Scaling Up Tic-Tac-Toe

Great - Now Make It Bigger

One of my stories for this week was to add a 4x4 option to my tic-tac-toe, which sounded intimidating at first. I was thinking I was going to need to update my scoring function, my mini-max's evaluate function, and the print board function. However, thanks to multimethods, it was surprisingly easy to add new features while keeping the code manageable.

Scoring and Evaluating the Board

Adding the ability to score and evaluate a 4x4 board sounded like one of the larger tasks involved for this story. Once I started looking at my code, though, I realized that both my score-board functions and eval-board functions were using my ->paths function. The game board itself was stored as a vector of nine elements, so that ->paths functions was taking that vector and turning it into a list of eight sub-lists, one for each way to get three in a row. Therefore, making this function able to also handle a 4x4 board would make both the score function and mini-max's evaluation function be able to handle the bigger board.

I started off by converting the ->paths function to a multimethod and making sure everything still worked before adding the new functionality. To do that, I created a defmulti called ->paths that dispatched based on the count of the board. Since the ->paths function only took one argument, which was the board, having the defmulti count the board vector and dispatch based on that meant I didn't need to change any code where ->paths was being called.

Then, I created a defmulti that dispatches off the value 9 and put all the code from the original ->paths function there, and then I deleted the original ->paths functions. After making sure all my tests still passed, I then added a defmulti that dispatched off a value of 16 and split the 16-element vector of a 4x4 board into the 10 possible ways to get four in a row.

Printing the Board

The process for being able to print the 4x4 board was almost identical. I converted the pre-existing print-board function to a multimethod that dispatches off a value of 9 and another one that dispatches off a value of 16. Now the print-board function could handle 3x3 or 4x4 without needing to change any code that calls the function.

Mini-max Has a Lot More to Think About

And just like that, after updating main to ask the user to select what size board they want, my tic-tac-toe could handle 4x4. However, any moves toward the beginning of the game takes mini-max a very long time to calculate due to the exponential increase in the game tree. My next adventure will be learning how to implement alpha-beta pruning to limit the amount of branches in the game tree mini-max traverses before making a decision.