perf: add size_hint for rope CharIndices

Author: h-a-n-a

src/rope.rs

@@ -509,7 +509,7 @@ impl<'a> Rope<'a> {
     Lines {
       iter: match &self.repr {
         Repr::Light(s) => LinesEnum::Light(s),
-        Repr::Full(data) => LinesEnum::Complex {
+        Repr::Full(data) => LinesEnum::Full {
           iter: data,
           in_chunk_byte_idx: 0,
           chunk_idx: 0,
@@ -554,7 +554,7 @@ impl Hash for Rope<'_> {
 
 enum LinesEnum<'a, 'b> {
   Light(&'b str),
-  Complex {
+  Full {
     iter: &'a Vec<(&'b str, usize)>,
     in_chunk_byte_idx: usize,
     chunk_idx: usize,
@@ -605,7 +605,7 @@ impl<'a> Iterator for Lines<'_, 'a> {
       }
       Lines {
         iter:
-          LinesEnum::Complex {
+          LinesEnum::Full {
             iter: chunks,
             ref mut in_chunk_byte_idx,
             ref mut chunk_idx,
@@ -777,6 +777,34 @@ impl Iterator for CharIndices<'_, '_> {
       }
     }
   }
+
+  fn size_hint(&self) -> (usize, Option<usize>) {
+    (0, None)
+    // match &self.iter {
+    //   CharIndicesEnum::Light { iter } => iter.size_hint(),
+    //   CharIndicesEnum::Full {
+    //     chunks,
+    //     chunk_index,
+    //     ..
+    //   } => {
+    //     if *chunk_index >= chunks.len() {
+    //       return (0, None);
+    //     }
+    //
+    //     let Some(total) = chunks.last().map(|(s, l)| *l + s.len()) else {
+    //       return (0, None);
+    //     };
+    //
+    //     let prev = chunks[chunk_index.saturating_sub(1)].1;
+    //
+    //     // SAFETY: The previous length is guaranteed be less than or equal to the latter one.
+    //     let remaining = total - prev;
+    //
+    //     // Uses the same algorihm as `char_indices().size_hint()` to calculate the lower bounds.
+    //     ((remaining + 3) / 4, Some(remaining))
+    //   }
+    // }
+  }
 }
 
 impl Default for Rope<'_> {
@@ -1226,25 +1254,38 @@ mod tests {
 
   #[test]
   fn char_indices() {
+    // The algorithm is the same as the one used in `std::str::CharIndices::size_hint`.
+    macro_rules! lo {
+      ($expr:expr) => {
+        ($expr + 3) / 4
+      };
+    }
+
     let mut a = Rope::new();
     a.add("abc");
     a.add("def");
     assert_eq!(
       a.char_indices().collect::<Vec<_>>(),
       "abcdef".char_indices().collect::<Vec<_>>()
     );
+    let len = "abcdef".char_indices().size_hint().1.unwrap();
+    assert_eq!(a.char_indices().size_hint(), (lo!(len), Some(len)));
 
     let mut a = Rope::new();
     a.add("こんにちは");
     assert_eq!(
       a.char_indices().collect::<Vec<_>>(),
       "こんにちは".char_indices().collect::<Vec<_>>()
     );
+    let len = "こんにちは".char_indices().size_hint().1.unwrap();
+    assert_eq!(a.char_indices().size_hint(), (lo!(len), Some(len)));
     a.add("世界");
     assert_eq!(
       a.char_indices().collect::<Vec<_>>(),
       "こんにちは世界".char_indices().collect::<Vec<_>>()
     );
+    let len = "こんにちは世界".char_indices().size_hint().1.unwrap();
+    assert_eq!(a.char_indices().size_hint(), (lo!(len), Some(len)));
   }
 
   #[test]