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!(r.total > 6);
    let r = d20::roll_dice("1d1-3").unwrap();
    assert_eq!(r.total, -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
   }
}

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 raw_stats: Vec<Roll> = d20::roll_dice("3d6").unwrap().into_iter().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);
}

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

extern crate d20;
fn main() {
    let rg = d20::roll_range(1,100).unwrap();
    assert!(rg >= 1 && rg <= 100);
}