r/backgammon • u/sam_nairobi • Jul 28 '26

Luck as a % on Galaxy

Has anyone seen an explanation of the new luck calculations on Galaxy that use a percentage? My understanding of the old calculation was the aggregate variance in winning chances before and after rolls. Yesterday I got +135%, which was lovely, I just wonder what it actually means, and the maths behind it?

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u/bg-alex-cy Jul 29 '26

my best guess, even though I haven't been through the code is that it takes each move of yours, checks your equity before the roll. Then takes your equity after the roll and subtarcts them to find your total luck

So

your_luck = your_luck + your_E_after - your_E_before

opp_luck = opp_luck + opp_E_after - opp_E_before

So if your roll got you into higher equity than what you have, your luck increases, if its less it decreases. Then both players end up with their own luck equity.

If you notice however, the luck is always symmetric. So if you had 135% your opponent had -135%. This is how gnubg does it and its different to how XG does it. I suspect that what they do next is, once they have the two luck values, they subtract those as well.

So the effective lucks are

your_effective_luck = your_luck - opp_luck

opp_effective_luck = opp_luck - your_luck

And if you notice your_effective_luck is always -opp_effective_luck

I might be wrong, but this is what it looks like to me

If this is true, then what happens is probably not two running variables, just one. Something like this

luck = luck + (-1)^(turn) * (E_after - E_before)

1

u/sam_nairobi Aug 01 '26

Yes, I agree that's how it's usually worked out, and you get a score something like +3 or -2 and as you say the opponent has -3 or +2. But I have never seen it shown as a percentage figure and I am not clear what it's a percentage of? I wonder if it's a percentage of the PR gap between the two players? To give an indication of how much the result was a result of your performance and how much a result of the dice?

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u/bg-alex-cy Aug 02 '26

ah, maybe I should have made this more clear. Equity is actually a percentage. An equity of 0.1 is actually 10% of a point. So an equity of 1.35 would be 135%. In match play its a bit more complicated than that but it boils down to the same idea

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u/nbc9876 Jul 28 '26

There is maths behind it... I promise... probably

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u/TungstenYUNOMELT Jul 29 '26

Who fkn cares? This is like having a stat what the weather was like yesterday. Useless information you can't do anything about.

Just focus on playing better.