___________________________________________
title: Decompiling Interactive Fiction
tags: zmachine interactivefiction zork
date: 2015-08-21
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A couple of weeks ago a friend of mine pointed me to the web-based
interactive fiction piece called Aisle. As I played it, I immediately
started wondering how I could get a text blob of all the possible
paths instead of trying to figure out all the actions required to
input into the interpreter.

  [Aisle]: http://iplayif.com/?story=http://parchment.toolness.com/if-archive/games/zcode/Aisle.z5.js

Finding the Source

I took a look at the source and discovered Aisle was using a tool
called Parchment, which is designed for interactive web fiction.
Looking at the javascript source of Aisle, it is basically a
http://parchment.toolness.com/if-archive/games/zcode/Aisle.z5.js in a
javascript function passed to Parchment.

  [Parchment]: https://github.com/curiousdannii/parchment
  [http://parchment.toolness.com/if-archive/games/zcode/Aisle.z5.js]: base64%20text%20blob

Since Linux has a base64(1) utility, I figured I’d strip out the
javascript and decode the text to see if I could read the text blob
and thus read every possible scenario in the story.

  [base64(1)]: http://man7.org/linux/man-pages/man1/base64.1.html

    % sed -e "s/processBase64Zcode('//g" Aisle.z5.js \
    | sed -e "s/');//g" \
    | base64 --decode > Aisle.z5

At first I did this without output redirection, and it flooded my
terminal with binary control characters, essentially frying it. I
killed the tmux window, and started again, this time looking to see if
the file type was known.

    % file Aisle.z5
    Aisle.z5: Infocom (Z-machine 5, Release 1 / Serial 990528)

A quick search on Infocom and Z-Machine, shows that Z-Machine was
developed for interactive fiction in 1979 for Infocom text adventure
games, of which Zork was the first and where the Z comes from. There’s
a whole lot of interesting history online about Z-Machine if anyone
wants to know more (spoiler: it’s related to UCSD Pascal P-Machine).

  [Z-Machine]: https://en.wikipedia.org/wiki/Z-machine
  [history]: http://inform7.com/if/interpreters/
  [UCSD Pascal]: https://en.wikipedia.org/wiki/UCSD_Pascal

Decoding Z-Machine

Now that the raw source was available, the next step was actually
finding useful text. A search found an open source tool called Frotz
that can play these files, and the next thing I know I have Aisle
running in a terminal and it works just like it does in the browser.

  [Frotz]: http://frotz.sourceforge.net/

Some addition searches didn’t really uncover much on how to de-compile
a Z-Machine file. Nothing in the Debian repos, and some additional man
pages for Frotz didn’t lead to much.

Finally I found Z-Code Tools. There was no compiled binaries for
Linux, and if there were, they probably don’t work anymore on a newer
version of Linux. I downloaded the source and a make later made the
binaries magically appear. The freshly compiled txd tool finally
yielded the results I wanted:

  [Z-Code Tools]: http://inform-fiction.org/zmachine/zcode.html

    % ./txd /var/tmp/Aisle.z5 | grep gnocchi | head -5
    gnocchiyou
    S139: "You pick up a bag of gnocchi and turn it over. The doughy balls weigh your eyes; gnocchi, women, a woman, statues, a slow motion crash of flesh on show her some gnocchi and then you eat it and live happily ever after." intelligent. Leaving the gnocchi you walk over and drop to a knee. "Will you

I put the full text dump of Aisle into a gist if anyone is interested
in taking a look.

  [gist]: https://gist.github.com/ecliptik/1ce9c21f04c984c705b9

There are many other tools included in ztools that can provide
additional information, such as listing a dictionary of available
actions and commands.

Final Thoughts

Interactive fiction is just more than a big blob of text. Since the
early days of Z-Machine, these stories have not only included
fascinating stories, but also puzzles and other methods of fully
immersing a reader. Fully exploring and creating Z-Machine and
Interactive fiction is best left to to the experts if you’d like to
learn more.

  [experts]: http://inform7.com/learn/

It also appears the Z-code Tools are not packaged in Debian, and this
may be a fun project to do sometime.

  [Debian]: http://www.debian.org