Heirloom Software: the Past as Adventure

That kind of really good test coverage frees your hands. It allowed us to make rapid progress on the other prime goal, which was to turn the obfuscated source we started with into a readable work of art that fully revealed the design intentions and astonishing cleverness of the original.

So, all the cryptic magic numbers had to go. The goto-laden spaghetti code had to be restructured into something Don Woods in 2017 wouldn't feel he needed to apologize for. In general, what we aimed to transform the source code into was something we could believe Crowther and Woods—two of the most brilliant hackers of their time—would have written in 1977 if they had then had the tools and best practices of 2017 at their fingertips.

Our most (ahem) adventurous move was to scrap the custom text-database format that Crowther and Woods had used to describe the vocabulary of the game and the topology of Colossal Cave.

This—the "complex, declaratively-specified data structure" I mentioned earlier—was the single cleverest feature of the design, and it went all the way back to Crowther's very first version. The dungeon's topology is expressed by a kind of pseudo-code broadly resembling the microcode found underneath a lot of processor architectures; movement consists of dispatching to the sequence of opcodes corresponding to the current room and figuring out which one to fire depending not only on the motion verb the user entered but also on conditionals in the pseudo-code that can test for the presence or absence of objects and their state.

Good luck grokking that from the 2.5 code we started with though. Here are the first two rules as they originally appeared in adventure.text, comprising ten opcodes:


3
1       2       2       44      29
1       3       3       12      19      43
1       4       5       13      14      46      30
1       145     6       45
1       8       63
2       1       12      43
2       5       44
2       164     45
2       157     46      6
2       580     30

Here's how those rules look, transformed to the YAML markup that our restoration, Open Adventure now uses:


- LOC_START:
    travel: [
      {verbs: [ROAD, WEST, UPWAR], action: [goto, LOC_HILL]},
      {verbs: [ENTER, BUILD, INWAR, EAST], action:
       ↪[goto, LOC_BUILDING]},
      {verbs: [DOWNS, GULLY, STREA, SOUTH, DOWN], action:
       ↪[goto, LOC_VALLEY]},
      {verbs: [FORES, NORTH], action: [goto, LOC_FOREST1]},
      {verbs: [DEPRE], action: [goto, LOC_GRATE]},
    ]
- LOC_HILL:
    travel: [
      {verbs: [BUILD, EAST], action: [goto, LOC_START]},
      {verbs: [WEST], action: [goto, LOC_ROADEND]},
      {verbs: [NORTH], action: [goto, LOC_FOREST20]},
      {verbs: [SOUTH, FORES], action: [goto, LOC_FOREST13]},
      {verbs: [DOWN], action: [speak, WHICH_WAY]},
    ]

The concept of using a Python helper to compile a declarative markup like this to C source code to be linked to the rest of the game was maybe just barely thinkable when Adventure 2.5 was written. YAML didn't exist at all until six years later.

But...designer's intent. That's much easier to see in the YAML version than in what it replaced. Therefore, given the purpose of heirloom restoration, YAML is better. Rather like stripping darkened varnish from a Rembrandt—the bright colors beneath may startle if you're used to the obscuring overlayer and think of it as definitive, but they are the truth of the work.

With our choices about what we could change so constrained, you might think the restoration was drudge work, but it wasn't like that at all. It was more like polishing a rough diamond—gradually seeing brilliance emerge from beneath an unprepossessing surface. The grottiness was largely—though not entirely—a consequence of the limitations of the tools Crowther and Woods had at hand. When we cleaned that up, we found genius with only a tiny sprinkling of bugs.

My dev team fixed those bugs, of course. We're hackers; that means we consider heirloom software a living heritage to be improved, not an idol to be worshiped. We certainly didn't think, for example, that Don Woods intended use of the verb "extinguish" on an oil-filled unlit urn to make the oil in it vanish.

Petr Vorpaev, reviewing a draft of this article, observed "Sometimes, we stripped bits of genius off, too. Because it was genius that was used to work around limitations that aren't there any more." He's thinking of a very odd feature of the 2.5 code—it worked around the absence of a string type in old FORTRAN by representing strings in a six-bit-per-character encoding packing five characters into a 32-bit word. That is, of course, a crazy thing to do in C, and we targeted it for removal early.

We added some minor features as well. For example, Open Adventure allows some command abbreviations that are standard in text-adventure games today but weren't supported in original ADVENT. By default, our version issues the > command prompt that also has been in common use for decades. And, you can edit your command input with Emacs keystrokes.

But, and this is crucial, all the new features are suppressed by an "oldstyle" option. If you choose that, you get a user experience that even a subject-matter expert would find difficult or impossible to distinguish from the 1995 and 1976–1977 originals.

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