Visually debugging a Minimax AI engine

While creating the Steam version of the popular board game Hive, it became clear that normal methods of on-screen debugging weren’t going to be in-depth enough.

Alpha-beta pruning of a small Minimax game tree.

Alpha-beta pruning of a small Minimax game tree.

Most AI for 2-player abstract-strategy games probably uses the tried-and-true Minimax algorithm. It works just how you’d think AI would work: you look at a tree of all of the possible board positions that you could get to (and all positions you get to from there, etc.) and score them. It can get a little more complex on top of that, but the basics are really straightforward.

While the Minimax algorithm is very general-purpose, the scoring function that you use to evaluate each position is game-specific. Debugging that scoring function (called a “heuristic evaluation function” in technical lingo) can be tricky, especially since it is very hard to see an entire game-tree at once. The small tree in the first picture above is used for teaching minimax, but is unrealistically small for real games. As an example, even a very simple game like Tic-Tac-Toe would have 3 possible moves right away (it would be 9 moves, but the board is symmetrical, so there are only 3 actual different moves). Looking at this tic-tac-toe tree, you can see that even this extremely-simple game’s tree grows quite large by the third layer.

Complexity of real Game Trees

Tic-Tac-Toe game tree

First two plies of Tic-Tac-Toe game tree

Let’s put that in perspective compared to other games: The number of moves that can be done per level is called a “branching factor“. In the tic-tac-toe example, the branching factor at the start of the game is 3. After moving, the branching factor is 2, 5, or 5, depending on which move is made. In chess, your first move can be one of 20 possibilities: 8 pawns (each with 2 different moves) and 2 knights which have 2 possible moves each. From there, the possibilities change very quickly. Due to this variability, when discussing games we tend to focus mainly on the average branching factor. The average branching factor of tic-tac-toe is 4, for chess it’s 35, the value for Hive is currently unknown but I’d estimate it between 40 and 50.

The need for a good visualizer

For a game such as Hive, to have the AI look 3 levels deep, you’d have (50^3) = 125,000 nodes on the third level of the tree. Even if we limit the branching factor (which we do in our AI) the number of nodes is quite large. At the time of this writing one of our AI levels limits to a branching factor of 30. So (30^3) = 27,000 nodes. Needless to say, that tree won’t fit on most computer screens, so it would be hard to debug it all at once.

Therefore, we need a different way to visualize what’s going on. When debugging a heuristic evaluation function, it’s important to know the score throughout the tree, how the score worked its way up each level, and it also helps to be able to see a detailed view of how the AI scored the nodes. Keep in mind that only the scores on the bottom level of the tree are actually used to be the final scores of the path to that node. However, when we limit the branching-factor, that involves giving a preliminary score to each node and sorting all of the sibling nodes in any given level of the tree, before traversing to their children. This way, even though many moves may be skipped in a given level of the tree, the odds are high that the most important moves are being evaluated. Due to this pre-scoring, it is helpful to have detailed scoring information on every node in the tree.

In addition to seeing the nodes in a current level, it would be helpful to have pointers to which layers of the tree are maximization or minimization steps so that you don’t have to keep as much information in your head. This way, you can examine any node in the tree and tell where it got its score from (eg: it’s children) and how its score is being used by its parent-node if it has one.

Our solution

Minimax AI Visualizer

Minimax AI Visualizer – click to enlarge

We wanted to be able to run the AI in our game’s debug-mode, then create a log-file and view it easily. After looking around, it seemed that a very simple solution would be to dump some JSON in a format that the Javascript InfoVis Toolkit (JIT) could load, then create a tool to render a “Space Tree” which expands and collapses as needed.

→→ Check out our Minimax AI Visualizer Tool in action. ←←

I started with the SpaceTree demo from JIT and just modified it from there. Features:

  • Loads data from a JSON file by default, but you can paste new JSON into a text-box to create a new tree (or use the text-box modify the existing tree).
  • Colorized to easily show which steps are Maximize steps or Minimize steps
  • Each node says what ply it represents to get to that node, and the final score that node ended up with.
  • The mouse scroll-wheel lets you zoom in and out
  • Hovering over a node brings up a detailed tool-tip bubble with the breakdown of each of the scoring-factors that were used to come up with the pre-scoring for a node – or the actual scoring, in the event that it’s a leaf-node.
  • Hovering over the Root Node brings up a tool-tip bubble with detailed info on the entire run of the AI: how many nodes were evaluated, how long the AI ran, how many total prune events there were, etc..
  • Each time there is a prune event (from the alpha-beta pruning), there will be one node which indicates the entire number of sibling nodes that were pruned at once.

If you want to use the same tool to visualize the progress of your own AI, all you need to do is have your code output JSON in the same format that’s used in the textarea below the graph, then paste it into that textarea and hit the “Load Tree” button.


Being able to more quickly track-down some of the weird decisions that the AI was making, let us drastically improve the AI in only a few days of work. The example that’s embedded in the Visualizer is from before most of the changes, but that shouldn’t matter since it’s just showing how it works. We’ve had some very good players helping us debug it, and one of the recent World Champions said that the top level of AI made him force a draw. We’re getting there!

If you want to see the AI in action, check out Hive on Steam.

If you have any questions about the Visualizer or about our AI, let me know in the comments!