Make public API, docs algorithm-agnostic

1 files changed, 229 insertions, 0 deletions

diff --git a/compiler/rustc_span/src/edit_distance.rs b/compiler/rustc_span/src/edit_distance.rs
new file mode 100644
index 00000000000..89f0386e3e9
--- /dev/null
+++ b/compiler/rustc_span/src/edit_distance.rs
@@ -0,0 +1,229 @@
+//! Edit distances.
+//!
+//! The [edit distance] is a metric for measuring the difference between two strings.
+//!
+//! [edit distance]: https://en.wikipedia.org/wiki/Edit_distance
+
+// The current implementation is the restricted Damerau-Levenshtein algorithm. It is restricted
+// because it does not permit modifying characters that have already been transposed. The specific
+// algorithm should not matter to the caller of the methods, which is why it is not noted in the
+// documentation.
+
+use crate::symbol::Symbol;
+use std::{cmp, mem};
+
+#[cfg(test)]
+mod tests;
+
+/// 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> {
+    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())
+        && a_char == b_char
+    {
+        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())
+        && a_char == b_char
+    {
+        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)
+}
+
+/// Provides a word similarity score between two words that accounts for substrings being more
+/// meaningful than a typical edit distance. The lower the score, the closer the match. 0 is an
+/// identical match.
+///
+/// Uses the edit distance between the two strings and removes the cost of the length difference.
+/// If this is 0 then it is either a substring match or a full word match, in the substring match
+/// case we detect this and return `1`. To prevent finding meaningless substrings, eg. "in" in
+/// "shrink", we only perform this subtraction of length difference if one of the words is not
+/// greater than twice the length of the other. For cases where the words are close in size but not
+/// an exact substring then the cost of the length difference is discounted by half.
+///
+/// Returns `None` if the distance exceeds the limit.
+pub fn edit_distance_with_substrings(a: &str, b: &str, limit: usize) -> Option<usize> {
+    let n = a.chars().count();
+    let m = b.chars().count();
+
+    // Check one isn't less than half the length of the other. If this is true then there is a
+    // big difference in length.
+    let big_len_diff = (n * 2) < m || (m * 2) < n;
+    let len_diff = if n < m { m - n } else { n - m };
+    let distance = edit_distance(a, b, limit + len_diff)?;
+
+    // This is the crux, subtracting length difference means exact substring matches will now be 0
+    let score = distance - len_diff;
+
+    // If the score is 0 but the words have different lengths then it's a substring match not a full
+    // word match
+    let score = if score == 0 && len_diff > 0 && !big_len_diff {
+        1 // Exact substring match, but not a total word match so return non-zero
+    } else if !big_len_diff {
+        // Not a big difference in length, discount cost of length difference
+        score + (len_diff + 1) / 2
+    } else {
+        // A big difference in length, add back the difference in length to the score
+        score + len_diff
+    };
+
+    (score <= limit).then_some(score)
+}
+
+/// Finds the best match for given word in the given iterator where substrings are meaningful.
+///
+/// A version of [`find_best_match_for_name`] that uses [`edit_distance_with_substrings`] as the
+/// score for word similarity. This takes an optional distance limit which defaults to one-third of
+/// the given word.
+///
+/// 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_with_substrings(
+    candidates: &[Symbol],
+    lookup: Symbol,
+    dist: Option<usize>,
+) -> Option<Symbol> {
+    find_best_match_for_name_impl(true, candidates, lookup, dist)
+}
+
+/// Finds the best match for a given word in the given iterator.
+///
+/// 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.
+///
+/// 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(
+    candidates: &[Symbol],
+    lookup: Symbol,
+    dist: Option<usize>,
+) -> Option<Symbol> {
+    find_best_match_for_name_impl(false, candidates, lookup, dist)
+}
+
+#[cold]
+fn find_best_match_for_name_impl(
+    use_substring_score: bool,
+    candidates: &[Symbol],
+    lookup: Symbol,
+    dist: Option<usize>,
+) -> Option<Symbol> {
+    let lookup = lookup.as_str();
+    let lookup_uppercase = lookup.to_uppercase();
+
+    // Priority of matches:
+    // 1. Exact case insensitive match
+    // 2. Edit distance match
+    // 3. Sorted word match
+    if let Some(c) = candidates.iter().find(|c| c.as_str().to_uppercase() == lookup_uppercase) {
+        return Some(*c);
+    }
+
+    let mut dist = dist.unwrap_or_else(|| cmp::max(lookup.len(), 3) / 3);
+    let mut best = None;
+    for c in candidates {
+        match if use_substring_score {
+            edit_distance_with_substrings(lookup, c.as_str(), dist)
+        } else {
+            edit_distance(lookup, c.as_str(), dist)
+        } {
+            Some(0) => return Some(*c),
+            Some(d) => {
+                dist = d - 1;
+                best = Some(*c);
+            }
+            None => {}
+        }
+    }
+    if best.is_some() {
+        return best;
+    }
+
+    find_match_by_sorted_words(candidates, lookup)
+}
+
+fn find_match_by_sorted_words(iter_names: &[Symbol], lookup: &str) -> Option<Symbol> {
+    iter_names.iter().fold(None, |result, candidate| {
+        if sort_by_words(candidate.as_str()) == sort_by_words(lookup) {
+            Some(*candidate)
+        } else {
+            result
+        }
+    })
+}
+
+fn sort_by_words(name: &str) -> String {
+    let mut split_words: Vec<&str> = name.split('_').collect();
+    // We are sorting primitive &strs and can use unstable sort here.
+    split_words.sort_unstable();
+    split_words.join("_")
+}