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Subject: Re: aspiration search question

Author: Vasik Rajlich

Date: 02:37:35 05/13/04

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On May 13, 2004 at 05:22:41, Daniel Shawul wrote:

>On May 13, 2004 at 05:13:10, José Carlos wrote:
>
>>On May 13, 2004 at 04:56:23, Daniel Shawul wrote:
>>
>>>On May 13, 2004 at 04:20:02, José Carlos wrote:
>>>
>>>>On May 13, 2004 at 03:35:00, Daniel Shawul wrote:
>>>>
>>>>>Hello
>>>>>
>>>>>My search is fail soft(i return the actual score)
>>>>>when i fail high at the root i widen the window by 3 pawns (300).
>>>>>
>>>>>                 if(score<=r_alpha)
>>>>>		 {
>>>>>			  r_beta=r_alpha;
>>>>>			  r_alpha=score-300; //r_alpha = -MATESCORE;
>>>>>		 }
>>>>>		 else if(score>=r_beta)
>>>>>		 {
>>>>>			  r_alpha=r_beta;
>>>>>			  r_beta=score + 300; //r_beta = MATESCORE;
>>>>>		 }
>>>>>If the search fail's high at 1.75 score but the real score was 10
>>>>>i get a score of 4.75 (1.75 + 3) in the next iteration. Why? my search is fail
>>>>>soft and the score returned should be independent of beta. If i change the 200
>>>>>to 300,score returned is 4.75??
>>>
>>>   a correction (1.75 + 2) = 3.75
>>>
>>>>
>>>>  Are you sure you're doing fail soft? I mean, you need some extra logic, like
>>>>starting off with -INFINITE in alpha nodes and increase the best score even in
>>>>fail lows. You also need to make sure of returning true bounds in qsearch. Also,
>>>>if you do some kind of forward prunning depeding on alpha and beta you won't be
>>>>able to return true scores.
>>>
>>>  i do futility pruning and other purnings but i always return
>>>   (score + margin) in all of the cases. Not alpha.
>>>
>>>for example in the following test position
>>>        rnb2rk1/ppq2p1p/4p1p1/3pP1B1/3P1Q2/2b2N2/P1P2PPP/2RK1B1R w - - 8 14
>>>
>>> 1&         34   -0.31    0.05  1. Qxf7+?
>>> 1          36   -0.10    0.05  1. Bd3
>>> 2         138   -0.44    0.09  1. Bb5
>>> 2         172   -0.24    0.09  1. Bf6
>>> 3         447    0.10    0.14  1. Bf6 Qa5
>>> 3         679    0.16    0.16  1. Bh6!
>>> 3         781    0.18    0.16  1. Bh6 Rd8
>>> 4        1899    0.15    0.20  1. Bh6 Rd8 2. Qf6 Nc6
>>> 5        3778    0.47    0.27  1. Bh6 Re8 2. Qf6
>>> 6       10330    0.71    0.33  1. Bh6 Rd8 2. Qf6
>>> 7       26798    1.06    0.44  1. Bh6!
>>> 7       86808    1.48    0.63  1. Bh6 Qe7 2. Bxf8 Qxf8 3. Qf6 Nd7
>>> 8      208184    1.20    0.95  1. Bh6 Qe7 2. Bxf8 Qxf8 3. Bd3 Nd7
>>>                                4. Ng5 Qg7
>>> 9      684592    1.40    2.27  1. Bh6 Nd7 2. Ng5 a6 3. f3
>>>10     3253930    1.42    9.00  1. Bh6 Qe7 2. Qe3 Qb4 3. Bxf8 Kxf8
>>>                                4. Qh6+ Kg8 5. Bd3 f6
>>>10     5270072    1.75   15.05  1. Bf6!
>>>10    11120355    3.75   33.33  1. Bf6!     //here score is 1.75+2=3.75
>>>                            //full window opened
>>>10    32259429   12.82   87.59  1. Bf6 Qxe5 2. Nxe5 Nd7 3. Nxd7 Bd2
>>>                                4. Kxd2 Bxd7 5. c4
>>>11&   32784106   12.47   89.17  1. Bf6? Qxe5
>>>
>>>   if i change margin to 3,the result will be something like this
>>>10    11120355    4.75   33.33  1. Bf6!
>>>10    32259429   12.82   87.59  1. Bf6 Qxe5 2. Nxe5 Nd7 3. Nxd7 Bd2
>>>                                4. Kxd2 Bxd7 5. c4
>>>11&   32784106   12.47   89.17  1. Bf6? Qxe5
>>>
>>>   I am very very sure i don't return alpha/beta anywhere in my search.
>>>I also tried turning off hashtable ,nullmove,iid etc but no success.
>>>
>>>best
>>>daniel
>>
>>  I don't find the effect you present here any strange. I can be caused by many
>>things:
>>  - Null move: you can cutoff with a much lower score than true score cause you
>>allow the opponent two moves in a row.
>>  - Hashing: cutting off by a stored bound from a different depth
>>  - Move ordering: if you search moves a, b and c with scores +1, +2, +10 you'll
>>cutoff with beta <= +1 in the first move, with beta <= +2 in the second.
>>  - Forward pruning: if score > beta + margin and (some conditions) return(beta
>>+ margin) or return(beta)-not true score-.
>>  - Lazy cutoffs
>>  Etc...
>
>    I do all of the things you said above but there is no return (alpha) or
>return (beta) in my search. The search somehow converges to beta. What i store
>in hashtables is score. Infact i tried turning off hashtable, null move ,  and
>pruning techniques but it all comes back to beta.
>

Yes, this is normal, and it's the reason why MTD (f) is a reasonable alternative
to PVS. The search is really good at doing the minimum amount of work.

Just glancing at the logs from my last run (I use MTD (f)), I see that 100% of
the fail-lows at the maximum depth returned the exact bounds value, and 73.3% of
the fail-highs did so.

These numbers are normal. It's also normal for soft fail-highs to be softer than
soft fail-lows, even at big remaining depths. (It's obvious why it should be so
at remaining depth == 0, but it also holds true throughout the search.)

Vas

>
>>
>>  If all of that is disabled, then make sure you have something like (simplified
>>search, and also in qsearch):
>>
>>search(alpha,beta,depth)
>>{
>>  int best = -INFINITY; // Make sure you start with -INF, not with alpha
>>  int score;
>>
>>  while(all moves)
>>  {
>>    score = -search();
>>    if (score >= beta)
>>      return(score);
>>    if (score >= best)
>>      best = score; // update this even when score <= alpha
>>  }
>>
>>  return(best)
>>}
>
>    That is exactly what i do in my search.
>
>>
>>  José C.



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