Author: Robert Hyatt
Date: 06:48:57 09/11/99
Go up one level in this thread
On September 11, 1999 at 05:27:21, Alessandro Damiani wrote:
>On September 11, 1999 at 00:34:46, Robert Hyatt wrote:
>
>>On September 10, 1999 at 17:57:38, Alessandro Damiani wrote:
>>
>>>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
>>
>>
>>I assume this means that for UPPER positions, you are just storing a random
>>move that you searched? Or are you using that "bestmove" trick to pick the
>>one with the highest value (even though it is still <= alpha?)
>>
>>validity checking isn't unreasonable, although if you are using 64 bits, I
>>don't think it is worth the effort. I _never_ get errors reported on such
>>moves, unless I introduce a bug... That is the only reason I actually leave
>>the "ValidateMove()" test in the code in fact...
>
>
>Yes, I store a random move at UPPER positions, but I don't use this move for
>move ordering. I need it for two reasons:
>
>1. Singular Extensions. This is very important!
>2. To check for hash errors (I am able to store only 52 bit of the hashcode, so
> I use the check for legality of the move to "add some bits")
>
>Alessandro
How do you use a 'random move' in singular extensions? I have played around
with this from time to time, but haven't yet decided to try it 'in production'
as I don't like how it behaves so far.
However, I don't quite see how a random move would be very important (your
words) for SE. If you aren't ready to discuss what you are doing, however,
that is perfectly ok. I'll explain what I have been twiddling around with once
I consider it 'working'...
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