Author: David Eppstein
Date: 09:33:42 08/03/99
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On August 03, 1999 at 10:24:34, Robert Hyatt wrote: >I can only speak for Crafty, but it spends more time on the move it is going >to play than it does on all the other moves combined... Ok, so here is another stupid idea to try to speed up the obvious moves. The reason you spend so much more search time on the part of the tree coming from the correct move is that you have to exactly resolve its search value, while alpha-beta lets you quickly get inexact cutoffs on the other moves. BUT, to play a good move, you don't really need to exactly resolve its value, you just need to know that it's better than the other moves. So, suppose you have a move that looks obvious, in that a shallow search gives it an evaluation of x while all other moves have an evaluation of y, y much smaller than x. Then if you see this sitution, do some deeper searches with a zero-width window at (x+y)/2. If the "obvious" move always fails high and the nonobvious ones fail low, then that's the move you should make. If something else happens, then you need to do a real alpha-beta search. Since all searches are zero-window with the window far away from the true evaluation of the line, they should all be fast, so you should be able to spend less time on them than you would with full alpha-beta to get to equal depth. One drawback of this approach, of course, is that you end up with no idea what the PV should be, but who cares about that if it plays the right moves?
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