Parsing and Visualizing Dice Results in C#

I created a simple webapp that allows you to enter standard dice notation, and get the probability of different results in pretty graphs.

Check out the project at diceresults.hdyar.com, and the source code on github.

Why?

A while ago, I was working on a 3D Dice Roller package for Unity. This is a more useful tool, and I do plan to finish it. In writing it, I had to put together a simple dice parser. The idea of polishing that never quite escaped me.

Later, my friend started working on a clone of some of the features of anydice, for a web-based RPG toolkiit they are still hacking on. We talked about how to implement things, and I just got that itch - I had to know if I could make it myself. How would I do it?

This is a one day jam project, and nothing to be taken seriously. (go use AnyDice, it’s great!). But it’s fun to explore parsers.

How It Works

The skeleton is in Blazor. I am using blazor for the server for the clayze project, so sticking with that was an easy decision, as it helps me learn the tool.

The interesting parts are the Dice Parser and the results calculation.

Parsing Dice Rolls in C#

I used the sprache parser combinator library to write the parser. I originally wrote the parser by hand, but it was bugging me out.

With Sprache, assembling a parser is pretty simple, the code just looks like this.

public static readonly Parser<Expression> NormalDiceRoll =
		from d in Number.Optional()
		from sep in DiceSep
		from f in Number
		select d.IsDefined ? new DiceRollExpression(d.Get(), f) : new DiceRollExpression(f);

Here, “Number” and “DiceSep” are other parsers. They all get combined together in a way that roughly matches the hierarchy of the language one is parsing. This expressiveness is what makes a parser combinator so powerful.

Calculating Possible Results of Dice Rolls

I treat every value as a dice roll, which are basically arrays of faces. A modifier number (the +3 in 1d20+3) is just a one-sided dice. I take all of these roll-segments and combine them together with their modifier (+,-,x,/).

Combining them is just brute force. In the roll 2d4, I take a [1,2,3,4] and add it to [1,2,3,4] by adding every value together, producing [1+1,1+2,1+3,1+4,2+1,2+2,2+3,2+4,3+1,3+2,3+3,3+4,4+1,4+2,4+3,4+4].

At this point, I imagined my old linear algebra professor reminding me there was a much easier way to do things. Probably.

That’s basically it. It’s brute-force method, and… it’s fine. Computers are fast. Besides, if you wanted to do this for real, than just use anydice