Author: Herman Hesse
Date: 14:40:46 07/30/99
Go up one level in this thread
On July 30, 1999 at 17:35:27, Ian Osgood wrote: >On July 29, 1999 at 20:54:08, Bruce Moreland wrote: > >> >>On July 29, 1999 at 18:58:12, Ian Osgood wrote: >> >>>Do other program authors curtail the search when there is a forced move at the >>>root? >>> >>>How do you detect that a root move may be forced? >>> >>>Could you compare the values of the best and second-best root moves after a >>>search iteration to detect a forced root move? (Granted, the second-best score >>>won't be accurate due to alpha-beta, but I figure that if the difference was >>>greater than a queen's value, you could still conclude that the best move was >>>forced.) >> >>The only sure way to do this is if there is only one legal move. > >So this would be considered root processing? Or is this detected at each >iteration as you detect longer mate lines for alternative moves? > >I'm a bit unclear about the alpha-beta return value: if your search of a non-PV >root move to depth N leads to mate in M<=N or cutoff in all branches searched, >will a near-mate score be returned, or will a near-alpha score be returned? > >Would other types of searches return a near-mate score in this case? > >>Any other technique is going to leave you open to cases where you can make >>mistakes, and I'm sure there are cases where you'll miss a win or make a losing >>move, and you won't do this if you'll think longer. >> >>If you decide that you can live with walking into losses and missing wins, the >>first question is why did you decide that you can live with this. >> >>One reason is to impress the humans, or avoid having them call you stupid. This >>is a valid reason, in my opinion. > >Very perceptive! I am concerned in this case with consumer-level play rather >than championship-level play. I think that many humans are annoyed when a >computer doesn't make an "obvious" forced move quickly. In fact, an occasional >blunder because of a quick recapture may be judged as human-like play! (One >could allow such behaviour to be disabled for a championship style of play.) > >>Another reason is that you save time on the clock this way, and in a computer vs >>computer game with both sides thinking on the opponent's time, you could >>initiate a sequence of instant moves this way, rather than walk into a situation >>where your opponent has a sequence of instance moves. I don't know if the >>strength gain is higher from catching mistakes, or having extra time or going >>for an instant-move sequence, but I'll bet on the latter. > >Which was latter? > >This brings up another thorny issue: good humans have an intuitive grasp of the >difficulty of a position, choosing to spend less time on easy situations and >more time on difficult or critical ones. Has anyone come up with a model for >this which a computer could understand as an aid to time allocation? Or do you >think this is a false distinction? > >>Now the question is deciding what is forced. An obvious clue is that the search >>sticks with one move more or less forever. Another clue might be that all of >>the other moves can be refuted in a small amount of time, but I haven't >>experimented with this. And if you need to restrict this because you are doing >>forced moves that aren't really forced, you can restrict it so that you only do >>this if the program wants to make a recapturing move. >> >>bruce > >I remember a discussion of search tree branch size as a move-ordering heuristic. >I wouldn't think it would be useful, because you might not get to the bad result >in a refuted branch until late in the search of that branch. > >I would think that recapturing is too narrow a measure. Wouldn't any unique >move which prevents a large score drop be considered forced? This might include >protective moves, capture avoidance, zwischenzug checks, etc. > >I also realize that you have to define "forced" in the context of a particular >search depth. > >(Thanks for all the responses! This is an interesting discussion.) > >Ian Chess Genius used to use a time management system (I am not sure if this still applies) where 'obvious' recaptures at the root used to cut off quickly. This would occasionally/often result in massive oversights when the postion turned from material to king involvement. Herman
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