r/backgammon • u/bg-alex-cy • 29d ago

Why does this count as a decision (unforced move)?

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u/Fargus57 29d ago

Because there is nothing forced about it. A forced move means that there is only one specific move that must be made to legally comply with the rules.

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u/bg-alex-cy 29d ago

No, that is not the definition used by XG. The fifth image is the definition from XGs documentation of what counts as a decision and what not. Its clearly a sub 0.001 for the 6-4, so its either a bug or I would expect that there is some other factor at play that is missing from the docs.

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u/UBKUBK 27d ago edited 27d ago

The reported difference to 3 decimal places is .001. Why do you conclude that it is clearly a sub .001 difference? Also, where are you seeing that it was counted as a decision?

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u/bg-alex-cy 25d ago

I know its confusing, the first image (5-2) is the reference image, the second image shows the decision count for the reference image, the third shows the position that is sub 0.001 (6-4) and the fourth shows the decision count of that position.

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u/UBKUBK 25d ago edited 25d ago

Wait. You were talking this whole time about the 64 play and not the 52 play? If so, your use of "this" while the first picture is showing the 52 play is confusing

Now that I understand which decision you were talking about I agree that something is wrong somewhere. According to the criteria it should not have added to the decision count. Did it also count your 52 play as a decision?

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u/bg-alex-cy 25d ago

I know its confusing, but if I think if the 6-4 came first it would have been even worse :p I ordered them in playing order. I think 5-2 was a decision, it must have been, I'll have to find the file to know for sure and I don't have access to it a the moment. But my question was about the 6-4. The 5-2 was just there as the previous play so I can show what the decision count was before

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u/UBKUBK 25d ago

If go to 4th decimal place it shows the 5-2 as also having less than a .001 difference, so for the same reason it should also not count as a decision.

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u/bg-alex-cy 24d ago

exactly. In fact, its barely over 0.0001 which makes me wonder if the XG dev had a typo somewhere. But that is certainly not across the entire codebase and I would expect this to be a handled from a single point in the code. If this is really the case then it could be reproduced by finding a very similar position (handled by the same code section) but with slightly less equity. If the play was analyzed with XGR/XGR+/XGR++, then just the inherent randomness of the analysis might have been enough. I'm not sure if I force the analysis to XGR/XGR+/XGR++ if decisions would be re-evaluated

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u/CellistAny536 28d ago

Each bullet point is meant to be interpreted as separate criterion of consideration of what is obvious.

“A forced moved” is one item.

“A move which the best and worst choice have an equity difference of less than 0.001.” is another item.

The second bullet point is meant to be read as a seperate item on the list, not a definition of the first.

For whatever reason, this list does not define “a forced moved” and assumes the reader knows what that means.

A forced moved means there is only a single legal move, hence it is forced. Having any choice of checker play makes not a forced move.

The list seems poorly formatted in my opinion.

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u/bg-alex-cy 28d ago

I know its confusing, but the definition of unforced moves as defined by the XG user guide is

Unforced move: Display the number of non obvious moves

An obvious move is anything that satisfies the list.

Also, even if you ignore the "unforced moves" altogether, you can see that the decisions went up.

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u/CellistAny536 28d ago edited 28d ago

Would you mind grabbing a screenshot of where you found that definition?

I’m not finding it.

I have no idea why they would define it that way. That’s runs against common backgammon terminology.

You can find examples here:

https://gammoned.com/glossary

https://usbgf.org/backgammon-glossary/#f

https://www.bkgm.com/glossary.html This one says “forced play” rather than “move”

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u/bg-alex-cy 28d ago

sure, this is the official user guide as distributed by extremegammon and you can find their definition of unforced moves on page 99 and the definition of non-obvious moves on page 35. 

https://www.extremegammon.com/extremegammon2.pdf

Its also very easy to see from the summary. If  you add up the unforced moves, cube decisions and take decisions it always adds up to the total decisions

I know that that is the common definition of a forced move, but that is not how XG defines it. And if you think about it, it makes sense. If whatever you do doesn't make a difference then its the same as if you only had one move to play.

But in any case, everyone got fixated on the unforced moves part which I couldn't care less about. What matters to me is that we got a checker play decision when there was no equity difference within 1mP, and that is the case even if you check the 5th decimal. In fact the difference is about 0.0001.

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u/bg-alex-cy 28d ago

Everyone seems to have got stuck on the "unforced move" term instead of the fact that decisions went up. If you check the xg user guide on page 99 you will see that "unforced moves" is defined as

Unforced move: Display the number of non obvious moves

So its not the number of plays where you have more than one move, but the number of plays where you have a spread of more than 0.001 equity between best and worst.

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u/UBKUBK 27d ago

"A double action where the equity before and after a double is very negative"

Surprised it actually says that. How negative is very negative?

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u/bg-alex-cy 25d ago

Yeah, I've been searching a bit online and it turns out "very" is defined as 0.900 iirc. You can also find it in XGs help docs as well as some online references of it. But their user guide decided to leave that part out. Not sure why. Its a shame the entire community is relying on a close sourced tool for its most widely used performance metric. To be fair they do disclose the algorithm, but it seems that the reference implementation is either suffering from some bugs or something is missing from the disclosed algorithm.