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...
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