perf: WithIndices use OnceCell

src/with_indices.rs

@@ -1,20 +1,17 @@
+use once_cell::sync::OnceCell;
+
 #[derive(Debug, Clone)]
 pub struct WithIndices<T: AsRef<str>> {
   /// line is a string reference
   pub line: T,
   /// the byte position of each `char` in `line` string slice .
-  pub indices_indexes: Box<[u32]>,
+  pub indices_indexes: OnceCell<Box<[u32]>>,
 }
 
 impl<T: AsRef<str>> WithIndices<T> {
   pub fn new(line: T) -> Self {
     Self {
-      indices_indexes: line
-        .as_ref()
-        .char_indices()
-        .map(|(i, _)| i as u32)
-        .collect::<Vec<_>>()
-        .into_boxed_slice(),
+      indices_indexes: OnceCell::new(),
       line,
     }
   }
@@ -26,17 +23,26 @@ impl<T: AsRef<str>> WithIndices<T> {
       return "";
     }
 
+    let indices_indexes = self.indices_indexes.get_or_init(|| {
+      self
+        .line
+        .as_ref()
+        .char_indices()
+        .map(|(i, _)| i as u32)
+        .collect::<Vec<_>>()
+        .into_boxed_slice()
+    });
+
     let str_len = self.line.as_ref().len() as u32;
-    let start = *self.indices_indexes.get(start_index).unwrap_or(&str_len);
-    let end = *self.indices_indexes.get(end_index).unwrap_or(&str_len);
+    let start =
+      indices_indexes.get(start_index).unwrap_or(&str_len).clone() as usize;
+    let end =
+      indices_indexes.get(end_index).unwrap_or(&str_len).clone() as usize;
     unsafe {
       // SAFETY: Since `indices` iterates over the `CharIndices` of `self`, we can guarantee
       // that the indices obtained from it will always be within the bounds of `self` and they
       // will always lie on UTF-8 sequence boundaries.
-      self
-        .line
-        .as_ref()
-        .get_unchecked(start as usize..end as usize)
+      self.line.as_ref().get_unchecked(start..end)
     }
   }
 }