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The Art of Unix Programming
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Unix Programming - Studying Cases - Case Study: Freeciv Data Files

Case Study: Freeciv Data Files

Freeciv is an open-source strategy game inspired by Sid Meier's classic CivilizationII. In it, each player begins with a wandering band of neolithic nomads and builds a civilization. Player civilizations may explore and colonize the world, fight wars, engage in trade, and research technological advances. Some players may actually be artificial intelligences; solitaire play against these can be challenging. One wins either by conquering the world or by being the first player to reach a technology level sufficient to get a starship to Alpha Centauri. Sources and documentation are available at the project site.

In Chapter7 we'll exhibit the Freeciv strategy game as an example of client-server partitioning, with the server maintaining shared state and the client concentrating on GUI presentation. But this game has another notable architectural feature; much of the game's fixed data, rather than being wired into the server code, is expressed in a property registry read in by the game server at startup time.

The game's registry files are written in a textual data-file format that assembles text strings (with associated text and numeric properties) into various internal lists of important data (such as nations and unit types) in the game server. The minilanguage has an include directive, so game data can be broken up into semantic units (different files) that are each separately editable. This design choice has been carried through to such an extent that it's possible to define new nations and new unit types simply by creating new declarations in the data files, without touching the server code at all.

The Freeciv server's startup parsing has an interesting feature that creates something of a conflict between two of Unix's design rules, and is therefore worth closer examination. The server ignores property names it doesn't know how to use. This makes it possible to declare properties that the server doesn't yet use without breaking the startup parsing. It means that development of the game data (policy) and the server engine (mechanism) can be cleanly separated. On the other hand, it also means startup parsing won't catch simple misspellings of attribute names. This quiet failure seems to violate the Rule of Repair.

To resolve this conflict, notice that it's the server's job to use the registry data, but the task of carefully error-checking that data could be handed off to another program to be run by human editors each time the registry is modified. One Unix solution would be a separate auditing program that analyzes either a machine-readable specification of the ruleset format or the source of the server code to determine the set of properties it uses, parses the Freeciv registry to determine the set of properties it provides, and prepares a difference report.[62]

The aggregate of all Freeciv data files is functionally similar to a Windows registry, and even uses a syntax resembling the textual portions of registries. But the creep and corruption problems we noted with the Windows registry don't crop up here because no program (either within or outside the Freeciv suite) writes to these files. It's a read-only registry edited only by the game's maintainers.

The performance impact of data-file parsing is minimized because for each file the operation is performed only once, at either client or server startup time.

[60] Regression testing is a method for detecting bugs introduced as software is modified. It consists of periodically checking the output of the changing software for some fixed test input against a snapshot of output captured at an earlier stage of the process and known (or assumed) to be correct.

[61] Actually, TERM is set by the system at login time. For actual terminals on serial lines, the mapping from tty lines to TERM values is set from a system configuration file at boot time; the details vary among Unixes. Terminal emulators like xterm(1) set this variable themselves.

[62] The ur-ancestor of such validator programs under Unix was lint, a validator for C code separate from the C compiler. Though GCC has absorbed its functions, old Unix hands are still apt to refer to the process of running a validator as ‘linting’, and the name survives in utilities such as xmllint.

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The Art of Unix Programming
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