public function get_availability_slug_by_collection( $tickets, $datetime = null ) {
			if ( ! $tickets ) {
				return;
			}

			if ( is_numeric( $datetime ) ) {
				$timestamp = $datetime;
			} elseif ( $datetime ) {
				$timestamp = strtotime( $datetime );
			} else {
				$timestamp = current_time( 'timestamp' );
			}

			$collection_availability_slug = 'available';
			$tickets_available = false;
			$slugs = [];

			foreach ( $tickets as $ticket ) {
				$availability_slug = $ticket->availability_slug( $timestamp );

				// if any ticket is available for this event, consider the availability slug as 'available'
				if ( 'available' === $availability_slug ) {
					// reset the collected slugs to "available" only
					$slugs = array( 'available' );
					break;
				}

				// track unique availability slugs
				if ( ! in_array( $availability_slug, $slugs ) ) {
					$slugs[] = $availability_slug;
				}
			}

			if ( 1 === count( $slugs ) ) {
				$collection_availability_slug = $slugs[0];
			} else {
				$collection_availability_slug = 'availability-mixed';
			}

			/**
			 * Filters the availability slug for a collection of tickets
			 *
			 * @param string Availability slug
			 * @param array Collection of tickets
			 * @param string Datetime string
			 */
			return apply_filters( 'event_tickets_availability_slug_by_collection', $collection_availability_slug, $tickets, $datetime );
		}