created_at2024-04-24 19:59:07.230432
updated_at2024-04-29 02:44:13.426866
descriptionDice rolling and dice expression (with a syntax similar to FoundryVTT) parsing library
Schuyler Cebulskie (Gawdl3y)




Crates.io Version docs.rs

Tyche is a library for parsing, rolling, and explaining the results of tabletop dice.
It also has a simple CLI app binary that evaluates a given expression.

The eventual goal is full compatibility with FoundryVTT's dice syntax with some convenient extensions.


  • Parsing dice expressions, simple or complex
  • Arithmetic
    • Addition: 2d6 + 2
    • Subtraction: 2d6 - 2
    • Multiplication: 2d6 * 2
    • Division (integer, rounded down): 2d6 / 2
    • Division (integer, rounded up): 2d6 \ 2
    • Standard mathematical order of operations
    • Parenthetical grouping: (2d6 + 2) * 2
  • Dice modifiers
    • Keep highest (advantage): 2d20kh, 2d20k
      • With specific amount to keep: 3d20kh2
    • Keep lowest (disadvantage): 2d20kl
      • With specific amount to keep: 3d20kl2
    • Reroll (once): 4d6r
      • With specific condition: 4d6r>4, 4d6r>=5, 4d6r<3, 4d6r<=2, 4d6r4
    • Reroll (recursive): 4d6rr
      • With specific condition: 4d6rr>4, 4d6rr>=5, 4d6rr<3, 4d6rr<=2, 4d6rr4
    • Explode (recursive): 4d6x
      • With specific condition: 4d6x>4, 4d6x>=5, 4d6x<3, 4d6x<=2, 4d6x4
    • Explode (once): 4d6xo
      • With specific condition: 4d6xo>4, 4d6xo>=5, 4d6xo<3, 4d6xo<=2, 4d6xo4
    • Minimum: 4d8min3
    • Maximum: 4d8max6
  • Dice modifier chaining (applied in the order they're specified): 4d6rr1x>4, 8d8min2kh4xo
  • Roller abstractions, allowing custom die rolling behavior



Run cargo add tyche or add the following to your project's Cargo.toml file:

tyche = "0.2.0"

Binary (CLI app)

Run cargo install tyche --features build-binary.
Assuming Cargo's bin directory is in your $PATH, use the app with tyche or tyche <dice expression>.

Library usage

There are three main types that you'll start with while using Tyche:

  • Dice: a struct containing a dice count, number of sides for each die, and modifiers that should be applied to any resulting rolls, representing a set of dice, e.g. 4d8x.
  • Expr: an AST-like tree structure enum of individual components of a dice expression, capable of representing complex sets of mathematical operations including dice, e.g. (2d6 + 2) * 2.
  • Roller: a trait for rolling individual die values and entire sets of Dice. Sometimes referred to as simply "RNG" for the sake of brevity. There are a few Roller implementations available out of the box:
    • FastRand: uses the fastrand crate for random number generation
    • Max: always rolls the max possible value for each die
    • Val: always rolls one specific value, ignoring dice sides
    • Iter: rolls a specific sequence of die values from an iterator, ignoring dice sides (useful for testing purposes)


All parsing requires the parse feature of the crate to be enabled (which it is by default).

Tyche uses the chumsky parser generator to parse all strings in a nearly zero-copy and wicked-fast fashion.

Most conveniently, parsing can be done by utilizing the standard FromStr implementations for the relevant types (Dice, Expr, Modifier, and Condition):

use tyche::{
	dice::modifier::{Condition, Modifier},
	Dice, Expr,

let expr: Expr = "4d6rr<3 + 2d8 - 4".parse()?;
let dice: Dice = "4d6rr<3".parse()?;
let modifier: Modifier = "rr<3".parse()?;
let cond: Condition = "<3".parse()?;

Alternatively, you can directly use the parsers for each type via its associated GenParser implementation or the functions in the tyche::parse module.

