zip_eq: add DoubleEndedIterator implementation

src/zip_eq_impl.rs

@@ -63,3 +63,19 @@ where
     J: ExactSizeIterator,
 {
 }
+
+impl<I, J> DoubleEndedIterator for ZipEq<I, J>
+where
+    I: DoubleEndedIterator,
+    J: DoubleEndedIterator,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        match (self.a.next_back(), self.b.next_back()) {
+            (None, None) => None,
+            (Some(a), Some(b)) => Some((a, b)),
+            (None, Some(_)) | (Some(_), None) => {
+                panic!("itertools: .zip_eq() reached end of one iterator before the other")
+            }
+        }
+    }
+}

tests/quick.rs

@@ -587,6 +587,13 @@ quickcheck! {
         TestResult::passed() // won't come here
     }
 
+    fn zip_eq_rev(a: Vec<i32>, b: Vec<i32>) -> bool {
+        let len = std::cmp::min(a.len(), b.len());
+        let a = &a[..len];
+        let b = &b[..len];
+        itertools::equal(zip_eq(a, b).rev(), zip(a, b).rev())
+    }
+
     fn equal_positions(a: Vec<i32>) -> bool {
         let with_pos = a.iter().positions(|v| v % 2 == 0);
         let without = a.iter().enumerate().filter(|(_, v)| *v % 2 == 0).map(|(i, _)| i);