UCCOM Ping Pong

Everyone starts at \(R_0 = 1000\). Each match counts as one result. Before it, player \(A\)’s expected score against \(B\) is

$$E_A = \frac{1}{1 + 10^{(R_B - R_A)/400}}, \qquad E_B = 1 - E_A$$

The winner \(W\) takes points from the loser \(L\), with \(K = 40\) for a best of 3 (\(K = 20\) for a single game) and the result rounded to the nearest 2:

$$\Delta = 2\cdot\operatorname{round}\!\left(\frac{K\,(1 - E_W)}{2}\right), \qquad R_W' = R_W + \Delta, \qquad R_L' = R_L - \Delta$$

So beating an equal player (\(E_W = \tfrac12\)) is \(\pm 20\), an upset is worth more, and a win you were expected to get is worth less. The margin doesn’t change the rating, and total points are conserved.

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