[RFC] Add decay system for openskill

[djonn]

Add decay of rating for openskill with the following logic before using a rating:

• The player receives NO penalty if they HAVE played a match in the last 21 days.
• The penalty the player receives is scaled to how long they have been inactive - between 3 weeks and 12 weeks - capped at 12 weeks.
• Mu (skill level) can decay to the midpoint between default mu (1000) and pre-decay mu at the lowest
• Mu cannot decay to a value higher than pre-decay mu
• Sigma (uncertainty) can decay to the quarter-point between pre-decay sigma and default sigma (500) at the highest
• Sigma cannot decay to a value lower than pre-decay sigma

Not sure how good that explanation is ⬆️, but I can't think of a better way to explain it. HMU @ the office if you want me to try and explain it to you.

Also the decay factors for mu and sigma are just a guess for something that could work, so maybe they need some work 🤷

I'm not sure if I like the complexity it adds (with this implementation). Fx the fact that it has to be called in 3 separate locations.

[Faustvii]

I do not play any games for 40 days. My current rating goes down (decay). Then I play a match after the 40 days, now I will start from the "decay" rating and we will calculate from that going forward, so the "decay" is part of the calculation because of my gap. IE I do not "jump" back to my original rating before the decay and then add the new result.

Is that a correct understanding?

[djonn]

Is that a correct understanding?

That is correct