Author: Robert Hyatt
Date: 11:01:42 09/10/99
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As promised, here is some data. The 'old' column is the standard released version of crafty 16.18. Which on positions where all moves have a score <= alpha, _no_ move is stored in the hash table, although the bound is stored for later use. 'new' uses the simple trick mentioned by Ed, namely setting best to -inf, then raising it every time a score comes back that is higher, and also remembering the move that went with that score. That move is now stored in the hash table when this node has been finished. I ran 3 kopec positions, three that aren't tactically motivated, and two that are (4/5) plus wac 230 which is a deep tactical shot that combines both types of themes... Until it finds the deep shot, it appears to be a pure positional problem, then suddenly at depth=16 it finds that Rxb4 fails high along a precise tactical line. So three normal type positions, three tactical positions. pos old new 1 4.897M 5.018M (kopec 2, 11 plies) 2 10.713M 13.563M (kopec 3, 11 plies) 3 14.739M 15.175M (kopec 4, 11 plies) 4 25.478M 30.032M (kopec 5, 11 plies) 5 9.272M 9.083M (kopec 22, 12 plies) 6 39.971M 41.749M (wac 230, 16 plies) The results seem quite clear to me. Trying to figure out a 'best move' at fail-low nodes doesn't work. Using such moves before the capture search blows the tree up. Sometimes only a little, other times drastically. As I said, I don't see _any_ way to reliably find the best move at a fail low position. you might think this trick works, but it doesn't. This is just a fact of life with alpha/beta. The search returns _bounds_ that can't be compared to each other to say that "just because this move has a bound <= 200, while this move has a bound <=250, clearly the bound <= 250 move is better. Alpha/Beta just doesn't work that way. Minimax will. But trying this made the trees larger in every case but 1, at least for me. I'll be happy to test any other positions anyone suggests, of course.
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