resent in the comment type whitelist.
	 *
	 * @param int $comment_id ID of the comment.
	 * @return boolean Whether the comment type is allowed.
	 */
	public function is_comment_type_allowed( $comment_id ) {
		$comment = get_comment( $comment_id );

		if ( isset( $comment->comment_type ) ) {
			return in_array( $comment->comment_type, $this->get_whitelisted_comment_types(), true );
		}
		return false;
	}

	/**
	 * Initialize the module in the sender.
	 *
	 * @access public
	 */
	public function init_before_send() {
		add_filter( 'jetpack_sync_before_send_wp_insert_comment', array( $this, 'expand_wp_insert_comment' ) );

		foreach ( $this->get_whitelisted_comment_types() as $comment_type ) {
			foreach ( array( 'unapproved', 'approved' ) as $comment_status ) {
				$comment_action_name = "comment_{$comment_status}_{$comment_type}";
				add_filter(
					'jetpack_sync_before_send_' . $comment_action_name,
					array(
						$this,
						'expand_wp_insert_comment',
					)
				);
			}
		}

		// Full sync.
		add_filter( 'jetpack_sync_before_send_jetpack_full_sync_comments', array( $this, 'expand_comment_ids' ) );
	}

	/**
	 * Enqueue the comments actions for full sync.
	 *
	 * @access public
	 *
	 * @param array   $config               Full sync configuration for this sync module.
	 * @param int     $max_items_to_enqueue Maximum number of items to enqueue.
	 * @param boolean $state                True if full sync has finished enqueueing this module, false otherwise.
	 * @return array Number of actions enqueued, and next module state.
	 */
	public function enqueue_full_sync_actions( $config, $max_items_to_enqueue, $state ) {
		global $wpdb;
		return $this->enqueue_all_ids_as_action( 'jetpack_full_sync_comments', $wpdb->comments, 'comment_ID', $this->get_where_sql( $config ), $max_items_to_enqueue, $state );
	}

	/**
	 * Retrieve an estimated number of actions that will be enqueued.
	 *
	 * @access public
	 *
	 * @param array $config Full sync configuration for this sync module.
	 * @return int Number of items yet to be enqueued.
	 */
	public function estimate_full_sync_actions( $config ) {
		global $wpdb;

		$query = "SELECT count(*) FROM $wpdb->comments";

		$where_sql = $this->get_where_sql( $config );
		if ( $where_sql ) {
			$query .= ' WHERE ' . $where_sql;
		}

		// TODO: Call $wpdb->prepare on the following query.
		// phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared
		$count = $wpdb->get_var( $query );

		return (int) ceil( $count / self::ARRAY_CHUNK_SIZE );
	}

	/**
	 * Retrieve the WHERE SQL clause based on the module config.
	 *
	 * @access public
	 *
	 * @param array $config Full sync configuration for this sync module.
	 * @return string WHERE SQL clause, or `null` if no comments are specified in the module config.
	 */
	public function get_where_sql( $config ) {
		if ( is_array( $config ) ) {
			return 'comment_ID IN (' . implode( ',', array_map( 'intval', $config ) ) . ')';
		}

		return '1=1';
	}

	/**
	 * Retrieve the actions that will be sent for this module during a full sync.
	 *
	 * @access public
	 *
	 * @return array Full sync actions of this module.
	 */
	public function get_full_sync_actions() {
		return array( 'jetpack_full_sync_comments' );
	}

	/**
	 * Count all the actions that are going to be sent.
	 *
	 * @access public
	 *
	 * @param array $action_names Names of all the actions that will be sent.
	 * @return int Number of actions.
	 */
	public function count_full_sync_actions( $action_names ) {
		return $this->count_actions( $action_names, array( 'jetpack_full_sync_comments' ) );
	}

	/**
	 * Expand the comment status change before the data is serialized and sent to the server.
	 *
	 * @access public
	 * @todo This is not used currently - let's implement it.
	 *
	 * @param array $args The hook parameters.
	 * @return array The expanded hook parameters.
	 */
	public function expand_wp_comment_status_change( $args ) {
		return array( $args[0], $this->filter_comment( $args[1] ) );
	}

	/**
	 * Expand the comment creation before the data is serialized and sent to the server.
	 *
	 * @access public
	 *
	 * @param array $args The hook parameters.
	 * @return array The expanded hook parameters.
	 */
	public function expand_wp_insert_comment( $args ) {
		return array( $args[0], $this->filter_comment( $args[1] ) );
	}

	/**
	 * Filter a comment object to the fields we need.
	 *
	 * @access public
	 *
	 * @param \WP_Comment $comment The unfiltered comment object.
	 * @return \WP_Comment Filtered comment object.
	 */
	public function filter_comment( $comment ) {
		/**
		 * Filters whether to prevent sending comment data to .com
		 *
		 * Passing true to the filter will prevent the comment data from being sent
		 * to the WordPress.com.
		 * Instead we pass data that will still enable us to do a checksum against the
		 * Jetpacks data but will prevent us from displaying the data on in the API as well as
		 * other services.
		 *
		 * @since 4.2.0
		 *
		 * @param boolean false prevent post data from bing synced to WordPress.com
		 * @param mixed $comment WP_COMMENT object
		 */
		if ( apply_filters( 'jetpack_sync_prevent_sending_comment_data', false, $comment ) ) {
			$blocked_comment                   = new \stdClass();
			$blocked_comment->comment_ID       = $comment->comment_ID;
			$blocked_comment->comment_date     = $comment->comment_date;
			$blocked_comment->comment_date_gmt = $comment->comment_date_gmt;
			$blocked_comment->comment_approved = 'jetpack_sync_blocked';
			return $blocked_comment;
		}

		return $comment;
	}

	/**
	 * Whether a certain comment meta key is whitelisted for sync.
	 *
	 * @access public
	 *
	 * @param string $meta_key Comment meta key.
	 * @return boolean Whether the meta key is whitelisted.
	 */
	public function is_whitelisted_comment_meta( $meta_key ) {
		return in_array( $meta_key, Settings::get_setting( 'comment_meta_whitelist' ), true );
	}

	/**
	 * Handler for filtering out non-whitelisted comment meta.
	 *
	 * @access public
	 *
	 * @param array $args Hook args.
	 * @return array|boolean False if not whitelisted, the original hook args otherwise.
	 */
	public function filter_meta( $args ) {
		if ( $this->is_comment_type_allowed( $args[1] ) && $this->is_whitelisted_comment_meta( $args[2] ) ) {
			return $args;
		}

		return false;
	}

	/**
	 * Expand the comment IDs to comment objects and meta before being serialized and sent to the server.
	 *
	 * @access public
	 *
	 * @param array $args The hook parameters.
	 * @return array The expanded hook parameters.
	 */
	public function expand_comment_ids( $args ) {
		list( $comment_ids, $previous_interval_end ) = $args;
		$comments                                    = get_comments(
			array(
				'include_unapproved' => true,
				'comment__in'        => $comment_ids,
				'orderby'            => 'comment_ID',
				'order'              => 'DESC',
			)
		);

		return array(
			$comments,
			$this->get_metadata( $comment_ids, 'comment', Settings::get_setting( 'comment_meta_whitelist' ) ),
			$previous_interval_end,
		);
	}
}