1 files changed, 100 insertions, 0 deletions
diff --git a/apps/dav/lib/CalDAV/EventComparisonService.php b/apps/dav/lib/CalDAV/EventComparisonService.php
new file mode 100644
index 00000000000..63395e7ce1c
--- /dev/null
+++ b/apps/dav/lib/CalDAV/EventComparisonService.php
@@ -0,0 +1,100 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2022 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+namespace OCA\DAV\CalDAV;
+
+use OCA\DAV\CalDAV\Schedule\IMipService;
+use Sabre\VObject\Component\VCalendar;
+use Sabre\VObject\Component\VEvent;
+
+class EventComparisonService {
+
+	/** @var string[] */
+	private const EVENT_DIFF = [
+		'RECURRENCE-ID',
+		'RRULE',
+		'SEQUENCE',
+		'LAST-MODIFIED'
+	];
+
+
+	/**
+	 * If found, remove the event from $eventsToFilter that
+	 * is identical to the passed $filterEvent
+	 * and return whether an identical event was found
+	 *
+	 * This function takes into account the SEQUENCE,
+	 * RRULE, RECURRENCE-ID and LAST-MODIFIED parameters
+	 *
+	 * @param VEvent $filterEvent
+	 * @param array $eventsToFilter
+	 * @return bool true if there was an identical event found and removed, false if there wasn't
+	 */
+	private function removeIfUnchanged(VEvent $filterEvent, array &$eventsToFilter): bool {
+		$filterEventData = [];
+		foreach (self::EVENT_DIFF as $eventDiff) {
+			$filterEventData[] = IMipService::readPropertyWithDefault($filterEvent, $eventDiff, '');
+		}
+
+		/** @var VEvent $component */
+		foreach ($eventsToFilter as $k => $eventToFilter) {
+			$eventToFilterData = [];
+			foreach (self::EVENT_DIFF as $eventDiff) {
+				$eventToFilterData[] = IMipService::readPropertyWithDefault($eventToFilter, $eventDiff, '');
+			}
+			// events are identical and can be removed
+			if ($filterEventData === $eventToFilterData) {
+				unset($eventsToFilter[$k]);
+				return true;
+			}
+		}
+		return false;
+	}
+
+	/**
+	 * Compare two VCalendars with each other and find all changed elements
+	 *
+	 * Returns an array of old and new events
+	 *
+	 * Old events are only detected if they are also changed
+	 * If there is no corresponding old event for a VEvent, it
+	 * has been newly created
+	 *
+	 * @param VCalendar $new
+	 * @param VCalendar|null $old
+	 * @return array<string, VEvent[]|null>
+	 */
+	public function findModified(VCalendar $new, ?VCalendar $old): array {
+		$newEventComponents = $new->getComponents();
+
+		foreach ($newEventComponents as $k => $event) {
+			if (!$event instanceof VEvent) {
+				unset($newEventComponents[$k]);
+			}
+		}
+
+		if (empty($old)) {
+			return ['old' => null, 'new' => $newEventComponents];
+		}
+
+		$oldEventComponents = $old->getComponents();
+		if (is_array($oldEventComponents) && !empty($oldEventComponents)) {
+			foreach ($oldEventComponents as $k => $event) {
+				if (!$event instanceof VEvent) {
+					unset($oldEventComponents[$k]);
+					continue;
+				}
+				if ($this->removeIfUnchanged($event, $newEventComponents)) {
+					unset($oldEventComponents[$k]);
+				}
+			}
+		}
+
+		return ['old' => array_values($oldEventComponents), 'new' => array_values($newEventComponents)];
+	}
+}