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diff --git a/apps/dav/lib/CalDAV/Integration/ExternalCalendar.php b/apps/dav/lib/CalDAV/Integration/ExternalCalendar.php
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+++ b/apps/dav/lib/CalDAV/Integration/ExternalCalendar.php
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+<?php
+/**
+ * @copyright 2020, Georg Ehrke <oc.list@georgehrke.com>
+ *
+ * @author Georg Ehrke <oc.list@georgehrke.com>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+namespace OCA\DAV\CalDAV\Integration;
+
+use Sabre\CalDAV;
+use Sabre\DAV;
+
+/**
+ * Class ExternalCalendar
+ *
+ * @package OCA\DAV\CalDAV\Integration
+ * @since 19.0.0
+ */
+abstract class ExternalCalendar implements CalDAV\ICalendar, DAV\IProperties {
+
+	/** @var string */
+	private const PREFIX = 'app-generated';
+
+	/**
+	 * @var string
+	 *
+	 * Double dash is a valid delimiter,
+	 * because it will always split the calendarURIs correctly:
+	 * - our prefix contains only one dash and won't be split
+	 * - appIds are not allowed to contain dashes as per spec:
+	 * > must contain only lowercase ASCII characters and underscore
+	 * - explode has a limit of three, so even if the app-generated
+	 *   calendar uri has double dashes, it won't be split
+	 */
+	private const DELIMITER = '--';
+
+	/** @var string */
+	private $appId;
+
+	/** @var string */
+	private $calendarUri;
+
+	/**
+	 * ExternalCalendar constructor.
+	 *
+	 * @param string $appId
+	 * @param string $calendarUri
+	 */
+	public function __construct(string $appId, string $calendarUri) {
+		$this->appId = $appId;
+		$this->calendarUri = $calendarUri;
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	final public function getName() {
+		return implode(self::DELIMITER, [
+			self::PREFIX,
+			$this->appId,
+			$this->calendarUri,
+		]);
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	final public function setName($name) {
+		throw new DAV\Exception\MethodNotAllowed('Renaming calendars is not yet supported');
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	final public function createDirectory($name) {
+		throw new DAV\Exception\MethodNotAllowed('Creating collections in calendar objects is not allowed');
+
+	}
+
+	/**
+	 * Checks whether the calendar uri is app-generated
+	 *
+	 * @param string $calendarUri
+	 * @return bool
+	 */
+	public static function isAppGeneratedCalendar(string $calendarUri):bool {
+		return strpos($calendarUri, self::PREFIX) === 0 && substr_count($calendarUri, self::DELIMITER) >= 2;
+	}
+
+	/**
+	 * Splits an app-generated calendar-uri into appId and calendarUri
+	 *
+	 * @param string $calendarUri
+	 * @return array
+	 */
+	public static function splitAppGeneratedCalendarUri(string $calendarUri):array {
+		$array = array_slice(explode(self::DELIMITER, $calendarUri, 3), 1);
+		// Check the array has expected amount of elements
+		// and none of them is an empty string
+		if (\count($array) !== 2 || \in_array('', $array, true)) {
+			throw new \InvalidArgumentException('Provided calendar uri was not app-generated');
+		}
+
+		return $array;
+	}
+
+	/**
+	 * Checks whether a calendar-name, the user wants to create, violates
+	 * the reserved name for calendar uris
+	 *
+	 * @param string $calendarUri
+	 * @return bool
+	 */
+	public static function doesViolateReservedName(string $calendarUri):bool {
+		return strpos($calendarUri, self::PREFIX) === 0;
+	}
+}