Author: Robert Hyatt
Date: 07:07:36 01/23/00
Go up one level in this thread
On January 22, 2000 at 20:43:22, Peter McKenzie wrote: >On January 22, 2000 at 17:40:10, Robert Hyatt wrote: > >>On January 22, 2000 at 03:12:53, David Blackman wrote: >> >>>On January 21, 2000 at 11:31:06, Robert Hyatt wrote: >>> >>>>The solution to the horizon effect is depth. And extensions. The most common >>>>horizon effect type of move is a check, which constrains the opponent to react >>>>to the check, and removes 2 plies from the depth. Extend on the check and you >>>>cut the loss by 1 ply. A capture/recapture is the next most likely cause, as >>>>a capture must either be followed by the recapture, a different capture to >>>>maintain material balance, or a check. Again you lose 2 plies. And extending >>>>on a capture/recapture pair will recover one of those plies. The capture, >>>>check, get out of check, recapture group of moves is harder of course... >>> >>>Yes. You need extensions. Depth won't solve it on its own, but will make the >>>program stronger. >>> >>>Recently i have tried a program that usually gets to 14 ply fairly quickly, but >>>has almost no extensions. It plays reasonably well most of the time, but a >>>couple of times i've seen it make obvious tactical errors. I mean obvious to me, >>>without computer assistance, and i'm a 1500 player. >>> >>>Careful analysis of the positions showed it was the horizon effect. A simple >>>tactic of 3 to 6 plies apparent depth was being missed because the computer >>>could play a series of meaningless and perhaps slightly bad delaying moves to >>>push the problem out past 14 plies so it couldn't see the main tactic. >> >> >>reaching a depth of 14 plies should hide most horizon effect problems from any >>but the very strong tactical players. But getting to 14 plies sounds impossible >>for a primitive program, without some sort of selectiveness... and _that_ will >>certainly cause tactical oversights... > >I think that 14 ply using nullmove pruning (R=2) and no extensions (not even >check) and no fancy quiescence would still be prone to some quite bad tactical >mistakes. > >Take the following classic type of position for example: > [D]5k2/1p5r/3pp3/p2p4/1b1P1P2/qP1Q1NP1/P1P3N1/1K6 w - - > >Its obvious to a human that after Qxh7?? Bc3, white will be mated. The tricky >thing for a program is to wade thru. the mindless checks by the white queen, and >of course the null movers have a little trouble with the Qb2# threat. > >I think a vanilla 14ply null mover without any extensions would have a tough >time avoiding Qxh7 though - even at depth 14. I think a 14 ply search will see this. It takes Crafty 10 plies to see Qxh7 drop to draw, and it drops every move after that. At depth=10 Crafty switches to something else. I would expect a 14 ply search to get a 'sniff' of this... And yes, some positions with no extensions will look ugly... but think of all the good positional things a 14 ply search would see..
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