Author: Robert Hyatt
Date: 10:17:57 09/10/99
Go up one level in this thread
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...
This page took 0.02 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.