The Gobblet game is a three-dimensional Tic-Tac-Toe variant where players have to align goblets that come in different size, bigger pieces having the ability to cover smaller ones.

This repo contains a Julia implementation of the game, along with a perfect strategy for the junior version that is played on a 3x3 board with goblets coming in three different sizes. We prove that the first player can always secure a win, provided that their first move is to place a small or large goblet (as opposed to a medium-sized one).

The junior game has about three billion possible states. Therefore, it can be solved by exhaustive search. Doing so on a personal computer requires a careful implementation though. To speed-up the search process, we use the following tricks:

• We augment the standard Value Iteration algorithm by maintaining both a lower and an upper bound on the value of each state. When these two coincide, the state is labelled as solved and does not have to be updated using the Bellman equation in future iterations. Because most states get solved in early iterations, using this trick yields a ≈15x overall speedup.
• Instead of using a hash table to store the value function, we build an explicit bijection between the set of game states and the [0, 2881473966] integer range. This allows a compact representation of the value function as a bit vector that can easily fit into memory.

Finally, we note that Julia makes writing high-performance code surprisingly painless and natural.

To start a game, just use ./run.sh. The optimal strategy is computed and stored at first launch. It should take about six hours on a standard desktop computer.