A landmark case in Japan could force tech giants to reveal how their algorithms work.

Last month, a Tokyo court ruled in favour of Hanryumura – a BBQ restaurant chain operator – in an antitrust case brought against Kakaku.com, operator of Tabelog, Japan's largest restaurant review platform.

Hanryumura claimed that Kakaku altered the way user scores were tallied in a way that hurt sales at its restaurants. The restaurant operator received $284,000 in damages but...