected function parseEventInfo(?VCalendar $calendar = null)
    {
        $uid = null;
        $organizer = null;
        $organizerName = null;
        $organizerForceSend = null;
        $sequence = null;
        $timezone = null;
        $status = null;
        $organizerScheduleAgent = 'SERVER';

        $significantChangeHash = '';

        // Now we need to collect a list of attendees, and which instances they
        // are a part of.
        $attendees = [];

        $instances = [];
        $exdate = [];

        $significantChangeEventProperties = [];

        foreach ($calendar->VEVENT as $vevent) {
            $eventSignificantChangeHash = '';
            $rrule = [];

            if (is_null($uid)) {
                $uid = $vevent->UID->getValue();
            } else {
                if ($uid !== $vevent->UID->getValue()) {
                    throw new ITipException('If a calendar contained more than one event, they must have the same UID.');
                }
            }

            if (!isset($vevent->DTSTART)) {
                throw new ITipException('An event MUST have a DTSTART property.');
            }

            if (isset($vevent->ORGANIZER)) {
                if (is_null($organizer)) {
                    $organizer = $vevent->ORGANIZER->getNormalizedValue();
                    $organizerName = isset($vevent->ORGANIZER['CN']) ? $vevent->ORGANIZER['CN'] : null;
                } else {
                    if (strtoupper($organizer) !== strtoupper($vevent->ORGANIZER->getNormalizedValue())) {
                        throw new SameOrganizerForAllComponentsException('Every instance of the event must have the same organizer.');
                    }
                }
                $organizerForceSend =
                    isset($vevent->ORGANIZER['SCHEDULE-FORCE-SEND']) ?
                    strtoupper($vevent->ORGANIZER['SCHEDULE-FORCE-SEND']) :
                    null;
                $organizerScheduleAgent =
                    isset($vevent->ORGANIZER['SCHEDULE-AGENT']) ?
                    strtoupper((string) $vevent->ORGANIZER['SCHEDULE-AGENT']) :
                    'SERVER';
            }
            if (is_null($sequence) && isset($vevent->SEQUENCE)) {
                $sequence = $vevent->SEQUENCE->getValue();
            }
            if (isset($vevent->EXDATE)) {
                foreach ($vevent->select('EXDATE') as $val) {
                    $exdate = array_merge($exdate, $val->getParts());
                }
                sort($exdate);
            }
            if (isset($vevent->RRULE)) {
                foreach ($vevent->select('RRULE') as $rr) {
                    foreach ($rr->getParts() as $key => $val) {
                        // ignore default values (https://github.com/sabre-io/vobject/issues/126)
                        if ('INTERVAL' === $key && 1 == $val) {
                            continue;
                        }
                        if (is_array($val)) {
                            $val = implode(',', $val);
                        }
                        $rrule[] = "$key=$val";
                    }
                }
                sort($rrule);
            }
            if (isset($vevent->STATUS)) {
                $status = strtoupper($vevent->STATUS->getValue());
            }

            $recurId = isset($vevent->{'RECURRENCE-ID'}) ? $vevent->{'RECURRENCE-ID'}->getValue() : 'master';
            if (is_null($timezone)) {
                if ('master' === $recurId) {
                    $timezone = $vevent->DTSTART->getDateTime()->getTimeZone();
                } else {
                    $timezone = $vevent->{'RECURRENCE-ID'}->getDateTime()->getTimeZone();
                }
            }
            if (isset($vevent->ATTENDEE)) {
                foreach ($vevent->ATTENDEE as $attendee) {
                    if ($this->scheduleAgentServerRules &&
                        isset($attendee['SCHEDULE-AGENT']) &&
                        'CLIENT' === strtoupper($attendee['SCHEDULE-AGENT']->getValue())
                    ) {
                        continue;
                    }
                    $partStat =
                        isset($attendee['PARTSTAT']) ?
                        strtoupper($attendee['PARTSTAT']) :
                        'NEEDS-ACTION';

                    $forceSend =
                        isset($attendee['SCHEDULE-FORCE-SEND']) ?
                        strtoupper($attendee['SCHEDULE-FORCE-SEND']) :
                        null;

                    if (isset($attendees[$attendee->getNormalizedValue()])) {
                        $attendees[$attendee->getNormalizedValue()]['instances'][$recurId] = [
                            'id' => $recurId,
                            'partstat' => $partStat,
                            'forceSend' => $forceSend,
                        ];
                    } else {
                        $attendees[$attendee->getNormalizedValue()] = [
                            'href' => $attendee->getNormalizedValue(),
                            'instances' => [
                                $recurId => [
                                    'id' => $recurId,
                                    'partstat' => $partStat,
                                ],
                            ],
                            'name' => isset($attendee['CN']) ? (string) $attendee['CN'] : null,
                            'forceSend' => $forceSend,
                        ];
                    }
                }
                $instances[$recurId] = $vevent;
            }

            foreach ($this->significantChangeProperties as $prop) {
                if (isset($vevent->$prop)) {
                    $propertyValues = $vevent->select($prop);

                    $eventSignificantChangeHash .= $prop.':';

                    if ('EXDATE' === $prop) {
                        $eventSignificantChangeHash .= implode(',', $exdate).';';
                    } elseif ('RRULE' === $prop) {
                        $eventSignificantChangeHash .= implode(',', $rrule).';';
                    } else {
                        foreach ($propertyValues as $val) {
                            $eventSignificantChangeHash .= $val->getValue().';';
                        }
                    }
                }
            }
            $significantChangeEventProperties[] = $eventSignificantChangeHash;
        }

        asort($significantChangeEventProperties);

        foreach ($significantChangeEventProperties as $eventSignificantChangeHash) {
            $significantChangeHash .= $eventSignificantChangeHash;
        }
        $significantChangeHash = md5($significantChangeHash);

        return compact(
            'uid',
            'organizer',
            'organizerName',
            'organizerScheduleAgent',
            'organizerForceSend',
            'instances',
            'attendees',
            'sequence',
            'exdate',
            'timezone',
            'significantChangeHash',
            'status'
        );
    }
}
