Author: Alessandro Damiani
Date: 13:21:48 09/10/99
Go up one level in this thread
On September 10, 1999 at 15:58:45, Ed Schröder wrote:
>On September 10, 1999 at 13:17:57, Robert Hyatt wrote:
>
>>On September 10, 1999 at 11:29:04, Alessandro Damiani wrote:
>>
>>>On September 10, 1999 at 09:36:51, Robert Hyatt wrote:
>>>
>>>>On September 10, 1999 at 08:01:35, Alessandro Damiani wrote:
>>>>
>>>>>On September 10, 1999 at 07:48:44, Ed Schröder wrote:
>>>>>
>>>>>>On September 10, 1999 at 00:19:37, Robert Hyatt wrote:
>>>>>>
>>>>>>>Here is an interesting position given to me by Steffen Jakob:
>>>>>>>
>>>>>>> /p/P5p/7p/7P/4kpK/// w
>>>>>>>
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> 8 | | | | | | | | |
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> 7 | *P| | | | | | | |
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> 6 | P | | | | | | *P| |
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> 5 | | | | | | | | *P|
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> 4 | | | | | | | | P |
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> 3 | | | | | *K| *P| K | |
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> 2 | | | | | | | | |
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> 1 | | | | | | | | |
>>>>>>> +---+---+---+---+---+---+---+---+
>>>>>>> a b c d e f g h
>>>>>>>
>>>>>>>
>>>>>>>Obviously black is getting crushed. He has one move, Kh3, which leads to a
>>>>>>>mate in 6. Steffen asked me to try this and Crafty found a mate in 4, which
>>>>>>>doesn't exist. I spent the entire day debugging this thing and here is what
>>>>>>>I found:
>>>>>>>
>>>>>>>If you recall the discussion here a couple of weeks ago, I reported that I store
>>>>>>>absolute mate scores (EXACT scores) in the hash table, and that I adjust them
>>>>>>>so that they are always stored as "mate in N from the current position". This
>>>>>>>has always worked flawlessly for me, and still does.
>>>>>>>
>>>>>>>For bounds, I once tried adjusting the bounds as well, but found quirks, and
>>>>>>>left them alone. Wrong answer. To fix this mate in 4 problem, I decided to
>>>>>>>adjust the bounds as well, but I now set any bound value that is larger than
>>>>>>>MATE-300, by reducing it to exactly MATE-300, but still using the "LOWER"
>>>>>>>flag to say that this is the lowest value this position could have. For bound
>>>>>>>values < -MATE+300, I set them to exactly -MATE+300 and leave the flag as is.
>>>>>>>
>>>>>>>This position is cute. Because not only is it a mate in 6, but there are
>>>>>>>transpositions that lead to mate in 7, mate in 8, and there are shorter (but
>>>>>>>non-forced) mates in 4 and 5. And there are stalemates, and positions with
>>>>>>>1 legal move, and so forth.
>>>>>>>
>>>>>>>You ought to find the following variation as one mate in 6:
>>>>>>>
>>>>>>>Kh3, f2, Kg2, Ke2, Kg3, f1=Q, Kh2, g5, hg, Kf3, g6, Qg2#
>>>>>>>
>>>>>>>If you find a shorter mate, it is wrong. If you find a longer mate, you
>>>>>>>are probably just extending like mad on checks (crafty finds a mate in 8 at
>>>>>>>shallow depths (9 plies, 2 secs on my PII/300 notebook), and doesn't find the
>>>>>>>mate in 6 until depth 10, 3 seconds.
>>>>>>>
>>>>>>>It is a good test as the transpositions are real cute with white's king caught
>>>>>>>in a tiny box, but with several different moves that triangulate and transpose
>>>>>>>into other variations...
>>>>>>>
>>>>>>>If you get it right, you have either handled the bounds right, or else you are
>>>>>>>very lucky. IE Crafty 16.17 gets this dead right. But if I disable the eval,
>>>>>>>it goes bananas, yet the eval is not important when mate is possible.
>>>>>>>
>>>>>>>Have fun...
