roman 0.1.6

Convert between integers and roman numerals
Documentation 
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//! Conversion between integers and roman numerals.

static ROMAN: &'static [(char, i32)] = &[
    ('I', 1), ('V', 5), ('X', 10), ('L', 50), ('C', 100), ('D', 500), ('M', 1000) ];
static ROMAN_PAIRS: &'static [(&'static str, i32)] = &[
    ("M", 1000), ("CM", 900), ("D",  500), ("CD", 400),
    ("C", 100),  ("XC", 90),  ("L",  50),  ("XL", 40),
    ("X", 10),   ("IX", 9),   ("V",  5),   ("IV", 4),
    ("I", 1) ];

/// The largest number representable as a roman numeral.
pub static MAX: i32 = 3999;

/// Converts an integer into a roman numeral.
///
/// Works for integer between 1 and 3999 inclusive, returns None otherwise.
///
/// # Example
///
/// ```
/// let x = roman::to(14);
/// assert_eq!(x.unwrap(), "XIV");
/// ```
///
pub fn to(n: i32) -> Option<String> {
    if n <= 0 || n > MAX { return None }
    let mut out = String::new();
    let mut n = n;
    for &(name, value) in ROMAN_PAIRS.iter() {
        while n >= value {
            n -= value;
            out.push_str(name);
        }
    }
    assert!(n == 0);
    Some(out)
}

#[test]
fn test_to_roman() {
    let roman = "I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII XVIII XIX XX XXI XXII".split(' ');
    for (i, x) in roman.enumerate() {
        let n = (i+1) as i32;
        assert_eq!(to(n).unwrap(), x);
    }
    assert_eq!(to(1984).unwrap(), "MCMLXXXIV");
}

/// Converts a roman numeral to an integer.
///
/// Works for integer between 1 and 3999 inclusive, returns None otherwise.
///
/// # Example
///
/// ```
/// let x = roman::from("XIV");
/// assert_eq!(x.unwrap(), 14);
/// ```
///
pub fn from(txt: &str) -> Option<i32> {
    let n = match from_lax(txt) {
        Some(n) => n,
        None    => return None
    };
    match to(n) {
        Some(ref x) if *x == txt => Some(n),
        _ => None
    }
}

fn from_lax(txt: &str) -> Option<i32> {
    let (mut n, mut max) = (0, 0);
    for c in txt.chars().rev() {
        let it = ROMAN.iter().find(|x| { let &(ch, _) = *x; ch == c } );
        if it.is_none() { return None }
        let &(_, val) = it.unwrap();
        if val < max {
            n -= val;
        } else {
            n += val;
            max = val;
        }
    }
    Some(n)
}

#[test]
fn test_from() {
    assert!(from("i").is_none());
}

#[test]
fn test_to_from() {
    for n in 1..MAX {
        assert_eq!(from(&to(n).unwrap()).unwrap(), n);
    }
}