View source for A-code (Dave Platt)

{{Software infobox
|Type=Authoring system
|Style=Parser
|Multimedia=No multimedia as such, but browser builds can support games making use of all
HTML capabilities, giving latent multimedia support.
|Developer=Dave Platt, Mike Arnautov
|Home page=https://mipmip.org/acode/
|Download=https://mipmip.org/acode/
|Format=Other
|System=Windows, Mac, Linux, iOS, Other
|Version=12.86
|Date=2022/01/04
|Status=Stable
|Notes=A-code can be used on any platform supporting (a) command line and (b) an ANSI C compiler/linker.
}}
Dave Platt developed the A-code language (not to be confused with the Level 9
A-code system!) for writing his influential "Adventure 3" (now known as
Adventure550) expansion of the original Crowther/Woods Adventure game. In
doing so, he broke from the traditional format of Adventure and its expanded
versions, which relying on a custom executable processing a standard format
data file.

THE A-CODE LANGUAGE

The A-code language is a "Polish prefix notation" notation one, where each
executable statement consists of an instruction keyword (a "minor directive"
or "minor opcode" in Platt's usage), optionally followed by one or more
arguments to which the instruction is to be applied. Thus e.g. "add count, 1"
is an instruction to increment the value of the the entity "count" by one.
Game entities (variables, objects, locations, texts, code chunks...) are
declared using "major directives". Full description of the current version of
the language can be found in https://mipmip.org/acode/acode-language.html

IMPLEMENTATIONS

Platt's implementation of the system consisted of a "munger" which took A-code
source of a game and created a tokenised pseudo-binary. The game was then
played by running a separate "executive" interpreter, interpreting the
psudo-binary. Both programs were written in Fortran 77.

The Current implementation, supported and developed by Mike Arnautov takes an
alternative approach. It uses an translator ("acdc") to convert A-code source
into derived ANSI C source. This derived source can be then build into
standalone executables (using command line console or browser for display and
input), executables suitable for CGI or PHP wripper on-line implementations,
QT5 executables. It can be further translated by Emscripten into stand-alone
HTML/JavaScript web pages.

NOTABLE FEATURES

The Platt version of A-code had some unusual features, such as e.g. "text   
switches", effectively replacing some code with declarative syntax of game  
texts, or "procedure groups", where separate chunks of code could bear the
same name and be executed in order of their declaration. Both these (and some
others) were subsequently built upon in later versions by Mike Arnautov. Thus
the basic text switch mechanism got developed into fairly complex "text
morphing" (see e.g. https://mipmip.org /acode/acode-texts.html) and to provide
powerful means for adding "wizard mode" for game debugging.

As another feature of note, with a minimum of discipline, A-code games can be
written to guarantee upward compatibility of saved games. This is invaluable 
particularly in the beta-testing stage of game development, since objects,
locations and general functionality can me added (or fixed) without
invalidating game saves generated by a previous version. (See
https://mipmip.org /acode/acode-upward.html)

PARSER

While under the hood A-code parser is a simple verb/noun one (currently no
adverbs, instruments or adjectives), it is nevertheless capable of parsing
complex sentences, such as e.g. "take all except keys and food then go out".
It supports natural use of "it" and also collective nouns "all" and
"treasure", features undo and redo. It provides automatic abbreviation of all
vocabulary words to the least unambiguous length as well as single-typo error
correction. And its vocabulary structure is effectively a 3D one, making it
possible automatically to echo back at the player more appropriate versions of
command words used.

EXISTING GAMES

As already noted, A-code was designed by Dave Platt specifically for writing
Adv550. It was subsequently used by Mike Goetz for the well known Adv580
version of the game. (The A-code source of this was recently recovered from
Goetz himself, thereby scotching earlier belief that Adv580 followed the
implementation model of the original Adventure.)

Mike Arnautov used A-code in 1970s to merge Platt's Adv550 with Luckett's and
Pike's Adventure II (now known as Adv440) into Adv660. At some stage in 1990s
Mike implemented Roger Firth's "Cloak of Darkness" nano-adventure in A-code.  
And in 2003 Mike further expanded Adv660 into Adv770. Finally, in 2020, as an
exercise, Mike created an A-code implementation of the original Adventure
which took the advantage of greatly improved player interaction made possible
by the current version of A-code.
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