1 files changed, 35 insertions, 5 deletions
diff --git a/src/iterators/flat_pairs.rs b/src/iterators/flat_pairs.rs
index 52a2074..9b92f55 100644
--- a/src/iterators/flat_pairs.rs
+++ b/src/iterators/flat_pairs.rs
@@ -25,7 +25,7 @@ pub struct FlatPairs<'i, R> {
     /// # Safety
     ///
     /// All `QueueableToken`s' `input_pos` must be valid character boundary indices into `input`.
-    queue: Rc<Vec<QueueableToken<R>>>,
+    queue: Rc<Vec<QueueableToken<'i, R>>>,
     input: &'i str,
     start: usize,
     end: usize,
@@ -35,12 +35,12 @@ pub struct FlatPairs<'i, R> {
 /// # Safety
 ///
 /// All `QueueableToken`s' `input_pos` must be valid character boundary indices into `input`.
-pub unsafe fn new<R: RuleType>(
-    queue: Rc<Vec<QueueableToken<R>>>,
-    input: &str,
+pub unsafe fn new<'i, R: RuleType>(
+    queue: Rc<Vec<QueueableToken<'i, R>>>,
+    input: &'i str,
     start: usize,
     end: usize,
-) -> FlatPairs<'_, R> {
+) -> FlatPairs<'i, R> {
     FlatPairs {
         queue,
         input,
@@ -102,6 +102,13 @@ impl<'i, R: RuleType> FlatPairs<'i, R> {
     }
 }
 
+impl<'i, R: RuleType> ExactSizeIterator for FlatPairs<'i, R> {
+    fn len(&self) -> usize {
+        // Tokens len is exactly twice as flatten pairs len
+        (self.end - self.start) >> 1
+    }
+}
+
 impl<'i, R: RuleType> Iterator for FlatPairs<'i, R> {
     type Item = Pair<'i, R>;
 
@@ -122,6 +129,11 @@ impl<'i, R: RuleType> Iterator for FlatPairs<'i, R> {
 
         Some(pair)
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = <Self as ExactSizeIterator>::len(self);
+        (len, Some(len))
+    }
 }
 
 impl<'i, R: RuleType> DoubleEndedIterator for FlatPairs<'i, R> {
@@ -214,4 +226,22 @@ mod tests {
         assert_eq!(pair.line_col(), (1, 5));
         assert_eq!(pair.line_col(), pair.as_span().start_pos().line_col());
     }
+
+    #[test]
+    fn exact_size_iter_for_pairs() {
+        let pairs = AbcParser::parse(Rule::a, "abc\nefgh").unwrap().flatten();
+        assert_eq!(pairs.len(), pairs.count());
+
+        let pairs = AbcParser::parse(Rule::a, "我很漂亮efgh").unwrap().flatten();
+        assert_eq!(pairs.len(), pairs.count());
+
+        let pairs = AbcParser::parse(Rule::a, "abc\nefgh").unwrap().flatten();
+        let pairs = pairs.rev();
+        assert_eq!(pairs.len(), pairs.count());
+
+        let mut pairs = AbcParser::parse(Rule::a, "abc\nefgh").unwrap().flatten();
+        let pairs_len = pairs.len();
+        let _ = pairs.next().unwrap();
+        assert_eq!(pairs.count() + 1, pairs_len);
+    }
 }