abs(beta)<VALUE_MATE_IN_PLY_MAX

[Pierre Bokma]

In stockfish the condition:

abs(beta)<VALUE_MATE_IN_PLY_MAX

is used as a criteria to do static null move pruning, null move search, razoring etc.

If this condition is not met these techniques are skipped.

I find it hard to understand what it means. If i see it correctly it means that these methodes should be skipped if the opponent has a mate in another branche of the tree.

can anybody please explain this condition?

Thanks in advance

[mcostalba]

can anybody please explain this condition?

It means that the pruning is not done in position in which side to move can give mate (beta >= VALUE_MATE_IN_PLY_MAX) or can receive mate (beta <= -VALUE_MATE_IN_PLY_MAX).

[zamar]

1) Mate scores returned from null move are not reliable (zugzwang issues), so we ignore them. If all scores, which could cause a fail high, are mate scores, there is no point in doing null move search.

Other way around, doing null move is not a reliable way to escape mate (again zugzwang issues).

2) Razoring makes use of newvalue : = value +- delta. When value is a mate score, "+- delta" doesn't make sense and could cause newvalue go out of bounds.

[Pierre Bokma]

Thanks Joona for this answer can you please clearify what you explained? I am going through the stockfish code and i am learning all the time

[zamar]

Thanks Joona for this answer can you please clearify what you explained?

Sorry, maybe I explained badly, but I don't think that I would be able to give any better explanation.

I am going through the stockfish code and i am learning all the time

Keep it going. Sooner or later it will make sense. And as your knowledge goes, you'll be able to ask more detailed questions.

[Desperado]

Hello Pierre,

without any context to Stockfishs code i would interprete a condition like

abs(beta)<VALUE_MATE_IN_PLY_MAX

in the following way.


A: values
===============

values are defined in most cases like this:

eg: range of all values: [-30000 ,to 30000] or any other bounds like
so: range of all values: [-bound ,to bound]

B: mate values
===============

b1)
===

now think of your search which has a max ply to go (eg: 256 plies).
then the mate values are

[-bound+256](eg:[-29744]) get mated in max (the search cannot deliver a higher mate)
[ bound-256](eg:[ 29744]) mating in max (the search cannot deliver a higher mate)

now the condition again: i guess that is a condition from a nullwindow where alpha==beta-1


b2) in words
=========

if(beta > -29744) cannot be mated (this is normally a check against alpha,but alpha==beta-1 i guess)

if(beta < 29744) cannot mate

b3) in code
========

if(beta > -29744 && beta < 29744)

it becomes

if(absN(beta) < 29744)

Hope it helps a bit...

Michael