Author: Alessandro Damiani
Date: 14:57:38 09/10/99
Go up one level in this thread
On September 10, 1999 at 17:38:31, Robert Hyatt wrote:
>On September 10, 1999 at 17:05:33, Alessandro Damiani wrote:
>
>>[..]
>>>>>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
>
>
>There are only three choices here.
>
>(1) best <= alpha. We don't know anything about any of the moves. Yet I
>believe Ed (and I thought you as well) said that if you take this 'best'
>move and store it in the hash, it works well. In my test, it didn't.
>
>(2) alpha < best < beta. This is a move with an _exact_ score that is
>correct. Also known as a PV-candidate move normally.
>
>(3) alpha < beta < best. This is a fail high move. It is better than the
>best that is allowable. It is _not_ the best move at this level however, it is
>just good enough to produce a score >= beta and terminate the search early.
>This is the case that feeds (1) above at the previous ply. If we try _another_
>move here we might get an even _bigger_ score here, and back up an even worse
>score to the previous ply. But alpha/beta makes us stop _now_.
This is the postcondition of AlphaBeta. In all three cases I store the best move
in the transposition table. BUT (I wrote that somewhere in an answer!) I don't
use that move in the case the score belonging to that move was an upper bound.
The code in Fortress:
[[
precondition: depth>0 && (height, bound, wasSingular) is the information from
the transposition table.
// there is no best move when the score was an upper bound.
// But we need the move for Singular Extensions, if it was singular
if (height>0 && bound==UPPER && !wasSingular) HashMove[ply]= 0;
here: code for move ordering.
]]
Conclusion: I do the same thing as you do, Bob.
I store the best move to have an additional check to avoid hash errors, when the
hashtable is looked up (move must be pseudo-legal).
I don't understand what Ed says.
Alessandro
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