Improve Osa implementation

This reduces the binary size of osa_distance by more than 25% while
improving the performance.

CHANGELOG.md

@@ -4,6 +4,12 @@ This project attempts to adhere to [Semantic Versioning](http://semver.org).
 
 ## [Unreleased]
 
+### Changed
+
+- improve OSA implementation
+  - reduce runtime
+  - reduce binary size by more than `25%`
+
 ### Fixed
 
 - Fix transposition counting in Jaro and Jaro-Winkler.

src/lib.rs

@@ -8,6 +8,7 @@ use std::collections::HashMap;
 use std::error::Error;
 use std::fmt::{self, Display, Formatter};
 use std::hash::Hash;
+use std::mem;
 use std::str::Chars;
 
 #[derive(Debug, PartialEq)]
@@ -288,18 +289,11 @@ pub fn normalized_levenshtein(a: &str, b: &str) -> f64 {
 /// assert_eq!(3, osa_distance("ab", "bca"));
 /// ```
 pub fn osa_distance(a: &str, b: &str) -> usize {
-    let a_len = a.chars().count();
     let b_len = b.chars().count();
-    if a == b {
-        return 0;
-    } else if a_len == 0 {
-        return b_len;
-    } else if b_len == 0 {
-        return a_len;
-    }
-
-    let mut prev_two_distances: Vec<usize> = (0..=b_len).collect();
-    let mut prev_distances: Vec<usize> = (0..=b_len).collect();
+    // 0..=b_len behaves like 0..b_len.saturating_add(1) which could be a different size
+    // this leads to significantly worse code gen when swapping the vectors below
+    let mut prev_two_distances: Vec<usize> = (0..b_len + 1).collect();
+    let mut prev_distances: Vec<usize> = (0..b_len + 1).collect();
     let mut curr_distances: Vec<usize> = vec![0; b_len + 1];
 
     let mut prev_a_char = char::MAX;
@@ -309,7 +303,7 @@ pub fn osa_distance(a: &str, b: &str) -> usize {
         curr_distances[0] = i + 1;
 
         for (j, b_char) in b.chars().enumerate() {
-            let cost = if a_char == b_char { 0 } else { 1 };
+            let cost = usize::from(a_char != b_char);
             curr_distances[j + 1] = min(
                 curr_distances[j] + 1,
                 min(prev_distances[j + 1] + 1, prev_distances[j] + cost),
@@ -322,12 +316,15 @@ pub fn osa_distance(a: &str, b: &str) -> usize {
             prev_b_char = b_char;
         }
 
-        prev_two_distances.clone_from(&prev_distances);
-        prev_distances.clone_from(&curr_distances);
+        mem::swap(&mut prev_two_distances, &mut prev_distances);
+        mem::swap(&mut prev_distances, &mut curr_distances);
         prev_a_char = a_char;
     }
 
-    curr_distances[b_len]
+    // access prev_distances instead of curr_distances since we swapped
+    // them above. In case a is empty this would still contain the correct value
+    // from initializing the last element to b_len
+    prev_distances[b_len]
 }
 
 /* Returns the final index for a value in a single vector that represents a fixed