Author: Slater Wold
Date: 10:47:39 02/05/04
Go up one level in this thread
On February 05, 2004 at 13:24:01, Robert Hyatt wrote: >On February 05, 2004 at 12:54:08, Slater Wold wrote: > >>On February 05, 2004 at 12:50:44, Robert Hyatt wrote: >> >>>On February 05, 2004 at 12:46:04, Slater Wold wrote: >>> >>>>On February 05, 2004 at 12:22:44, Bob Durrett wrote: >>>> >>>>>Referenced by: >>>>> >>>>>http://www.talkchess.com/forums/1/message.html?345569 >>>>> >>>>>> An alpha cutoff is what happens when you search the second move, >>>>>>> and you prove that if you play that move, your opponent has a move >>>>>>> he can play that will produce a score less than your "lower bound" >>>>>>> you established for the first move. There is no need to search >>>>>>> further. >>>>>>> >>>>>>> For example, after that +1 on the first move, you try the second >>>>>>> move and after trying the first move the opponent has in reply to >>>>>>> that move, you discover you _lose_ a pawn. The score is -1.0... >>>>>>> There is no need to search other opponent moves to produce a >>>>>>> score even lower than -1.00, because you already know this move >>>>>>> is at _least_ -1.00 and possibly worse, while the first move is >>>>>>> +1.00. You stop searching this move and move on to your third >>>>>>> choice... >>>> >>>>I haven't looked at many programs, other than TSCP, source, but I have a few >>>>question about this also... >>>> >>>>When do most engines call the qsearch? After selecting a move believed to be >>>>correct, of at the end of each search tree? >>> >>>When remaining depth = zero. IE if you want to do a normal 3 ply search, and >>>you have no search extensions, I make a move at the root, subtract 1 from depth, >>>and recursively call search. There I make a move for the opponent, subtract 1 >>>from depth and if it is > 0 (2-1 > 0 so true here) I again call search, >>>otherwise I call the q-search. At ply=3 I make a move for the program again, >>>subtract one from depth and now call the q-search since depth==0 after the last >>>subtraction. >>> >>>I did 3 full plies of looking at everything, but ply-4 and beyond are captures >>>only. >>> >>>> >>>>The reason I ask is, say you get a cutoff, because the 2nd move produced -1.0 >>>>(as in above), but after 4 checks and a capture, you regain that pawn & better >>>>position. >>> >>>Alpha/Beta is called a "depth-first" search strategy, because you _never_ stop >>>searching before you reach a tip position, therefore in the above you regain the >>>material before reaching the tip. >> >> >>So the tip, is where captures and checks stop? > > >tip or more commonly "leaf"/"leaves". Yes Got'cha. Thanks for clearing that up. I think I remember seeing in TSCP source, to extend on checks, but not captures. But then again, I could be mistaken. I never knew extending on captures and checks was a 'part' of Alpha/Beta. Makes sense though. I guess I always took it for granted that was qsearch's function. But I can see now, where that wouldn't be real helpful, since you don't call eval() after qsearch. :) How much time in Crafty is spent in qsearch? Do you do captures or captures & checks? Thanks again Bob!
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