d20

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.

Usage

let r = d20::roll_dice("3d6 + 4").unwrap();
assert!(r.total > 6);

let r = d20::roll_dice("1d1-3").unwrap();
assert_eq!(r.total, -2);

// Malformed expressions return a descriptive error instead of panicking.
assert!(d20::roll_dice("roll four chickens and add six ferrets").is_err());

Every failure mode is reported through the d20::D20Error enum, and no input — however malformed — will ever panic. Counts, sides, and modifiers are supported up to the documented MAX_DICE / MAX_SIDES / MAX_MODIFIER limits; values beyond them return a clear error rather than overflowing.

Inspecting Results

A Roll exposes the per-term breakdown via terms: Vec<TermResult>. Each TermResult is either a Dice { multiplier, sides, rolls } recording the individual die results, or a Modifier(i32) constant — so the data is self-describing (modifiers are not faked as single-element rolls):

use d20::TermResult;

let r = d20::roll_dice("3d6 + 2").unwrap();
for term in &r.terms {
    match term {
        TermResult::Dice { multiplier, sides, rolls } => {
            println!("{multiplier}d{sides} rolled {rolls:?} (subtotal {})", term.subtotal());
        }
        TermResult::Modifier(n) => println!("modifier {n}"),
    }
}
assert_eq!(r.total, r.terms.iter().map(TermResult::subtotal).sum());

Iterating Roll

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

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

use d20::*;

let raw_stats: Vec<Roll> = d20::roll_dice("3d6").unwrap().rolls().take(6).collect();

println!("\nCHARACTER STATS:");
println!("  STR: {}", raw_stats[0].total);
println!("  INT: {}", raw_stats[1].total);
println!("  WIS: {}", raw_stats[2].total);
println!("  DEX: {}", raw_stats[3].total);
println!("  CON: {}", raw_stats[4].total);
println!("  CHA: {}", raw_stats[5].total);

Deterministic Rolls

For reproducible results (tests, replays), supply your own RNG via roll_dice_with_rng / roll_range_with_rng:

use rand::SeedableRng;
use rand::rngs::StdRng;

let mut rng = StdRng::seed_from_u64(42);
let a = d20::roll_dice_with_rng("3d6", &mut rng).unwrap();
let b = d20::roll_dice_with_rng("3d6", &mut StdRng::seed_from_u64(42)).unwrap();
assert_eq!(a, b); // same seed => same roll

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!((1..=100).contains(&rg));