Tuesday, 6 October 2015

Abstractions in Code, Details in Metadata

I've programmed many games - each one was special in its own way. One in particular stands out early in my university studies, a top down shooter. It was not graphics, gameplay, or sound that made it stand out however. It was the lesson it taught me about software development.

Level One

With the deadline for completion of the project looming, time was running out. The core game engine was complete but other than the first level there was nothing else for the player to do. With more marks awarded for various components I decided to add a second level.

At the time the game consisted of a source file called level.cs. This contained parts of functionality explicit to every level that I would need. It also contained code specific to the first level. My solution was to extract a base class and introduce level.cs and level1.cs. This worked. The addition of level two was not as easy. The second level required a considerable amount of additional code, despite the shared functionality. A slow feedback cycle of change, compile, and test, made this addition even more tedious. With the test phase consuming much of my time.

Hopefully you can see where this is going. While I never added a third level, the same problem exists. In fact for every additional level the problem would get worse.


The lesson I learned here was that a game engine should be abstract, while the details of the level should be data that is configured outside of the code. This allows anyone to make levels for the game. Levels can be unique rather than constrained to how the programmers coded them to be, introducing novel gameplay elements constrained only by the imagination of the designers.

This concept is not unique to games programming. I would learn a few years later that this is a well known and advised practice - The Pragmatic Programmer summarises that abstractions should live in code, while details lives in metadata (data about data).

"Program for the general case, and put the specifics outside the compiled code base."

Those of you with a keen sense for code smells may be thinking about another issue with this story, and yes, you're right. The base class caused issues. The use and misuse of inheritance will be the subject of a future post.

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