>>>>>>>
>>>>>>>I did... :)
>>>>>>
>>>>>>A simple solution: do not store a position in the hash table if there is
>>>>>>no best-move. It solves the mate-cases and also repetition cases. Also
>>>>>>there is no speed loss of the search.
>>>>>>
>>>>>>Ed
>>>>>
>>>>>Do you mean by "no best-move"
>>>>> bestmove == 0
>>>>>or
>>>>> best<=alpha, after having searched all moves (best: minimax score)?
>>>>>
>>>>>What I do:
>>>>> if bestmove == 0 then don't store anything, just return the score (mate or
>>>>> stalemate).
>>>>>
>>>>>Alessandro
>>>>
>>>>
>>>>that doesn't make sense to me. If _every_ move at one node in the tree returns
>>>>alpha for the score, which is the best move? And since you don't have one, you
>>>>don't store anything? That hurts performance, because the next time you
>>>>encounter this position, you get to search it again, while I discover that the
>>>>last time I searched it I returned alpha, so I can just do that now and not
>>>>search anything...
>>>
>>>No, no. My answer was misleading. What I mean is explained by the following code
>>>(the code is simpilied!). I have marked the important things by an "****". It is
>>>assumed that
>>> - when the king is removed from board its position is -1 ( < 0)
>>> - alpha, beta < INF
>>>
>>>Alessandro
>>>
>>>int AlphaBeta (int alpha, int beta, int depth) {
>>>
>>>//**************************************
>>>// legality check:
>>>
>>> if (myKingSquare<0) return -INF;
>>>
>>>//**************************************
>>>
>>> if (depth==0) return Quiescence(alpha,beta);
>>>
>>> // here use info from the transposition table
>>>
>>> best= -INF; bestmove= 0; startalpha= alpha;
>>> i= 0; n= GenMoves();
>>> while (i!=n && best<beta) {
>>> // m[i] is the current move
>>>
>>> make(m[i]);
>>> value= -AlphaBeta(-beta,-alpha,depth-1);
>>> unmake(m[i]);
>>>
>>> if (value>best) {
>>> best= value; bestmove= m[i];
>>> if (best>alpha) alpha= best;
>>> };
>>> i++;
>>> };
>>>
>>>//**********************************************
>>>// no best move => mate or stalemate
>>>
>>> if (bestmove==0) {
>>> if InCheck(Me) return -MATE+ply;
>>> return STALEMATE;
>>> };
>>>
>>>//**********************************************
>>>
>>> // here update the transposition table
>>>
>>> return best;
>>>}
>>
>>
>>Same question as before. The above simply doesn't work as you think it
>>does. Here is why.
>>
>>at ply=N you set best to -inf, and then step thru each move and do a search
>>after making it. And you pass that search a value for alpha and beta that is
>>used to terminate the search when it can prove that the score below that move
>>is >= beta (which at our ply=N node means the move we tried is <= alpha.)
>>
>>So lets assume that after we search the first move, we get a score back that
>>is obviously > -infinity, but < alpha. You remember that move as "best". But
>>the problem here is that the 'proof' search stopped as soon as it found a score
>>> beta. It didn't try _all_ moves to get the largest score > beta, just the
>>first score > beta... which is why we refer to the search as returning a bound.
>>At least as low, but maybe even lower.
>>
>>So you end up with a bunch of random bounds that are all <= alpha, and you take
>>the largest one and assume that is the best move and store that move in the hash
>>table? I will run some tests as this is easy to do, but when I tried this a few
>>years ago, my tree got _bigger_. And when I looked into why, I found myself
>>searching nonsense moves from the hash table _before_ I got to the winning
>>captures (the first of which was a move that would refute the move at the
>>previous ply.)
>>
>>Easy to test. I'll supply some data in a bit, just for fun...
>
>For one moment forget about alpha and beta, you are on the wrong track as
>alpha and beta are not a part at all of the code. You need an extra stack
>that is set to -INF at each ply. Then before you do A/B you do the bestmove
>calculation for that ply. Involved variables: SCORE and STACK, no alpha beta.
>
>Ed
I think the best way to explain is to write a small piece of code in pseudo C,
else we talk around the point.
Alessandro
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