You just have to play enough hands so that luck goes away,” said Burch. “Cepheus loses to no one over a long enough time. Before every decision, it checks its cards against the database and uses probability to play the best possible game, whether that means calling, raising or folding its hand. Instead, Cepheus plays the hand it is dealt in the best way possible. It is impossible to make a program that wins every hand, because an opponent can always be dealt a pair of aces or another lucky hand.

“But every time it plays a hand it comes up with a better strategy.”

“Cepheus starts off with a strategy that’s quite terrible,” said Burch. The game involves two players who bet fixed amounts with a limit on the number of raises allowed. The program plays a variant of poker called heads-up limit hold’em, made famous by Michael Craig’s 2005 book, The Professor, the Banker and the Suicide King.

“For every single possible situation you could get into, it has a description for how you should play,” said Neil Burch, a computer scientist who helped develop Cepheus at the University of Alberta Computer Poker Research Group. At the end of the marathon training session, the database contained 11 terabytes of information on calls, raises and folds for every hand a player could have.Ĭepheus learns from an algorithm that essentially minimises its regrets: the program reviews every decision made and then learns which moves paid off and which cost it the hand. With each game Cepheus played, the program built up a database of cards dealt, betting decisions and outcomes. The feat required the number-crunching power of four thousand computer processors, each handling six billion hands every second. To learn the game, Cepheus spent two months playing the equivalent of more than a billion billion hands of Texas hold’em, which is more poker games than have been played in the entirety of human history.

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