Author: Alessandro Damiani
Date: 14:05:33 09/10/99
Go up one level in this thread
[..]
>>>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
>
>
>OK... here is what I did:
>
>Normal alpha/beta first:
>
>int Search(int alpha, int beta, etc...) {
> best=-infinity;
> bestmove=0;
> foreach (move in movelist) {
> MakeMove();
> value=-Search(-beta,-alpha,etc.)
> if (value > best) {
> best=value;
> bestmove=current.move;
> }
> if (value > alpha) {
> if (value >= beta) {
> return(value);
> }
> alpha=value;
> }
> }
> HashStore(bestmove,alpha, etc...)
>}
>
>
>
>So what I did was to simply take the score for each search made after trying
>one of the moves at this ply, and remember the 'best' score and associated move.
>
>All I am saying is "this does not work". It is a characteristic of the alpha/
>beta search. It isn't a "it might work if ..." it simply won't work. Because
>the searches below this node (again, assuming this is a fail-low node where _no_
>move produces a score > alpha, which is the case where I claim there is never a
>best move to try here) return a bound on the value for that move. And I have no
>idea how to choose between a move with a bound <=200 and another move with a
>bound <= 150. Because the first could have a score well below 200. I simply
>don't know as I told the search below this node to stop whenever you find a
>score > X, where X is my negated alpha bound.
>
>Now, we have code. Did I misunderstand what you are saying? If not, then I
>can certainly explain further why that 'best' and 'bestmove' above are no good
>in this context. You can think of "best" as a random number that is <= alpha,
>nothing more. Which means "bestmove" is a random move chosen from the moves
>searched at this ply. And that is _not_ the move we want to try first when we
>get back to this position and it is suddenly not a fail-low position where all
>moves have to be tried, but rather it is a fail high position where the best
>move will let us cut off quickly...
I never said something against that. When I read Ed's answers I see what you
say: fail-soft AlphaBeta. What you say is what I think all the time:
best: score of the position after all moves are searched.
best<=alpha => the real score of the position is <=best. There is no information
about the best move.
bestmove==0 => best==-INF (after searching all moves)
I think, Ed should write down a small piece of code.
Alessandro
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