#
cargo/util/
edit_distance.rs

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    use std::{cmp, mem};

    /// Finds the [edit distance] between two strings.
    ///
    /// Returns `None` if the distance exceeds the limit.
    ///
    /// [edit distance]: https://en.wikipedia.org/wiki/Edit_distance
    pub fn edit_distance(a: &str, b: &str, limit: usize) -> Option<usize> {
        // Comparing the strings lowercased will result in a difference in capitalization being less distance away
        // than being a completely different letter. Otherwise `CHECK` is as far away from `check` as it
        // is from `build` (both with a distance of 5). For a single letter shortcut (e.g. `b` or `c`), they will
        // all be as far away from any capital single letter entry (all with a distance of 1).
        // By first lowercasing the strings, `C` and `c` are closer than `C` and `b`, for example.
        let a = a.to_lowercase();
        let b = b.to_lowercase();

        let mut a = &a.chars().collect::<Vec<_>>()[..];
        let mut b = &b.chars().collect::<Vec<_>>()[..];

        // Ensure that `b` is the shorter string, minimizing memory use.
        if a.len() < b.len() {
            mem::swap(&mut a, &mut b);
        }

        let min_dist = a.len() - b.len();
        // If we know the limit will be exceeded, we can return early.
        if min_dist > limit {
            return None;
        }

        // Strip common prefix.
        while let Some(((b_char, b_rest), (a_char, a_rest))) = b.split_first().zip(a.split_first()) {
            if a_char != b_char {
                break;
            }
            a = a_rest;
            b = b_rest;
        }
        // Strip common suffix.
        while let Some(((b_char, b_rest), (a_char, a_rest))) = b.split_last().zip(a.split_last()) {
            if a_char != b_char {
                break;
            }
            a = a_rest;
            b = b_rest;
        }

        // If either string is empty, the distance is the length of the other.
        // We know that `b` is the shorter string, so we don't need to check `a`.
        if b.len() == 0 {
            return Some(min_dist);
        }

        let mut prev_prev = vec![usize::MAX; b.len() + 1];
        let mut prev = (0..=b.len()).collect::<Vec<_>>();
        let mut current = vec![0; b.len() + 1];

        // row by row
        for i in 1..=a.len() {
            current[0] = i;
            let a_idx = i - 1;

            // column by column
            for j in 1..=b.len() {
                let b_idx = j - 1;

                // There is no cost to substitute a character with itself.
                let substitution_cost = if a[a_idx] == b[b_idx] { 0 } else { 1 };

                current[j] = cmp::min(
                    // deletion
                    prev[j] + 1,
                    cmp::min(
                        // insertion
                        current[j - 1] + 1,
                        // substitution
                        prev[j - 1] + substitution_cost,
                    ),
                );

                if (i > 1) && (j > 1) && (a[a_idx] == b[b_idx - 1]) && (a[a_idx - 1] == b[b_idx]) {
                    // transposition
                    current[j] = cmp::min(current[j], prev_prev[j - 2] + 1);
                }
            }

            // Rotate the buffers, reusing the memory.
            [prev_prev, prev, current] = [prev, current, prev_prev];
        }

        // `prev` because we already rotated the buffers.
        let distance = prev[b.len()];
        (distance <= limit).then_some(distance)
    }

    /// Find the closest element from `iter` matching `choice`. The `key` callback
    /// is used to select a `&str` from the iterator to compare against `choice`.
    pub fn closest<'a, T>(
        choice: &str,
        iter: impl Iterator<Item = T>,
        key: impl Fn(&T) -> &'a str,
    ) -> Option<T> {
        // Only consider candidates with an edit distance of 3 or less so we don't
        // suggest out-of-the-blue options.
        iter.filter_map(|e| Some((edit_distance(choice, key(&e), 3)?, e)))
            .min_by_key(|t| t.0)
            .map(|t| t.1)
    }

    /// Version of `closest` that returns a common "suggestion" that can be tacked
    /// onto the end of an error message.
    pub fn closest_msg<'a, T>(
        choice: &str,
        iter: impl Iterator<Item = T>,
        key: impl Fn(&T) -> &'a str,
    ) -> String {
        match closest(choice, iter, &key) {
            Some(e) => format!("\n\n\tDid you mean `{}`?", key(&e)),
            None => String::new(),
        }
    }

    #[test]
    fn test_edit_distance() {
        use std::char::{from_u32, MAX};
        // Test bytelength agnosticity
        for c in (0u32..MAX as u32)
            .filter_map(from_u32)
            .map(|i| i.to_string())
        {
            assert_eq!(edit_distance(&c, &c, usize::MAX), Some(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!(edit_distance(a, b, usize::MAX), Some(1));
        assert_eq!(edit_distance(b, a, usize::MAX), Some(1));
        assert_eq!(edit_distance(a, c, usize::MAX), Some(2));
        assert_eq!(edit_distance(c, a, usize::MAX), Some(2));
        assert_eq!(edit_distance(b, c, usize::MAX), Some(1));
        assert_eq!(edit_distance(c, b, usize::MAX), Some(1));
    }