{"id":867,"date":"2018-10-14T17:08:48","date_gmt":"2018-10-14T15:08:48","guid":{"rendered":"http:\/\/hudecekpetr.cz\/?p=867"},"modified":"2018-10-14T17:08:52","modified_gmt":"2018-10-14T15:08:52","slug":"a-formal-grammar-for-magic-the-gathering","status":"publish","type":"post","link":"https:\/\/hudecekpetr.cz\/cs\/a-formal-grammar-for-magic-the-gathering\/","title":{"rendered":"A formal grammar for Magic: the Gathering"},"content":{"rendered":"

<\/p>

I wrote an ANTLR4 formal grammar for Magic: the Gathering cards.<\/p>\n

It turns this<\/p>\n

\"A<\/p>\n

Or, more specifically, it turns this<\/p>\n

Reach<\/em>
\n Undergrowth \u2014 When you cast this spell, reveal the top X cards of your library, where X is the number of creature cards in your graveyard. You may put a green permanent card with converted mana cost X or less from among them onto the battlefield. Put the rest on the bottom of your library in a random order.<\/em><\/p>\n

Into this:<\/p>\n

\"\"<\/a><\/p>\n

You can try it out here:\u00a0https:\/\/soothsilver.github.io\/mtg-grammar\/<\/a><\/strong>\u00a0and you can download the grammar and the source code from\u00a0https:\/\/github.com\/Soothsilver\/mtg-grammar<\/a>.<\/p>\n

<\/p>\n

It handles all 273 cards of\u00a0Guilds of Ravnica<\/em>, the most recent Standard set as of now.<\/p>\n

What’s it good for? For nothing, because the Wizards Fan Content Policy<\/a> prohibits fans from using Wizards‘ „gameplay“ in their content, which, I suspect, includes Magic: the Gathering rules.<\/p>\n

But suppose we weren’t limited by the policy. The Magic Arena<\/a> team isn’t. They are using a parser, similar to the one I created, to generate code from rules text.<\/p>\n

Traditionally in video games, game components such as cards, spells, and abilities are scripted. Here’s Affectionate Indrik:<\/p>\n

\"\"<\/a><\/p>\n

Here’s how the code for Affectionate Indrik’s ability looks like in an unnamed Magic: the Gathering game:<\/p>\n

Ability ability = new EntersBattlefieldTriggeredAbility(
\nnew FightTargetSourceEffect()
\n.setText(„you may have it fight target creature you don’t control“),
\ntrue
\n);
\nability.addTarget(new TargetCreaturePermanent(filter));
\nthis.addAbility(ability);<\/span><\/p>\n

And here’s how it looks like after parsing from rules text:<\/p>\n

\"\"<\/a><\/p>\n

The idea is that, from this syntax tree, you can generate the code above automatically. A syntax tree is no longer arbitrary English text incomprehensible to a rules engine, it’s an object that can be, in theory, unambiguously converted into code. The hope is that this way, it will be easier to add new cards (because nobody needs to script them) and that there will be less bugs (because nobody can make bugs in the scripts because the code is created automatically). There could be „bugs“ in the rules text, but that’s unlikely. Wizards of the Coast pays attention to editing. I’ve only encountered a single editing error in the entire Guilds of Ravnica<\/em> set (there’s a missing space between the end of the rules text and the reminder text for surveil in the card Mission Briefing<\/a>).<\/p>\n

But of course, the rub is that there could be bugs in the parser or the semantic analysis or the code generation. That has already happened. Look at Beamsplitter Mage:<\/p>\n

\"\"<\/a><\/p>\n

It reads, in part,\u00a0„Whenever [..], if you control one or more other creatures that spell could target, choose one of those creatures. […]“<\/p>\n

So the parser sees the phrase „those creatures“. Well, asks the parser, I wonder what those creatures are, I should look at the preceding sentence to figure that out. It does and it\u00a0reads „one or more other creatures that spell could target“ and joyfully presents to the player as possible choices all creatures that spell could target.<\/p>\n

What the parser failed to\u00a0notice are the words „if you control“: „those creatures“ actually refers to „one or more other creatures under your control that the spell could target“. Here’s a Benjamin Finkel, a Magic Arena developer, explaining it:<\/p>\n

Comment<\/a> from discussion Beamsplitter mage incorrect on mtga<\/a>.<\/div>\n