I’ve been tinkering with my project in the past week so much that it’s time for a small update again. The last time I wrote I was starting to get to know Haskell better. I also wrote some Python scripts to create some XML data to be parsed and serialized by my Haskell code. Since then I’ve written a small Haskell program that reads in the XML data (with a slightly improved XML serializer) and creates small soccer cups. Since the results are generated randomly, the next steps for my project would be both simulating match outcomes in a more realistic way and programming the actual soccer match.
This is where I have to think some more about the actual software architecture of my game. The ones who played Sensible World of Soccer may remember how the game seemed to have two major modes, the “organisatory” mode with menus and league tables, and the “match” mode with the actual, well, match. (Later soccer games probably have a same kind of division.) I’ve been playing with the idea of dividing my game to two separate processes, one for the organization and other for the match. In the past few days I’ve been reading The Art of Unix Programming (a book I sincerely recommend, even if you’re programming for other OSes), which inspires me regarding modularity in software even more.
The basic principle I’ve been thinking of is that the organizational part creates a temporary file with the data about the clubs that will play against each other, time, place, etc. and starts the match process with this file as a parameter. Match process then reads in the file and plays the match based on the data. After the match is finished, the match process writes another file describing the result and maybe possible player injuries (if I ever get that far). The reason for this is simply making the programming work easier; I can fiddle around with one part without being able to break the other one, and I can even program the two parts in different languages without problems.
While planning the architecture I actually went even a bit farther than that. When the match starts, I’ll have more than one process doing the work. Simply following the Rule of Modularity, I’ll have a small server-client structure, with the server having the information about the match and doing all the AI, and client drawing the match and handling the controls as well as other user interface.
And that’s not even enough modularity for me. The server process should have a frontend that actually communicates with the client and handles the physics and player movements based on the input from the client(s) and from the other part of the server – the AI process, which calculates the AI actions and passes the AI “input” to the physics part of the server. I actually wanted to go as far as dividing the client to two processes, one process handling user input and another drawing graphics, but that would probably make the implementation too difficult.
So, in a nutshell, the server part consists of two processes, AI and physics. The client part sends input to the server (physics part) and draws the match. The physics part of the server receives input from the clients and AI process and calculates the player positions as well as the rest of the match data, sending the data to both clients and the AI process.
At this point a reader might wonder why make it so complicated. Well, I want it to be modular, and I don’t think a plugin architecture would really be enough. I want the game to be easily customizable, so that one can e.g. write his own AI component and use it with the rest of the game, as long as he follows the protocol. Or, put another way, the client side I will program will probably be ugly as hell, and if someone wants to make it better he can just exchange that part without having to reprogram the whole game. The architecture also makes the game playable on older hardware, if a lightweight client is implemented as well. (I’m thinking of ncurses.) In the end it’s basically the same principle as in Freeciv, with the server having the game data and client displaying it (but with even more modularity).
The other reason why to divide the game in processes is that I want to make using several programming languages as easy as possible. That way, people wanting to hack with the game are not restricted to the programming language(s) I’ve chosen. For example, I’ve done a bit of programming AI in C in the past and I started to get the feeling that a linearly increasing complexity of the AI makes the code exponentially complex. Still, I’m sure there are people that can program better AI in C than what I can do in Haskell, and it might as well be that I’ll decide to use C later after all. I also don’t think I want to program everything in e.g. Haskell (which seems to fit well for AI), especially the client side. (There are SDL and OpenGL bindings for Haskell though.) In the end, I will probably use quite weird programming language combinations and I don’t want to mess with foreign function interfaces too much, so separating everything into processes feels like a good idea.
One may also wonder how badly the performance of the game is hit when it is divided to so (relatively) many processes. I looked at Beej’s Guide to Unix Interprocess Communication (my game should be cross-platform in the end, though), and it seems like the most efficient way to do this is through shared memory. Since I’m not really sure if that plays well with higher programming languages like Haskell or Python that I also planned on using on my project and since I thought having the communication streamed would be the easiest way I decided to go for good ol’ BSD sockets. They seem reasonably fast and another bonus with them is that the possible inclusion of multiplayer in the game would be smaller hassle.
Another point from the book mentioned above I also want to follow is first getting the program to work, then getting it to work well. Probably the biggest reason why the software I’ve written until now was never really ready to be released is that I always wanted to make it huge from the start. (Actually, if you look at this project, I’m still doing it.) Dividing the project to so clearly dividable modules makes it possible for me to actually try to program each module in a quick and dirty way just to get a prototype done, and improve each module separately later.
In other words, the next step in my project is to write small functionality for each part of my project and make them work with each other. Since I’m lazy, I thought about doing the client side either with Pygame (the SDL bindings to Python) or just reusing the C++/SDL code I made a couple of months ago that displays a simple soccer pitch with some triangles as players. I’d like to do the AI part with Haskell, since I can imagine how pure functions and lazy evaluation make that task easier. And besides, I think it’s a great language (once you get the hang of it). The third process of my project involves physics, i.e. moving the ball and the players depending on their actions, which may not be a very difficult thing to accomplish and I might as well program it in C.
Of course, another difficult thing is getting the parts work well with each other and designing the protocols between them. That means I still have some planning ahead before I actually get to writing the code, but when I do, I hope it actually doesn’t take forever before I have something to show. When I do have something finished (whenever that may be), I’ll set up a (darcs) repository for the readers to have a look. In any case, when I have some new design or code to write about, I’ll keep you posted.