oast_Notification $notification Notification to clear the dismissal of.
	 *
	 * @return bool
	 */
	public function clear_dismissal( $notification ) {

		if ( $notification instanceof Yoast_Notification ) {
			$dismissal_key = $notification->get_dismissal_key();
		}

		if ( is_string( $notification ) ) {
			$dismissal_key = $notification;
		}

		if ( empty( $dismissal_key ) ) {
			return false;
		}

		// Remove notification dismissal for all users.
		$deleted = delete_metadata( 'user', 0, $dismissal_key, '', true );

		return $deleted;
	}

	/**
	 * Add notification to the cookie
	 *
	 * @param Yoast_Notification $notification Notification object instance.
	 */
	public function add_notification( Yoast_Notification $notification ) {

		// Don't add if the user can't see it.
		if ( ! $notification->display_for_current_user() ) {
			return;
		}

		$notification_id = $notification->get_id();

		// Empty notifications are always added.
		if ( $notification_id !== '' ) {

			// If notification ID exists in notifications, don't add again.
			$present_notification = $this->get_notification_by_id( $notification_id );
			if ( ! is_null( $present_notification ) ) {
				$this->remove_notification( $present_notification, false );
			}

			if ( is_null( $present_notification ) ) {
				$this->new[] = $notification_id;
			}
		}

		// Add to list.
		$this->notifications[] = $notification;
	}

	/**
	 * Get the notification by ID
	 *
	 * @param string $notification_id The ID of the notification to search for.
	 *
	 * @return null|Yoast_Notification
	 */
	public function get_notification_by_id( $notification_id ) {

		foreach ( $this->notifications as & $notification ) {
			if ( $notification_id === $notification->get_id() ) {
				return $notification;
			}
		}

		return null;
	}

	/**
	 * Display the notifications
	 *
	 * @param bool $echo_as_json True when notifications should be printed directly.
	 *
	 * @return void
	 */
	public function display_notifications( $echo_as_json = false ) {

		// Never display notifications for network admin.
		if ( function_exists( 'is_network_admin' ) && is_network_admin() ) {
			return;
		}

		$sorted_notifications = $this->get_sorted_notifications();
		$notifications        = array_filter( $sorted_notifications, array( $this, 'is_notification_persistent' ) );

		if ( empty( $notifications ) ) {
			return;
		}

		array_walk( $notifications, array( $this, 'remove_notification' ) );

		if ( $echo_as_json ) {
			$notification_json = array();
			foreach ( $notifications as $notification ) {
				$notification_json[] = $notification->render();
			}

			echo json_encode( $notification_json );

			return;
		}

		foreach ( $notifications as $notification ) {
			echo $notification;
		}
	}

	/**
	 * Remove notification after it has been displayed
	 *
	 * @param Yoast_Notification $notification Notification to remove.
	 * @param bool               $resolve Resolve as fixed.
	 */
	public function remove_notification( Yoast_Notification $notification, $resolve = true ) {

		$index = false;

		// Match persistent Notifications by ID, non persistent by item in the array.
		if ( $notification->is_persistent() ) {
			foreach ( $this->notifications as $current_index => $present_notification ) {
				if ( $present_notification->get_id() === $notification->get_id() ) {
					$index = $current_index;
					break;
				}
			}
		}
		else {
			$index = array_search( $notification, $this->notifications, true );
		}

		if ( false === $index ) {
			return;
		}

		if ( $notification->is_persistent() && $resolve ) {
			$this->resolved++;
			$this->clear_dismissal( $notification );
		}

		unset( $this->notifications[ $index ] );
		$this->notifications = array_values( $this->notifications );
	}

	/**
	 * Get the notification count
	 *
	 * @param bool $dismissed Count dismissed notifications.
	 *
	 * @return int Number of notifications
	 */
	public function get_notification_count( $dismissed = false ) {

		$notifications = $this->get_notifications();
		$notifications = array_filter( $notifications, array( $this, 'filter_persistent_notifications' ) );

		if ( ! $dismissed ) {
			$notifications = array_filter( $notifications, array( $this, 'filter_dismissed_notifications' ) );
		}

		return count( $notifications );
	}

	/**
	 * Get the number of notifications resolved this execution
	 *
	 * These notifications have been resolved and should be counted when active again.
	 *
	 * @return int
	 */
	public function get_resolved_notification_count() {

		return $this->resolved;
	}

	/**
	 * Return the notifications sorted on type and priority
	 *
	 * @return array|Yoast_Notification[] Sorted Notifications
	 */
	public function get_sorted_notifications() {

		$notifications = $this->get_notifications();
		if ( empty( $notifications ) ) {
			return array();
		}

		// Sort by severity, error first.
		usort( $notifications, array( $this, 'sort_notifications' ) );

		return $notifications;
	}

	/**
	 * AJAX display notifications
	 */
	public function ajax_get_notifications() {
		$echo = filter_input( INPUT_POST, 'version' ) === '2';

		// Display the notices.
		$this->display_notifications( $echo );

		// AJAX die.
		exit;
	}

	/**
	 * Remove storage when the plugin is deactivated
	 */
	public function deactivate_hook() {

		$this->clear_notifications();
	}

	/**
	 * Save persistent notifications to storage
	 *
	 * We need to be able to retrieve these so they can be dismissed at any time during the execution.
	 *
	 * @since 3.2
	 *
	 * @return void
	 */
	public function update_storage() {

		$notifications = $this->get_notifications();

		/**
		 * Filter: 'yoast_notifications_before_storage' - Allows developer to filter notifications before saving them.
		 *
		 * @api Yoast_Notification[] $notifications
		 */
		$notifications = apply_filters( 'yoast_notifications_before_storage', $notifications );

