Migrate ratings to Codeforces

[xiaowuc1]

DMOJ currently uses TopCoder ratings. TopCoder ratings are unique in that they have a volatility metric which bounds the rating changes that an individual has (individuals with lower volatility change rating less than individuals with higher volatility).

This issue is a proposal to migrate the rating system to that used by Codeforces, which does away with the notion of volatility. In reality, this issue is asking for volatility to be removed from the rating system, and migrating to Codeforces is a convenient way to achieve this.

Although there is nothing inherently wrong with TopCoder's rating system, it does not feel well-suited to DMOJ. The main reason for this is that the community is small and volatility will result in rating differences going to zero over time as volatility converges to zero, which punishes individuals who stay on DMOJ for a long time and markedly improve. By comparison, on TopCoder, volatility does not appear to converge to zero which is why rating changes are not affected by this.

[xiaowuc1]

It turns out that TopCoder ratings can be manipulated adversarially (see https://codeforces.com/blog/entry/87848).

It is unlikely that this matters in practice.