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Subject: Re: mate threat extension/null move

Author: Stuart Cracraft

Date: 21:01:15 09/27/04

Go up one level in this thread


On September 27, 2004 at 18:17:19, Robert Hyatt wrote:

>On September 25, 2004 at 16:35:05, Stuart Cracraft wrote:
>
>>On September 25, 2004 at 16:02:08, Andrew Platt wrote:
>>
>>>On September 25, 2004 at 10:30:52, Rick Bischoff wrote:
>>>
>>>>Hi,
>>>>
>>>>I am confused about the mate threat extension.  I have the following code:
>>>>
>>>>search ( depth, alpha, beta )
>>>>
>>>>->check extension
>>>>->else pawn extension
>>>>
>>>>->if depth <= 0 do the queiscent
>>>>
>>>>->if checkReps return draw
>>>>->if insufficnetMaterial return draw
>>>>
>>>>->Probe the hash table and return if good
>>>>
>>>>->if not in check and depth>3 and last move was not null and material is ok:
>>>>->-> do null move
>>>>->-> x = -search(depth - 3, -beta, -beta +1 )
>>>>->-> undo null move
>>>>->-> if ( x >= beta ) return beta
>>>>->-> if ( x < (-MATE+MAXPLY) ) cout << "found mate"
>>>>
>>>>->normal alpha/beta stuff
>>>>
>>>>end search
>>>>
>>>>"found mate" never gets printed out in WAC141 unless I change the null move to a
>>>>full width search... Why?  Well, I shouldn't say never, but I let it run to ply
>>>>9.
>>>
>>>if (x >= beta) return beta
>>>
>>>means that you don't return the actual scores, just the cutoff. This is fine
>>>until you try to detect threats. Then you need to return the actual scores
>>>because only those will have the mate in them.
>>>
>>>In WAC 141 you should hit a bunch of mate threat extensions on the way to the
>>>lowest ply because each time your Rooks take a pawn, and then the bishop, you
>>>should trigger one. In my search it still isn't enough to counteract the drop
>>>into qsearch where I don't consider checks. I've given up on generating the mate
>>>score in a short time right now (I have plenty of other problems!). It's enough
>>>that the first few plies in the PV are correct so if it actually hit it, it
>>>would find it. Of course that only works here because it turns out that the
>>>Queen sacrifice is good more material as well as mate!
>>>
>>>Andy.
>>
>>So do you solve WAC 141 in a reasonable amount of time?
>>I don't and my code returns all values, does a null move
>>with -beta,-alpha, checks if the returning value is
>>equal to -MATE+ply+2 to extend (but doesn't extend it if
>>already in check), and I return -MATE+ply when in check
>>in the main search. In quiescence, I pass back to the main
>>search if in check, but I don't search checking moves unless
>>they are captures.
>>
>>Anyway, with the above, WAC 141 is out of reach and no one
>>has been able to help it!
>
>
>If you do mate threat right, you should find this quickly.  Here is Crafty on a
>single-CPU 2.8ghz PIV:
>
>                8->   1.58  -1.20   1. Kf1 a5 2. Rxh5 gxh5 3. Rxh5 Re1+
>                                    4. Kxe1 Nxh5 5. Qg5+ Kf8 6. Qh6+ Kg8
>                                    7. Qxh5
>                9     3.91  -1.20   1. Kf1 a5 2. Rxh5 gxh5 3. Rxh5 Re1+
>                                    4. Kxe1 Nxh5 5. Qg5+ Kf8 6. Qh6+ Kg8
>                                    7. Qxh5
>                9     4.38     +1   1. Qxf4!!
>                9     4.63     +3   1. Qxf4!!
>                9     5.11     +M   1. Qxf4!!
>                9    49.31  Mat06   1. Qxf4 Bxf4 2. Rxh5 gxh5 3. Rxh5 Bh6
>                                    4. Rxh6 Qh2+ 5. Kxh2 Kf8 6. Rh8#
>                9->  49.31  Mat06   1. Qxf4 Bxf4 2. Rxh5 gxh5 3. Rxh5 Bh6
>                                    4. Rxh6 Qh2+ 5. Kxh2 Kf8 6. Rh8#
>
>4 secs to find it.  49 secs to produce the mate score.
>
>With mate threat extension disabled (ext/mate=0 command) I see this:
>
>               10->  20.95  -1.31   1. Kf1 Kf8 2. Rxh5 gxh5 3. Rxh5 Re1+
>                                    4. Kxe1 Nxh5 5. Qh6+ Ke8 6. Qxh5 Bf4
>               11    40.48  -1.08   1. Kf1 a5 2. Rxh5 gxh5 3. Rxh5 Re1+
>                                    4. Kxe1 Nxh5 5. Qg5+ Kf8 6. Qxh5 Bb4+
>                                    7. Kd1 Qd6
>               11    43.03     +1   1. Qxf4!!
>               11    43.94     +3   1. Qxf4!!
>               11    52.35     +M   1. Qxf4!!
>               11     5:12  Mat06   1. Qxf4 Bxf4 2. Rxh5 gxh5 3. Rxh5 Bh6
>                                    4. Rxh6 Qh2+ 5. Kxh2 Kf8 6. Rh8#
>               11->   5:12  Mat06   1. Qxf4 Bxf4 2. Rxh5 gxh5 3. Rxh5 Bh6
>                                    4. Rxh6 Qh2+ 5. Kxh2 Kf8 6. Rh8#
>
>
>SO it finds the right move 2 plies quicker, 10X faster, with than without the
>mate threat extension...
>
>This with a simple q-search with no checks, no check evasion or anything else..

I obviously don't do mate threat right.

Taking the result of the null move with -beta,-beta+1 or -beta,-alpha, or
MINIMUM_INT,MAXIMUM_INT, and checking that result if < beta for
== -MATE+ply+2 and extending by 1 obviously is not the right way to
do mate threat. I tried endless variations of the above and nothing
speeds up WAC 141 over my 95 second solution on a 1ghz P3 so I'll
let it sit at this unless anyone can see through all that. I think I've
been fairly straightforward about not having been able to get this
mate threat working right. I know it is in various people's code
but I find deciphering another chess program often doesn't give the
insight I had thought and that silent suffering with my own code
is "where it's at".

Stuart



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