Author: Omid David Tabibi
Date: 09:03:37 10/12/03
Go up one level in this thread
On October 12, 2003 at 11:45:07, Tord Romstad wrote:
>On October 12, 2003 at 10:23:35, Omid David Tabibi wrote:
>
>>[D]6k1/5p2/3P2p1/7n/3QPP2/7q/r2N3P/6RK b - - 0 1
>>
>>If you do checks everywhere in quiescence, you should see this immediately.
>
>If I did *all* checks everywhere in qsearch, I should see it instantly, yes.
>But as you
>remarked in the article at the beginning of this thread, this is too expensive.
>Beyond
>the first ply of qsearch, I have very strong restrictions about when and which
>checks
>to generate.
>
>>After 1...Rxd2 2.Qxd2 all the rest of the moves are checks until you detect draw
>>by threefold repetition (maybe you've turned off repetition detection in
>>quiescence? or your max extensions limit is too shallow...). HIARCS finds the
>>move at the first iteration!
>
>I do repetition detection in quiescence, and I have no max extension limit.
>Looking
>closer on the position in question, it does seem a bit strange that I need such
>a long
>time to find the solution. The combination is not very deep. Perhaps I have a
>bug
>somewhere -- it's worth a closer look.
>
>>Falcon doesn't manage to solve number 12 either.
>
>Number 12 is very hard. But even solving number 10 and 11 in less than a
>second
>is very impressive, IMHO.
>
>>>You must have a very inefficient way of generating checks, I think.
>>
>>That's true. Only recently I added checks in quiescence to the engine, and so
>>still haven't written a gen_checks() functions. However, the kind of attack
>>tables I use result in a very speedy generation of captures, which results in a
>>very optimized captures only quiescence. Adding checking moves will slow down
>>the engine considerably anyway, even if I write a good gen_checks()...
>
>I am not so sure about that. Most of what you write above applies to my engine,
>too. The attack tables are useful when generating checks, too. And of course
>you
>do not generate checks before you have generated and searched all captures and
>they all failed low.
>
>>One thing I have to mention is that in the normal version I never check for
>>check evasions in quiescence. If the side to move is in check and doesn't have
>>any legal non-losing capture, I just return eval(). That's another reason why
>>the normal quiescence is so fast.
>
>This sounds extremely dangerous to me -- doesn't this imply that you will not
>always detect mates in qsearch?
No checkmate can possibly take place at the first ply of quiescence, since I do
the following in the main search:
...
makemove(move);
if (other side is in check)
extension += 1;
call_depth = depth - 1 + extension;
if (call_depth > 0)
score = -search(..., call_depth);
else
score = -quiescence(...);
...
So if the other side is in check the depth will be extended instead of calling
quiescence.
But within the quiescence no checkmate can be detected in the normal version.
>And doesn't this cause too many tactical mistakes?
It to causes problems mainly in null-move pruning. Assume you are at depth = 3
and use R = 2. Your calling depth is 3-2-1=0, i.e., you directly call quiescence
(after doing a null-move). Now it is the opponent's turn who checkmates you in
the first ply of quiescence. Using checks in quiescence the checkmate will be
detected, which will trigger a mate threat extension in the main search.
Otherwise (in the normal version) just eval() is returned and assuming it is
above beta we have a fail-high. All good and nice we are sure that our position
is good enough to justify a cutoff, while in fact we are mate in 1!
That's the main reason why checks in the first ply of quiescence contribute so
much to tactical strength.
>
>>>It seems like checks in the qsearch is one of those things that works well in
>>>some
>>>programs, and not in others. Crafty, for instance, seems to do very well
>>>without
>>>any checks whatsoever,
>>
>>I wouldn't say so from a tactical point of view. Whenever the game turned
>>tactical, Crafty didn't have any chance against Falcon with checks in
>>quiescence. But Crafty did search deeper and played a better positional game. I
>>must also add that Falcon uses a huge number of different extensions (I think
>>only HIARCS has more extensions), and so maybe adding checks in quiescence on
>>top of them all isn't such a good idea...
>
>Very interesting.
>
>>>but for me the results without checks are clearly worse.
>>>
>>>Other ideas that I have never been able to make work are recapture extensions
>>>and
>>>all sorts of nullmove pruning except plain R=3 (R=2, R=2.5, adaptive pruning and
>>>verified
>>>nullmove pruning are all clearly worse for me).
>>
>>In Falcon I conducted all the experiments I conducted on Genesis for the paper
>>verified null-move pruning, and got the same results. Plain R=3 was too risky
>>neglecting many tactical shots. I now use a modified version of verified
>>null-move pruning.
>>
>>But maybe plain R=3 didn't work for me because I didn't have checks in
>>quiescence, and so it resulted in a very inaccurate search. The only program
>>I've heard which uses plain R=3 is DIEP, which does conduct checks everywhere in
>>quiescence.
>
>This is very possible. I have experimented with values of R below 3, and with a
>minimalistic qsearch without checks, but never in combination. Probably yet
>another thing I should try ...
>
>Tord
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