Author: Robert Hyatt
Date: 07:20:31 12/23/00
Go up one level in this thread
On December 23, 2000 at 00:17:54, Uri Blass wrote: >On December 22, 2000 at 22:40:27, Robert Hyatt wrote: > >>On December 22, 2000 at 15:52:19, Ernst A. Heinz wrote: >> >>>>On December 22, 2000 at 14:45:03, Ernst A. Heinz wrote: >>>> >>>>>>Did you look at what happens if you play a4? Your kingside gets totally >>>>>>shredded, starting with Qxf7+... >>>>> >>>>>Yes, true -- but then Black's King is not standing there! >>>>> >>>>>It rather seems quite safe on d8 while White's King gets >>>>>into mighty trouble on d1 after the sequence a4 - Qxf7+ - >>>>>Kd8 - Qxg7: >>>>> >>>>>[d]2rk3r/6Q1/4p3/1pqpP3/p3b1PP/1B6/PPP2R1R/2K5 b - - 0 1 >>>>> >>>>>"DarkThought" quickly scores this as substantially positive >>>>>for Black, locking onto Qe3+ as the best move almost instantly. >>>>> >>>>>=Ernst= >>>> >>>>I have no doubt that black might actually be winning here. But that wasn't >>>>the point. You said you picked a4 at depth=10. For that to happen, you have >>>>to ignore king safety and sit in the middle of the board, with a queen at f7, >>>>a rook on the open file, the king rook hanging, the king can't move to connect >>>>the rooks, etc. >>>> >>>>IE at 10 ply it is all judgement, not "truth" as the search probably can't see >>>>the final outcome. >>> >>>"DarkThought" gets a fail-low on 0-0 in iteration #10 with the >>>score dropping from +1.1 in iteration #9 to just +0.39. Then, >>>it locks onto a4 and shows the correct PV for it already in >>>iteration #10 (with the white King being driven to d1 where it >>>is similarly exposed as the black King on d8). >>> >>>So, why should that mean to ignore king safety or all the other >>>evaluation stuff you are talking about above? >>> >>>To me, it simply looks like our search picks up the crucial >>>threats earlier (maybe, by means of more appropriate extensions >>>in this particular case) -- no more, no less. >>> >>>=Ernst= >> >> >>Note that I am not saying your eval is bad, your search is bad, nor am I saying >>mine is right. I am simply saying that in _this_ position, your program is >>probably not seeing the "truth" any more than any of the other programs that >>have been tested on this position see it. It appears, to me, that it becomes >>a matter of how the evaluation is tuned. IE you fail low on castling at depth >>10. Crafty likes Rc7 until depth=12 or so where it fails _high_ on castling. > >I can add that Crafty can see nothing wrong with 0-0 even at depth 15 after >hours. > > >I also cannot see something wrong after 0-0 with other programs. > >a4 seems to be even better than 0-0 but I do not see a reason that 0-0 should >fail low. > >Uri Failing low doesn't always mean losing material, or getting stomped. It sometimes simply means that the program can see deeply enough to trigger some eval term that is large enough to take the score outside the alpha/beta window. IE I use aspiration search in Crafty to augment PVS. I set the alpha/beta window for the current search centered around the alpha/beta window for the previous search, and correct for the odd/even score swings. Sometimes this turns into a fail high or fail low where the score only changes by .15 or some such small number... And of course, sometimes the score changes by a whole pawn, even though no material is getting lost.
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