Wednesday, March 28, 2012

Janowski model revisited

I was a bit incorrect in how I interpreted Janowski's paper.

The real way to think about his model is as a linear interpolation between the dead cube equity and the live cube equity.

The dead cube equity is always P*(W+L)-L, normalized to the current cube value. The live cube equity is piecewise linear, and has different forms for three cases:

1) Cube centered. 0<P<TP, equity runs from -L to -1; TP<P<CP, equity runs from -1 to 1; CP<P<1, equity runs from 1 to +W.
2) Cube owned by player. 0<P<CP, equity runs from -L to +1; CP<P<1, equity runs from +1 to +W.
3) Cube owned by opponent. 0<P<TP, equity runs from -L to -1; TP<P<1, equity runs from -1 to +W.

TP and CP here are the live cube take and cash points, so

TP = (L-1/2)/(W+L+1/2)
CP = (L+1)/(W+L+1/2)

Then the Janowski model equity is x * live cube equity + (1-x) * dead cube equity.

That's it. Everything else falls out of that, and in practice you don't really need to calculate initial double, redouble, too good points, or even take and cash points - you just compare cubeful equities.

For example, the take decision calculates the equity when the player owns the doubled cube and compares that to -(cube value). If the equity on take is greater than the equity on pass, you take.

Similarly, the double decision calculates the equity on double (which is the minimum of the equity when the opponent owns the doubled cube and +(cube value) for the pass) and compares it to the equity at the current cube value when the player owns the cube. If the doubled equity is bigger, you double.

My error before was around too-good points: in my implementation I assumed that the too-good point was calculated by comparing the dead cube equity with pass value, thinking that you're giving up the value of the cube by not doubling. But really you're not; you still have the option of doubling if the game starts to go against you. You should be comparing the model cubeful equity at the current cube against the value you'd get on a double (the minimum of the take and pass equities there).

This invalidates some of the statistical results I calculated for Janowski's model before, so I'll have to redo those.

Also it makes it easy to generalize to two cube life indexes: each player's interpolation of equity uses their appropriate index.

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