Author: Tom Likens
Date: 19:53:47 02/10/03
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On February 10, 2003 at 21:32:04, John Wentworth2 wrote: >Sorry for the stupid question but what the heck is Fail low and Fail High ? Hello John, No stupid questions, we all have to start somewhere. This explanation assumes a standard negamax search with initial bounds of <alpha, beta>, which is usually +/- some small epsilon around a presumed good score. A fail-high occurs when the value returned by the search is larger than beta. This indicates that the true value of the search is greater than the upper bound we initially supplied. We don't know how much better, just that it is better. It's usually OK to play this move if you are out of time. A fail-low is the converse situation. It indicates that the true value of the position is less than the lower bound we set (i.e. alpha). This can be trouble, because the search is telling us that it sees a problem, but it doesn't know know the true extent of the problem. Usually, this situation needs to be resolved. Most programs will spend extra search time trying to obtain the real score and hopefully find a better move. This can happen at the root if the program is searching the initial position with a +/- window around a presumed valid score instead of <-INF,+INF> (the so-called "aspiration search"). You could avoid this problem altogether by using <-INF,+INF> as your initial alpha/beta bound, but the total search time on average would take longer. regards, --tom
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