# Taking Risks on Rotations

I haven’t owned a gaming console since the Playstation 2, mainly because I’ve found PCs to be sufficient for my own purposes and I don’t do very much gaming in the first place. I already have a pretty good PC for some of my own work (such as programming and some image/video editing I’ve done in the past). Nonetheless, I do enjoy watching a few YouTube channels which feature playthroughs of games with commentary, and I’ve certainly seen a lot of news regarding Nintendo’s latest console, the Switch.

One of the launch titles for the console, 1-2-Switch is a minigame collection which largely serves to demonstrate the capabilities of the Switch’s controllers (or “Joy-Cons”). The controllers have buttons, motion sensing capabilities, a camera (“Eating Contest”) and some kind of precise vibration (“Ball Count”). Most of the minigames seem simple and somewhat fun, as well as relatively balanced (in fact most games don’t really feature a way for players to interfere with one another – several others also have players’ roles over the course of the entire minigame being largely symmetric). There are a few that seem unbalanced (“Samurai Training” – assuming equal probability of success the first player has a greater probability of winning), though there was one that was clearly unbalanced in favour of the second player, that being “Joy-Con Rotation”.

The game invites players to rotate their controller about a vertical axis as much as possible, without otherwise disturbing it (e.g. by tilting the controller) too much. Scores are measured in degrees of rotation, with “unsuccessful” turns (e.g. too much alternative movement) scoring 0. Players get three rotations in turn, and then the score from their three turns is added up. In a sense this gives the second player an advantage; on his final turn he needs to know how much to rotate the controller to win, and should aim for precisely that. The problem we’re analysing today is then: how do we make a decision on how much of a rotation to attempt if we’re the first player, and how much of an advantage does the game give the second player (at least as far as the final turn is concerned)?

Let’s make a couple of simplifying assumptions to this:

• Let $X_i$ be a continuous random variable over $[0, \infty)$ and some $i$s. The probability mass function of the $X_i$, $f_{X_i}(x)$ are assumed independent, continuous and tending to zero as $x$ tends to infinity. Furthermore, for a given value of $x$ representing the number of degrees player $i$ attempts to rotate the controller, we are successful iff $X_i \leq x$.
• We assume scores are continuous (in the actual game, they’re only precise down to the degree).

Now, assuming we aim for a given rotation $x$, the probability of a win is then approximately $f_{X_1}(x) \times (1 - f_{X_2}(x))$, which we seek to maximise. Thus, for a simple example, if the $X_i$ are exponentially distributed with parameters $\lambda_1$ and $\lambda_2$, we then seek to maximise the function

$g(x) = (1 - e^{-\lambda_1x}) \times e^{-\lambda_2x}$

This can be minimised, and yields a solution of $x^* = \frac{1}{\lambda_1} \log \frac{\lambda_1 + \lambda_2}{\lambda_2}$; this gives the first player a winning probability of

$\left( \dfrac{\lambda_1 + \lambda_2}{\lambda_2} \right)^{-\frac{\lambda_2}{\lambda_1}} \left( \dfrac{\lambda_1}{\lambda_1 + \lambda_2} \right)$

So if both players are equally skilled in terms of rotation ability (that is, $\lambda_1 = \lambda_2$) then the first player wins with probability of just $0.25$. Furthermore, we can find out how much better player 1 needs to be for an even game (that is, the value of $k$ for which $\lambda_1 = k\lambda_2$); we formulate

$(k+1)^{-\frac{1}{k}} \left(\dfrac{k}{k+1}\right) = 0.5$

which works out to about $k = 3.4035$. If this player went second, however, he would have an over $0.905$ probability of winning!

The multiple trial case is interesting, especially because it seems like the problem does not demonstrate optimal substructure; that is, maximising the probability you win for a given round doesn’t necessarily mean that that’s the correct way to play if going for more rounds. For a simple (if highly contrived) example; let’s suppose we play two rounds. Player one can (roughly) make rotations up to 50 degrees with probability 1, 200 degrees with probability 0.3, and more than that with probability 0. Player 2 can make rotations up to 49 degrees with probability 1, 52 degrees with probability 0.6, and more than that with probability zero. To win one round, you should spin 50 degrees (win probability: 0.4); but to win two, if you spin 50 degrees twice, your opponent could spin 52 once and 49 once (leaving a win probability of 0.4); going for 200s and hoping for at least one success gives you a win probability of 0.51.