Manually creating Dice

Programmatically constructing Dice to roll is painless, even with lots of chained modifiers, thanks to its use of the builder pattern.

use tyche::{dice::modifier::Condition, Dice};

// Simple set of dice, no modifiers: 2d20
let d2d20 = Dice::new(2, 20);

// Exploding dice: 4d6x
let d4d6x = Dice::builder()
	.explode(None, true)

// Chained modifiers: 6d6rr1x
let d6d6rr1x = Dice::builder()
	.reroll(Condition::Eq(1), true)
	.explode(None, true)

Rolling Dice

All rolling of dice is performed by a Roller implementation.
The most suitable "default" roller implementation is FastRand, which generates random numbers for die values using a fastrand::Rng instance.

use tyche::dice::roller::FastRand as FastRandRoller;

// Create a FastRand roller with the default thread-local fastrand::Rng
let mut roller = FastRandRoller::default();

// Create a FastRand roller with a custom-seeded fastrand::Rng
let rng = fastrand::Rng::with_seed(0x750c38d574400);
let mut roller = FastRandRoller::new(rng);

Once you have a roller, you can roll a single die at a time or a whole set of Dice with it:

use tyche::dice::{roller::FastRand as FastRandRoller, Dice, Roller};

let mut roller = FastRandRoller::default();

// Roll a single 20-sided die
let die = roller.roll_die(20);

// Roll two six-sided dice
let dice = Dice::new(2, 6);
let rolled = roller.roll(&dice, true)?;

Rolling a single die results in a DieRoll instance, whereas rolling a set of Dice returns a Rolled instance.

Working with rolled dice sets

Rolled is a struct that ties multiple DieRolls together with the Dice that were rolled to generate them so it can accurately describe what happened during the roll and application of any modifiers that were on the dice.

Using a Rolled result, you can easily total the results of all rolled dice and/or build a string that contains the original dice set along with a list of each individual die roll.

use tyche::{
	dice::{roller::FastRand as FastRandRoller, Dice, Roller},

// Build and roll 4d6kh2
let mut roller = FastRandRoller::default();
let dice = Dice::builder()
let rolled = roller.roll(&dice, true)?;

// Total the results
let total = rolled.total()?;

// Build a detailed string about the rolls
// The resulting string will look like "4d6kh2[2 (d), 5, 6, 4 (d)]"
let described = rolled.to_string();

// This is the same as the to_string() call above, except only two die rolls will be listed.
// The resulting string will look like "4d6kh2[2 (d), 5, 2 more...]"
let limited = rolled.describe(Some(2));

Working with individual die rolls

A DieRoll contains the final value of the die alongside information about any changes that were made to it and the source of said changes.

Added rolls

When a modifier (rather than the original dice set) causes the addition of a new DieRoll to a Rolled set, the roll's added_by field is set to Some(source_modifier). An added roll will be marked as such in strings.
Modifiers that can result in additional die rolls are:

  • Explode
  • Reroll
Dropped rolls

When a modifier causes the removal of a DieRoll from a Rolled set, the roll's dropped_by field is set to Some(source_modifier). A dropped roll will not be affected by any further modifiers, is not included when totaling the rolled set, and will be marked as dropped in strings.
Modifiers that can result in dropped die rolls are:

  • Reroll
  • Keep highest
  • Keep lowest
Changed rolls

When a modifier directly manipulates the value of a DieRoll in a Rolled set, the roll's changes field has a ValChange item added to it that describes the change made and the modifier that caused it.
Modifiers that can result in changed die rolls are:

  • Minimum
  • Maximum

Working with expressions

Expr trees are essentially always obtained from parsing an expression string, as manually creating them would be quite cumbersome.

Once you have an Expr variant, it can be evaluated to produce an Evaled expression tree. Evaled expression trees are nearly identical to their originating Expr tree, except any Dice variants have their dice rolled. This separation allows for describing an expression in a detailed way both before and after rolling dice it contains, in addition to a few other utilities.

use tyche::{
	dice::roller::FastRand as FastRandRoller,
	expr::{Describe, Expr},

// Parse a nice dice expression
let expr: Expr = "4d6 + 2d8 - 2".parse()?;

// This expression is definitely not deterministic (it contains dice sets)

// Build a string for the expression - in this case, it'll be identical to the original parsed string
assert_eq!(expr.to_string(), "4d6 + 2d8 - 2");

// Evaluate the expression, rolling any dice sets it contains
let mut roller = FastRandRoller::default();
let evaled = expr.eval(&mut roller)?;

// Build a detailed string about the evaluated expression
// The resulting string will look like "4d6[3, 2, 6, 2] + 2d8[5, 4] - 2"
let described = evaled.to_string();

// This is the same as the to_string() call above, except only two die rolls in each set will be listed.
// The resulting string will look like "4d6[3, 2, 2 more...] + 2d8[5, 4] - 2"
let limited = evaled.describe(Some(2));

// Calculate the final result of the expression
let total = evaled.calc()?;


All contributions are welcome! Try to keep PRs relatively small in scope (single feature/fix/refactor at a time) and word your commits descriptively.


Tyche is licensed under the LGPLv3 license.

Commit count: 121

cargo fmt