src/libsyntax/util/lev_distance.rs

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use std::cmp;
use symbol::Symbol;

/// 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.
/// Besides Levenshtein, we use case insensitive comparison to improve accuracy on an edge case with
/// a lower(upper)case letters mismatch.
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);

    let (case_insensitive_match, levenstein_match) = iter_names
    .filter_map(|&name| {
        let dist = lev_distance(lookup, &name.as_str());
        if dist <= max_dist {
            Some((name, dist))
        } else {
            None
        }
    })
    // Here we are collecting the next structure:
    // (case_insensitive_match, (levenstein_match, levenstein_distance))
    .fold((None, None), |result, (candidate, dist)| {
        (
            if candidate.as_str().to_uppercase() == lookup.to_uppercase() {
                Some(candidate)
            } else {
                result.0
            },
            match result.1 {
                None => Some((candidate, dist)),
                Some((c, d)) => Some(if dist < d { (candidate, dist) } else { (c, d) })
            }
        )
    });

    if let Some(candidate) = case_insensitive_match {
        Some(candidate) // exact case insensitive match has a higher priority
    } else {
        if let Some((candidate, _)) = levenstein_match { Some(candidate) } else { None }
    }
}

#[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);
}