		// No notifications to store, clear storage.
		if ( empty( $notifications ) ) {
			$this->remove_storage();

			return;
		}

		$notifications = array_map( array( $this, 'notification_to_array' ), $notifications );

		// Save the notifications to the storage.
		update_user_option( get_current_user_id(), self::STORAGE_KEY, $notifications );
	}

	/**
	 * Provide a way to verify present notifications
	 *
	 * @return array|Yoast_Notification[] Registered notifications.
	 */
	public function get_notifications() {

		return $this->notifications;
	}

	/**
	 * Get newly added notifications
	 *
	 * @return array
	 */
	public function get_new_notifications() {

		return array_map( array( $this, 'get_notification_by_id' ), $this->new );
	}

	/**
	 * Get information from the User input
	 *
	 * @param string $key Key to retrieve.
	 *
	 * @return mixed value of key if set.
	 */
	private static function get_user_input( $key ) {

		$filter_input_type = INPUT_GET;
		if ( 'POST' === strtoupper( $_SERVER['REQUEST_METHOD'] ) ) {
			$filter_input_type = INPUT_POST;
		}

		return filter_input( $filter_input_type, $key );
	}

	/**
	 * Retrieve the notifications from storage
	 *
	 * @return array Yoast_Notification[] Notifications
	 */
	private function retrieve_notifications_from_storage() {

		$stored_notifications = get_user_option( self::STORAGE_KEY, get_current_user_id() );

		// Check if notifications are stored.
		if ( empty( $stored_notifications ) ) {
			return;
		}

		if ( is_array( $stored_notifications ) ) {
			$notifications = array_map( array( $this, 'array_to_notification' ), $stored_notifications );
			// Apply array_values to ensure we get a 0-indexed array.
			$notifications = array_values( array_filter( $notifications, array( $this, 'filter_notification_current_user' ) ) );

			$this->notifications = $notifications;
		}
	}

	/**
	 * Sort on type then priority
	 *
	 * @param Yoast_Notification $a Compare with B.
	 * @param Yoast_Notification $b Compare with A.
	 *
	 * @return int 1, 0 or -1 for sorting offset.
	 */
	private function sort_notifications( Yoast_Notification $a, Yoast_Notification $b ) {

		$a_type = $a->get_type();
		$b_type = $b->get_type();

		if ( $a_type === $b_type ) {
			return WPSEO_Utils::calc( $b->get_priority(), 'compare', $a->get_priority() );
		}

		if ( 'error' === $a_type ) {
			return -1;
		}

		if ( 'error' === $b_type ) {
			return 1;
		}

		return 0;
	}

	/**
	 * Dismiss the notification
	 *
	 * @param Yoast_Notification $notification Notification to dismiss.
	 * @param string             $meta_value   Value to save in the dismissal.
	 *
	 * @return bool
	 */
	private static function dismiss_notification( Yoast_Notification $notification, $meta_value = 'seen' ) {
		// Dismiss notification.
		return ( false !== update_user_meta( get_current_user_id(), $notification->get_dismissal_key(), $meta_value ) );
	}

	/**
	 * Remove all notifications from storage
	 */
	private function remove_storage() {

		delete_user_option( get_current_user_id(), self::STORAGE_KEY );
	}

	/**
	 * Clear local stored notifications
	 */
	private function clear_notifications() {

		$this->notifications = array();
	}

	/**
	 * Filter out non-persistent notifications.
	 *
	 * @param Yoast_Notification $notification Notification to test for persistent.
	 *
	 * @since 3.2
	 *
	 * @return bool
	 */
	private function filter_persistent_notifications( Yoast_Notification $notification ) {

		return $notification->is_persistent();
	}

	/**
	 * Filter out dismissed notifications
	 *
	 * @param Yoast_Notification $notification Notification to check.
	 *
	 * @return bool
	 */
	private function filter_dismissed_notifications( Yoast_Notification $notification ) {

		return ! $this->maybe_dismiss_notification( $notification );
	}

	/**
	 * Convert Notification to array representation
	 *
	 * @param Yoast_Notification $notification Notification to convert.
	 *
	 * @since 3.2
	 *
	 * @return array
	 */
	private function notification_to_array( Yoast_Notification $notification ) {

		return $notification->to_array();
	}

	/**
	 * Convert stored array to Notification.
	 *
	 * @param array $notification_data Array to convert to Notification.
	 *
	 * @return Yoast_Notification
	 */
	private function array_to_notification( $notification_data ) {

		return new Yoast_Notification(
			$notification_data['message'],
			$notification_data['options']
		);
	}

	/**
	 * Filter notifications that should not be displayed for the current user
	 *
	 * @param Yoast_Notification $notification Notification to test.
	 *
	 * @return bool
	 */
	private function filter_notification_current_user( Yoast_Notification $notification ) {
		return $notification->display_for_current_user();
	}

	/**
	 * Checks if given notification is persistent.
	 *
	 * @param Yoast_Notification $notification The notification to check.
	 *
	 * @return bool True when notification is not persistent.
	 */
	private function is_notification_persistent( Yoast_Notification $notification ) {
		return ! $notification->is_persistent();
	}

	/**
	 * Write the notifications to a cookie (hooked on shutdown)
	 *
	 * Function renamed to 'update_storage'.
	 *
	 * @deprecated 3.2 remove in 3.5
	 * @codeCoverageIgnore
	 */
	public function set_transient() {
		_deprecated_function( __METHOD__, 'WPSEO 3.2' );
	}
}