garando_syntax2 0.1.0

Backport of libsyntax
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use crate::symbol::Symbol;
use std::cmp;

/// To find the Levenshtein distance between two strings
pub fn lev_distance(a: &str, b: &str) -> usize {
    // cases which don't require further computation
    if a.is_empty() {
        return b.chars().count();
    } else if b.is_empty() {
        return a.chars().count();
    }

    let mut dcol: Vec<_> = (0..b.len() + 1).collect();
    let mut t_last = 0;

    for (i, sc) in a.chars().enumerate() {
        let mut current = i;
        dcol[0] = current + 1;

        for (j, tc) in b.chars().enumerate() {
            let next = dcol[j + 1];
            if sc == tc {
                dcol[j + 1] = current;
            } else {
                dcol[j + 1] = cmp::min(current, next);
                dcol[j + 1] = cmp::min(dcol[j + 1], dcol[j]) + 1;
            }
            current = next;
            t_last = j;
        }
    }
    dcol[t_last + 1]
}

/// To find the best match for a given string from an iterator of names
/// As a loose rule to avoid the obviously incorrect suggestions, it takes
/// an optional limit for the maximum allowable edit distance, which defaults
/// to one-third of the given word
pub fn find_best_match_for_name<'a, T>(
    iter_names: T,
    lookup: &str,
    dist: Option<usize>,
) -> Option<Symbol>
where
    T: Iterator<Item = &'a Symbol>,
{
    let max_dist = dist.map_or_else(|| cmp::max(lookup.len(), 3) / 3, |d| d);
    iter_names
        .filter_map(|&name| {
            let dist = lev_distance(lookup, &name.as_str());
            if dist <= max_dist {
                // filter the unwanted cases
                Some((name, dist))
            } else {
                None
            }
        })
        .min_by_key(|&(_, val)| val) // extract the tuple containing the minimum edit distance
        .map(|(s, _)| s) // and return only the string
}

#[test]
fn test_lev_distance() {
    use std::char::{from_u32, MAX};
    // Test bytelength agnosticity
    for c in (0..MAX as u32)
        .filter_map(|i| from_u32(i))
        .map(|i| i.to_string())
    {
        assert_eq!(lev_distance(&c[..], &c[..]), 0);
    }

    let a = "\nMäry häd ä little lämb\n\nLittle lämb\n";
    let b = "\nMary häd ä little lämb\n\nLittle lämb\n";
    let c = "Mary häd ä little lämb\n\nLittle lämb\n";
    assert_eq!(lev_distance(a, b), 1);
    assert_eq!(lev_distance(b, a), 1);
    assert_eq!(lev_distance(a, c), 2);
    assert_eq!(lev_distance(c, a), 2);
    assert_eq!(lev_distance(b, c), 1);
    assert_eq!(lev_distance(c, b), 1);
}