hashing question

[adams161]

First i want to thank all who gave me info on multi proccesing chess programs.

I'm looking at my hashing code and i'm not sure i'm doing it 100%. the code to determine hash keys is working but i'm not sure on alpha hashing. I check that the depth is > = to the current depth. if alpha changes i hash the depth, that alpha changed, and the new value for the score or alpha. When i check hash keys at the top of search if alpha changed i return the score for the new alpha.

If it creates a beta cuttof i store that and when i check the hash lookup if depth >=depth ( must be true for all hash plays) i return beta.

If nothing changes alpha or triggers a beta cuttof I do a fail low and return alpha when i check the hash key.

So there are 3 possible values that can be hashed. fail high, fail low, or play the score that i got on my last search. And depth has to be >=current depth.

Does this sound right?

Mike

[adams161]

i guess these are the cases im worried i'm handling right.

fail high, score when hashed was >beta at that node that was hashed does that mean anytime you get that node again return beta or do you check the value to make sure the value is greater than beta still on the current node your doing a hash lookup.

fail low, alpha didnt change when this node was hashed, but do you check anything when you get this postion again and find your hash entry or just return alpha. for example if the alpha when this node was hashed was higher than the current alpha in the node of lookup does that make a difference?

if you find a value that is beteen alpha and beta and it wasnt a fail high or a fail low do you just return that score , do you have to search it, does it matter if you just return the score and dont check that the score is currently between alpha and beta on the node of lookup as long as it was when you hashed it as an alpha changing node?

thanks
Mike Adams

[bob]

i guess these are the cases im worried i'm handling right.

fail high, score when hashed was >beta at that node that was hashed does that mean anytime you get that node again return beta or do you check the value to make sure the value is greater than beta still on the current node your doing a hash lookup.

fail low, alpha didnt change when this node was hashed, but do you check anything when you get this postion again and find your hash entry or just return alpha. for example if the alpha when this node was hashed was higher than the current alpha in the node of lookup does that make a difference?

if you find a value that is beteen alpha and beta and it wasnt a fail high or a fail low do you just return that score , do you have to search it, does it matter if you just return the score and dont check that the score is currently between alpha and beta on the node of lookup as long as it was when you hashed it as an alpha changing node?

thanks
Mike Adams

First, take the three cases where you store an entry...

(1) the score you are storing is inside the alpha/beta window. You store this entry as EXACT.

(2) the score you are storing is >= beta. Here you store the score, with a flag of LOWER because the score is at _least_ that high, and could be higher.

(3) the score you are storing is <= alpha. Here you store the score with a flag of UPPER because the score is at least that low and could be even lower.

Now you do a lookup. Note that all of these assume that the draft of the stored entry is >= remaining search depth, else you can't use the entry all...

(1) you retrieve an EXACT entry. You just return the stored score and you are done...

(2) you retrieve an LOWER entry. If the stored score is >= beta, then you just fail high as you normally would because you know that the score is no lower than the score you found from the table, and that score is >= the current beta value, so it is fail-high time.

(3) you retrieve an UPPER entry. If the stored score is <= alpha, then you just fail low as you normally would because you know that the score is no higher than the stored score, and the stored score is <= alpha which calls for a fail-low...

[Dann Corbit]

An aside about debugging hash:

1. Create a full hash build from the current board that builds the same hash as your incremental hash but reexamines the entire board.
2. Do something like this:
#ifdef DEBUG
testhash = BuildFullHashFromBoard(board);
assert(testhash == hash);
#endif

And then when you run in debug mode, if there is a problem in building the hash incrementally, it will show up easily.

[bob]

An aside about debugging hash:

1. Create a full hash build from the current board that builds the same hash as your incremental hash but reexamines the entire board.
2. Do something like this:
#ifdef DEBUG
testhash = BuildFullHashFromBoard(board);
assert(testhash == hash);
#endif

And then when you run in debug mode, if there is a problem in building the hash incrementally, it will show up easily.

Actually you should have much more than that as a general debugging approach. I have a function (expensive) in Crafty that re-computes _everything_ (all bit boards, hash signatures (2), etc, to make sure the incremental versions are correct. It also sanity-tests the data structures (no two bitboards can have the same bit set since that would mean two pieces on the same square, etc) to catch other types of gross errors after a significant change is made...

I enable this with -DDEBUG, default is to compile it out.