Re: [Bug-gnubg] Bug: Double/Beaver

[Joern Thyssen]

On Fri, Aug 22, 2003 at 06:31:24AM -0400, Christopher D. Yep wrote
> 
> Btw, can you give me more information about how GNU makes cubeful checker 
> plays?  If I have the "cubeful chequer evaluation" boxes checked, I'm 
> guessing it means that every 0-ply evaluation, including every lookahead, 
> is performed according to the cube location (and value, if a match).  So 
> for example if at 6-ply a particular sequence of dice rolls involves the 
> cube alternating back and forth across the board (redouble/take each turn), 
> then for this particular sequence of dice rolls half the plies will be 
> evaluated cubeful based on player0 owning the cube and half evaluated 
> cubeful based on player1 owning the cube, correct?

Perhaps this bit can clarify some of your questions:

<URL:http://www.gnubg.org/win32/gnubg/gnubg.html#Cubeful%20equities>

gnubg will take any cube position into account, but the forward pruning
at the intermediate plies will be performed with the original cube value
and cube position.

This may give problems in positions where the gammon price changes
dramatically upon turning the cube. Examples, are initial doubles in
money game with Jacoby rule, and many match scores.

> 
> For any position with a centered cube, the win/gammon/backgammon numbers 
> reported with Analyse-Hint are obtained by assuming that gnubg does *not* 
> double on the first roll, but with the cube being turned during the 
> lookahead whenever gnubg thinks it's appropriate.  

Correct.

> If I do a rollout on this position, there are two sets of numbers
> reported; 

Yes, unlike the evaluations we allow the rollout to branch, i.e., the
"no double" and "double, take" continuations will generally not be the
same for a given trial in the rollout. This leads to different gwcs,
typically when the gammon prices changes significantly after doubles.

> the win/gammon/backgammon numbers reported by the
> double/take entry are obtained by assuming that gnubg doubles on the
> first roll and plays cubeful after that.  Correct?

Yes.

> 
> For every N-ply evaluation and every rollout, if some of the nodes of the 
> lookahead end in double/pass before N-plies are reached, for the purpose of 
> the win/gammon/backgammon breakdown, gnubg uses the final 
> win/gammon/backgammon estimate for these nodes rather than continuing all 
> the way to N-ply, correct?

Rollouts: yes (the rollout is implicitly truncated at double pass)
Evaluations: no (the evaluation is *not* truncated)

> Finally, for an N-ply evaluation are all intermediate plies during 
> lookahead always evaluated at 0-ply (this is what Jellyfish 3.5 and Snowie 
> 3.2 do for example)?  

Yes

> If so, gnubg can be marginally improved by using 
> minimax as you note, 

I'm not sure if the improvement is "marginally". In your original
example the equity change is from -0.107 to -0.118, or around 10%, when
we choose another cube value/cube position for the forward pruning.

> so that e.g. a 6-ply evaluation uses 5, 4, 3, 2, and 
> 1-ply for the intermediate plies before using 0-ply for the final ply.  On 
> the other hand performing a higher ply evaluation (esp. 3-ply and above) 
> will become much slower.

Well, the theory is that using the *-minimax algorithm this will not be
the case. 

> If minimax is ever implemented, perhaps an option could be added allowing 
> the user to either (a) use minimax or (b) perform all intermediate 
> evaluations at 0-ply.

Certainly.

> Thanks for helping me understand all this.  This topic is of significant 
> interest to me.  I think it should be in the gnubg manual.  Once I 
> understand it fully I'd like to volunteer myself to write an entry for the 
> gnubg manual (unless someone else is already working on it; if so, let me 
> know).

I've made a few notes available from

<URL:http://www.gnubg.org/win32/gnubg/gnubg.html#Cubeful%20equities>

It would be great if you could expand the chapter. I can provide the
texi source for the chapter if you want to work on it.

Jørn

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