Tuesday, 14 July 2015

IncursionScript and the Accent Compiler Compiler

When I got a few minutes, I spent a bit more time looking at replacing the Accent usage in Incursion.  For the most part, Accent works pretty much the same as yacc or bison, but does so in a much more user friendly way.  Yacc and bison use BNF grammars, where Accent uses an EBNF grammar.  This allows minor use of some of the regular expression syntax to avoid painful rule finagling completely.

This StackOverflow post quotes the type of work you have to do (from another web page), to convert from EBNF to BNF.  Looking at the one off case, it only seems mildly inconvenient, but when you have several variations, it becomes painful.  Especially, if you're considering converting ~100k of rules for parsing IncursionScript.

  MONSTER { theMon->Attr[0] = theMon->Attr[1] =
            theMon->Attr[2] = theMon->Attr[3] =
            theMon->Attr[4] = theMon->Attr[5] = 0;
            CurrAttk = 0; CurrFeat = 0; theRes = theMon; }
  LITERAL<name>  { theMon->Name = name; }
  ':' cexpr3<n>    { theMon->MType[0] = n; }
    ( ',' cexpr3<n2> { theMon->MType[1] = n2; }
    ( ',' cexpr3<n3> { theMon->MType[2] = n3; } )? )?
    '{' (mon_entry)* '}' { theMon++; };
That was the initial part of a monster type definition. Note that the Accent grammar syntax for the left hand side of rules, embeds the parameter name in < and >, whereas bison has a much more limited support and uses [ and ]. Also, the right hand side of a rule can use the previously mentioned aspects of regular expression syntax. In this case ( and )? are used to indicate 0 or 1 occurrences of a match, and additionally + and * can be used similarly to indicate one or more, and none or more matches.  Note also that the parsed parameter values are used directly in the code blocks, without syntactical sugar, where bison requires that n be $n.

Any use of these regular expression syntaxes would when rewritten for bison, require more and more complicated contrivances and would break up the whole rule making the grammar less readable.  In theory, it should be possible to construct a bison grammar that parses the Accent syntax, with a matching lexer, and convert an Accent grammar to a more convoluted bison grammar.  The bison grammar for the Accent syntax isn't hard to do, in fact, the Accent author provides one in their documentation.  The bulk of the work would be in writing some subset of C parsing, to transform the code blocks, and then there's the flexible Accent parameter lists which bison cannot support at all.

Old with dated pre-ANSI code and using a GPLv2 license, requiring a commercial license to be able to be used freely, Accent is a not just a compelling alternative to bison.  It's almost impossible justify downgrading from it, if one ignores the unfortunate license.  But to become something where any inspired player, can open up a script and start modding, it needs to downgrade from it.

Translating the grammar from EBNF to BNF?  Not enough.  Bison doesn't offer the additional support required.  This solution would still require hours of painstaking rewriting of the custom Accent grammar syntax.

The only remaining alternative, is to write an Accent replacement.  Or to enhance bison or byacc, to support EBNF, and the additional functionality.  But then how many hours would that require?  It might be worth downgrading that 90k Accent grammar to a bison grammar after all.  Something to think about it.

That reminds me, I've searched everywhere three times and I can't find my Dragon parsing book.  It's gone.  I imagine it's with all my compact flash usb adapter, my micro sd card, my issue of Dragontales, and the other things I haven't noticed going missing yet.

Incursion progress report

For the most part, Incursion is pretty stable now.  There are still over a hundred "issues", but this is to be expected for an engine of it's depth.  The last work I did on it before the weekend, beyond a fix for a crash which someone kindly reported, was to try and get an example module put together.

That effort stalled, because what Incursion is, is a customised demonstration game, Halls of the Goblin King.

The source code does have pretty solid support for multiple modules, any of which can provide either new dungeons to explore, or new items, effects, classes, races or any other form of content.  But there are places through the code where it just looks at the first module with an accompanying comment saying just look at this, because it's the only module we have right now.

The source has unused support for multiple players, but it's not consistently used throughout the code, and that's ignoring the question of how multiple players in a turn-based game can even be done.

What I spent time on over the weekend, was just trying to better get a picture on more of the scope of the engine.  In order to do this, I started documenting the constants defined in the script, starting with those used for dungeons (see commit #c953cfe on Bitbucket).  These can refer to values, like the minimum and maximum room widths on the x axis, the density of torches in lit rooms and even references to other game content like features, regions, terrain and even other dungeons connected in some way.

The more time I spend learning more and more of Incursion's source code, the more it seems like the best engine to base a fantasy roguelike on, given certain considerations.  One is the expression relating to the last 10% of the project requiring 90% of the work, Incursion simply requires a little more work and some polishing before anyone who simply hopes to write some scripts and to make a unique roguelike, can do so.  Another is that the game must be OGL and as mentioned fantasy.

Even just spending a few idle hours searching for and commenting the usage of constants, while watching Wimbledon over the weekend, the flexibility and depth of the engine appeals to me.  And the work required to make what it can create less same-same, seems less of a burden.  By that, I refer to things like the depth of a dungeon scaling everything, with increasing depth being downward.  If you wanted to make a tower of ascending levels, then that should be a possibility.  And whether depth should imply a fixed scaling at all, should be another matter.

One idea I should note down, in case I forget to write it elsewhere, is to make it possible to hand edit loaded module data, or to hand author some.  This would not be a method of more easier development, but would lead to additional features like live visualisation of changes to things and how it affects level generation and so forth.  Just reloading the scripts isn't as possible, as they are compiled before use.