* @param array $config Full sync configuration for this sync module.
	 *
	 * @return array Number of items yet to be enqueued.
	 */
	public function estimate_full_sync_actions( $config ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
		return 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_constants' );
	}

	/**
	 * Sync the constants if we're supposed to.
	 *
	 * @access public
	 */
	public function maybe_sync_constants() {
		if ( get_transient( self::CONSTANTS_AWAIT_TRANSIENT_NAME ) ) {
			return;
		}

		set_transient( self::CONSTANTS_AWAIT_TRANSIENT_NAME, microtime( true ), Defaults::$default_sync_constants_wait_time );

		$constants = $this->get_all_constants();
		if ( empty( $constants ) ) {
			return;
		}

		$constants_checksums = (array) get_option( self::CONSTANTS_CHECKSUM_OPTION_NAME, array() );

		foreach ( $constants as $name => $value ) {
			$checksum = $this->get_check_sum( $value );
			// Explicitly not using Identical comparison as get_option returns a string.
			if ( ! $this->still_valid_checksum( $constants_checksums, $name, $checksum ) && ! is_null( $value ) ) {
				/**
				 * Tells the client to sync a constant to the server
				 *
				 * @param string The name of the constant
				 * @param mixed The value of the constant
				 *
				 * @since 4.2.0
				 */
				do_action( 'jetpack_sync_constant', $name, $value );
				$constants_checksums[ $name ] = $checksum;
			} else {
				$constants_checksums[ $name ] = $checksum;
			}
		}
		update_option( self::CONSTANTS_CHECKSUM_OPTION_NAME, $constants_checksums );
	}

	/**
	 * Retrieve all constants as per the current constants whitelist.
	 * Public so that we don't have to store an option for each constant.
	 *
	 * @access public
	 *
	 * @return array All constants.
	 */
	public function get_all_constants() {
		$constants_whitelist = $this->get_constants_whitelist();

		return array_combine(
			$constants_whitelist,
			array_map( array( $this, 'get_constant' ), $constants_whitelist )
		);
	}

	/**
	 * Retrieve the value of a constant.
	 * Used as a wrapper to standartize access to constants.
	 *
	 * @access private
	 *
	 * @param string $constant Constant name.
	 *
	 * @return mixed Return value of the constant.
	 */
	private function get_constant( $constant ) {
		return ( defined( $constant ) ) ?
			constant( $constant )
			: null;
	}

	/**
	 * Expand the constants within a hook before they are serialized and sent to the server.
	 *
	 * @access public
	 *
	 * @param array $args The hook parameters.
	 *
	 * @return array $args The hook parameters.
	 */
	public function expand_constants( $args ) {
		if ( $args[0] ) {
			$constants           = $this->get_all_constants();
			$constants_checksums = array();
			foreach ( $constants as $name => $value ) {
				$constants_checksums[ $name ] = $this->get_check_sum( $value );
			}
			update_option( self::CONSTANTS_CHECKSUM_OPTION_NAME, $constants_checksums );

			return $constants;
		}

		return $args;
	}

	/**
	 * Return Total number of objects.
	 *
	 * @param array $config Full Sync config.
	 *
	 * @return int total
	 */
	public function total( $config ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
		return count( $this->get_constants_whitelist() );
	}
}