Skip to content


Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time

Forbidden Island, in Kotlin

I've codified the rules of Forbidden Island in Kotlin.

This isn't a playable game. It's just the rules of the game, written in code.

You could use it to:

  • create a program that plays the game;
  • enjoy the fun and challenge of implementing the rules yourself; or
  • implement a game playable by humans. (Yawn)

Run a sample player

If you want to see a program in action, you can either start it with Maven:

mvn exec:java -Dexec.mainClass=""

Or compile and execute it using Kotlin's tools:

kotlinc src/main/kotlin/ -d forbidden_island.jar
kotlin -classpath forbidden_island.jar

Want to create a program that plays the game?

There is a play subpackge in src/main that contains a GamePlayer interface for developing automata which can 'play' games of Forbidden Island. The package also contains functions which can test the playing automaton and print the results of the test as a Markdown table.

You can see examples of GamePlayer implementations and how to use the various testing functions in the RandomGamePlayer and RulesBasedGamePlayer classes.

Submit your GamePlayer

If you've created an interesting GamePlayer and would like to share it with others, I'm keen to collect implementations in this repository via pull requests. Please follow these steps to submit:

  • place your source files in a subpackage of that is your github handle in lowercase;
  • test your GamePlayer using testGamePlayer with 100,000 games per category and print the results using printGamePlayerTestResult (this will likely take a long time; my implementation took 45 min. to run this many tests);
  • create a in your subpackage, describe the design and most interesting aspects of your GamePlayer implementation, and copy in your test results;
  • submit a pull request.

Want to implement the rules yourself?

The code was developed almost completely using test-driven design. This means that the repository contains a large suite of fairly comprehensive tests. That in turns means that, if you'd like to implement the rules yourself, you can avoid the drudgery of imagining and writing all the tests and can instead just hoe in coding the solution.

There is a branch named tests-only which contains all of the tests, but with all the non-trivial functions of the implementation replaced with TODO().

It's recommended that you get the tests passing in the following order, as each of these can be implemented without depending on any implementation from the following tests.

  • CollectionsTest
  • ModelTest
  • TreasureDeckTest
  • GameMapTest
  • GameSetupTest
  • GameInitialisationTest
  • GamePrinterTest
  • GameStateResultTest
  • GamePhaseTest
  • GameStateProgressionTest
  • GameStateAvailableActionsTest
  • FullGameTest

My tips for developing a game-playing automaton

From my experiecnce, creating a GamePlayer that gets any significant results seems to be a bit of a Herculean task, but also a little bit addictive, so be prepared to waste a whole lotta time!

It's probably best to play the actual game a few times to understand it before trying to solve it with code. The physical game is fun, quick, classy and collaborative, but there's also a highly-rated iPad version if you want to play solitaire.

Extend the ExplainingGamePlayer rather than the base GamePlayer interface, as this will give you extra information when you test your solution that you can use to iterate and optimise.

Iterate on your GamePlayer, running tests, printing the stats, and using them to guide your development.

In developing my RulesBasedGamePlayer, I started by trying to just make the game last longer, i.e. lose later, which meant focusing on increasing the number of actions per game. After I thought I'd coded all the obvious things to stop the game being lost, I started adding rules which encoded simple strategies for moving towards good places to be and doing good things.

Don't focus on win rates to start with, because it takes a lot of work before they will start to move off 0%. Instead, focus on one thing at a time. For example, if your player is losing 89% of games because of "BothPickupLocationsSankBeforeCollectingTreasure", that's probably a good place to focus in order to bring that number down.

For most development, you can just run tests against the 'Novice' starting flood level, as the flood level has an obvious and simple impact on the game. However, different numbers of players have a more complex impact on the game, so I'd recommend always testing with categories of 2, 3 and 4 players, and checking that each improvement you make to your solution makes games across all player numbers improve (or at least makes some far better than it makes the others worse).

Beware of the trade-off between the gamesPerCategory parameter to testGamePlayer and over-fitting of your solution. A small gamesPerCategory allows for faster iteration, but it can be easy to start creating a solution which is highly tuned to a small number of games but doesn't work as well on a larger sample. You can use the VarianceCheck to gather information about the statistical variance of your solution using different sample sizes of random games.

If you run out of ideas and want some inspiration for how to improve your solution, try using the stepThroughGame function and looking for choices your program is making that could be improved. As an example, I found a lot of my rules were competing against each other, so one rule would move a player off a tile, then the next action another rule would move them straight back. You can also use the debugGame function to run a game or many games quickly and print out the game state whenever a certain condition that you specify occurs.


Forbidden Island, in Kotlin






No releases published


No packages published