Adding some randomness to turn-based AI (1 of 2)

We make a habit of asking great Hive players to give us feedback on the AI for our Steam version of Hive . Now that the AI has become pretty formidable, a suggestion we started hearing a few times was that the AI wasn’t random enough. This allowed players to basically memorize outcomes and they found themselves replaying any moves that worked in the past, rather than playing to win.

As I mentioned in my ECGC 2013 talk, an important concern in “real-world”/commercial game AI – as opposed to just getting optimal results in a lab – is to make sure that your AI teaches players to get better at the game rather than how to get better at just beating your AI.

When to use Randomness

In perfect strategy games (those with no luck), a completely random move is highly unlikely to be good. Conversely, even with great AI: if the AI is imperfect, then having little-to-no-randomness makes it easy for players to memorize and exploit any weaknesses. Your first reaction might be to just assume we should make perfect AI. However, for sufficiently-complex games such as Hive and Chess, the possible outcomes are more numerous than the atoms in the universe, so we’re likely to be relegated to imperfect AI forever (or at least until we have decent quantum-computers).

Not only does complete randomness lead to weak decisions, but almost* any randomness generally leads to slightly worse decisions. If you aren’t using randomness, you always chose the best-scoring move. Due to this trade-off of efficacy vs. randomness, I would not recommend adding much randomness to your AI until it is already very good.

Randomness in Openings

Players who played dozens of games in a row against our AI quickly became cognizant of patterns and vulnerabilities in the AI’s openings. Once they discovered a way to make the AI have a weak opening they found themselves constantly replaying those openings and regardless of what happened after, the AI couldn’t recover from that bad of a start.

Since variation in openings was crucial to preventing the AI from getting in a rut, we added a fair amount of randomness.

How it was before:

Originally, we used a weighted probability to figure out which Hive tile to place. The second ply was always placing a Queen, off-center of the first piece (not inline). The third move and beyond were all calculated by our minimax engine.

How it is now:

Hive "C", "I", "L", and "Z" shaped openings.

Hive “C”, “I”, “L”, and “Z” shaped openings.

We use a weighted probability for the first and second pieces. The second piece is still quite likely to be a Queen (it’s a very solid choice) but it won’t always be a Queen. The location of the second placement is now completely random. In Randy Ingersoll’s book, Play Hive Like A Champion, he named the openings based on the shape of the tiles. Normal openings can be laid-out like a “C”, “I”, “L” or “Z” (there are also “F” and “J” openings, which are just rotated versions of the “L”). If a player chooses not to place their queen by the second move, then the opening is called an “X”. The “X” isn’t considered a very good opening, but all of the possible openings can now be done by the slightly random AI; the weightings are just tuned so that it will be pretty rare for the AI to play “X”.

In addition to the decent randomness on the second move, we also expanded it so that the third-move (which is computed using the minimax engine) has a high degree of randomness. 1/2 of the time, the best scoring move will not be chosen, and in 1/2 of the those instances, the second best scoring move won’t be chosen either. The next post will explain more about this type of randomness.

Randomness on all other moves!

This is a little more complicated/mathematical/technical so I’ve split it into another post to avoid tiring you out with this already-long post! It should be posted in the next day or two. edit: it’s posted now.


The addition of randomness to the AI engine has greatly increased the replay-value of the AI in Hive. Hopefully this concept can be useful to some other developers as well. This change is going to be released as a free update in the next couple of days, along with a significant batch of visual updates and about a half-dozen other AI improvements that we’ve made over the past week or so. Keep your eye out for the update on Steam! EDIT: The update has been released!

As always, please let us know if you have any feedback about the changes in Hive, or if you have any questions about this post! If you’ve implemented randomness in your own AI in another interesting way, please leave a comment for the other readers – and myself – to learn from.


*: “Almost” because you can get a small amount of randomness from randomly choosing between any moves that tie for the “best move”, but that’s unlikely to happen very often and will still yield nearly-identical moves in many cases – eg: moving one pawn in Chess instead of its mirror opposite.

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