Hello, welcome to my series of article about refactoring game legacy code. Game legacy code is no different than other types of code. What’s makes game different than others type of codes ? With a game, you are dealing with a event driven, simulated real-time architecture. They have different needs and requirements than a business, Web or rich client application. Games need to react fast to events such as ‘gun was triggered’, you have parts of your code that are called 30 or 60 times per second, loading times need to be short, etc.
There is a lot of things going on in a game, where in a typical business application you just need to collect some input, process it and persist into a database (good ‘ol CRUD).
The subject is my first complete game ever written. If you look at the projects sections of this site, you’ll see some failed attempts. The game is a Tetris clone called “That Game From Russia (you know it)”. It is written in C++ using SDL library. It was done in two weeks back in January 2009. If you have read my article Paradigm Shift, you know this was before I read many books and articles that changed my way of thinking about code, design and architecture.
The code can be found at http://code.google.com/p/gamefromrussia/
We start with SVN revision 5.
Current state of the architecture
The architecture is based on a basic state machine. There is a bit of duplicated code scattered around the code, like the input handling. The code is pretty tightly coupled. They are little to no abstraction in the code. SDL types are found in public API. When I created the code, I intended to do my Tetris in one day. Yeah silly I know. I didn’t intend to create a flexible architecture. I wanted to create a Tetris for fun thus I didn’t try to care that much about the craft of my code.
Anyway so you have the states (DropPiece, InitGame, Setup, GameOver, RemoveLines, SetNextPiece, PauseState, LinesRemoveAnimation) that define the core behavior of the game. Each state has a enter(), execute() and exit() methods. enter() initialize the state, execute() is called on every iteration of the game loop and exit() is called before changing to another state.
The “domain” or the core of the game are in the classes Block, BlockLine, Board, Piece. Block is a single block. BlockLine is a line of blocks in the Board. Board contains a list of block lines. Piece is a collection of blocks that represent the shapes in a Tetris game.
The other classes are support classes. SoundPlayer manages sound effects and music. TextPrinter manage drawing the current level, the score and the number of lines.
What we are trying to refactor ?
What I want to refactor is all the code related to the Piece handling. I want to make the rotation code easier to understand. Currently, for each type of piece, you have obscure code that move the blocs depending of the direction. It assume that the x, y is for the center block and the other blocks are moving according to this block. While developing this code, I had sketches on paper to help me map the blocks for each piece. I shouldn’t needed a piece of paper to manage the complexity of this code.
The right way to rotate a piece is to use a rotation matrix and apply it to each of the blocks coordinate. I found this when looking at other Tetris clone code. You don’t need to use a matrix explicitly thought. I will explain the solution a little deeper in a following article.
When you refactor code, you should keep in mind your main goal. When dealing with legacy code, you can be tempted to do large scale refactoring of your ugly, stupid code. Having a goal help you focus of refactoring code that matters for what you are trying to accomplish.
Why I am doing this ?
In this case, I want to train my refactoring, TDD, SOLID skills in a C++ code base in preparation for a much larger game project this fall. Also, I am to experiment some techniques illustrated in Michael C. Feathers book Working Effectively with Legacy Code.
Refactoring legacy code is hard
I don’t claim to be an expert in refactoring legacy code. In fact, it’s quite the opposite. I am still a beginner in refactoring legacy code. Up to this point, I only refactored one legacy code base I written in C# back in 2007. It is quite a hard journey depending of the quality of the code base. My current code base has a lot of inner, subtle dependencies that we will need to break over time.
In next episode, we’ll start doing some code cleanups to get a feel of the code base before setting up the tests in preparation for larger refactorings.