The mode of reinforcement learning that has been so successful in teaching computers to play backgammon is called temporal difference (TD) learning, which is based on the differences between temporally successive predictions.
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Each move by each notional player in the game (the computer plays both sides) is regarded as a time step, and there is a heuristic reward signal sent to the agent after each step and at the end of each game. The agent learns to predict the best move by adjusting the prediction at each time step to make it more closely match the prediction at the next time step. It is the difference between successive predictions which is the only measure of error, and the program is never explicitly instructed as to what is the best move.
Gerald Tesauro, an IBM researcher, is responsible for pioneering TD techniques with backgammon. His program, TD-Gammon, was developed after abandoning experiments with a supervised learning program called Neurogammon, in which the good and bad moves were hard-coded.
Neurogammon never reached an expert level of play, whereas TD-Gammon went on improving for 1,500,000 games and became a world-class player. Readers with long memories may recall that a version of TD-Gammon was included in the 1996 Family Funpak for OS2/Warp.
The next commercially available neural net program came in 1998, in the form of Fredrik Dahls Jellyfish, and this was soon followed by Olivier Eggers Snowie. The current version of Snowie is regarded as the state of the art in terms of its playing skills and analysis tools, and it is priced accordingly.
However, there is a free alternative in the form of GNU Backgammon. This was the brainchild of Gary Wong, who by 1999 had drawn on the work of Tesauro and others to produce a neural net backgammon player called Costello.
He donated his code to the GNU Project, and GNU Backgammon (as it became known) is still under development. It plays an extremely strong game and has not stopped learning.
A version of it plays on the First Internet Backgammon Server (FIBS), where it ranks in the top 20 of over 6,000 players.
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