Crate d20 [] [src]


D20 is a simple crate designed to evaluate roll expressions. A roll expression is an english-language string that reflects the intent of a dungeon or game master to perform a particular roll.

For example, in a tabletop game you may frequently hear phrases like "roll 2d10", or "roll 3d6 and add 5". These are roll expressions, and the components within them are what we call die roll terms. A die roll term is either a term that calls for the rolling of an n-sided die x times (e.g. 3d6) or a modifier that simply adds or subtracts a constant value from the larger expression.

Examples of valid roll expressions include:

  • 3d6
  • 2d10 + 5
  • 1d20-3
  • +6
  • -2
  • 3d10+5d100-21+7

Roll expressions can have arbitrary length and complexity, and it is perfectly legal for the final result of a roll expression to be negative after applying modifiers.


extern crate d20;

fn main() {
    let r = d20::roll_dice("3d6 + 4").unwrap();
    assert!( > 6);
    let r = d20::roll_dice("1d1-3").unwrap();
    assert_eq!(, -2);

    let r = d20::roll_dice("roll four chickens and add six ferrets");
    match r {
       Ok(_) => assert!(false), // this should NOT be ok, fail
       Err(_) => assert!(true), // bad expressions produce errors

Iterating Roll

A valid Roll can be converted into an open ended iterator via its into_iter() method, providing successive rolls of the given die roll expression.

Note that it will be necessary to constrain the iterator via take(n).

extern crate d20;
use d20::*;

fn main() {
    let v: Vec<Roll> = d20::roll_dice("3d6").unwrap().into_iter().take(3).collect();

    assert_eq!(v.len(), 3);
    assert!(v[0].total >= 3 && v[0].total <= 18);
    assert!(v[1].total >= 3 && v[1].total <= 18);
    assert!(v[2].total >= 3 && v[2].total <= 18);     

Range Rolls

If you are less concerned about dice rolls and require only a random number within a given range, roll_range() will do just that.

    let rg = d20::roll_range(1,100).unwrap();
    assert!(rg >= 1 && rg <= 100);



Represents the results of an evaluated die roll expression.


A RollIterator is created when into_iter() is called on a Roll.



Represents an individual term within a die roll expression. Terms can either be numeric modifiers like +5 or -2 or they can be terms indicating die rolls.



Evaluates the expression string input as a die roll expression (e.g. 3d6 + 4). The results are returned in a Result object that contains either a valid Roll or some text indicating why the function was unable to roll the dice / evaluate the expression.


Generates a random number within the specified range. Returns a Result containing either a valid signed 32-bit integer with the randomly generated number or some text indicating the reason for failure.