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Subject: Re: aspiration search question

Author: José Carlos

Date: 02:13:10 05/13/04

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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...

  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)
}

  José C.



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