Author: Peter McKenzie
Date: 11:06:46 01/23/00
Go up one level in this thread
On January 23, 2000 at 10:07:36, Robert Hyatt wrote: >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... What is the 10ply PV of crafty here? How many checks does it contain? If it contains more than 4 checks, doesn't that mean a 14ply search without check extensions wouldn't find it? Also, are you using a mate threat extension? > >And yes, some positions with no extensions will look ugly... but think of all >the good positional things a 14 ply search would see.. Yes, I think I'd like my program to be able to see 14ply :-)
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