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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,
kind: &str,
) -> String {
match closest(choice, iter, &key) {
Some(e) => format!(
"\n\nhelp: a {kind} with a similar name exists: `{}`",
key(&e)
),
None => String::new(),
}
}
#[test]
fn test_edit_distance() {
use std::char::{MAX, from_u32};
// 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));
}