Add implementation for DoubleEndedIterator for Zip with test

src/ziptuple.rs

@@ -98,6 +98,30 @@ macro_rules! impl_zip_iter {
                 $B: ExactSizeIterator,
             )*
         { }
+
+        #[allow(non_snake_case)]
+        impl<$($B),*> DoubleEndedIterator for Zip<($($B,)*)> where
+            $(
+                $B: DoubleEndedIterator + ExactSizeIterator,
+            )*
+        {
+            #[inline]
+            fn next_back(&mut self) -> Option<Self::Item> {
+                let ($(ref mut $B,)*) = self.t;
+                let size = *[$( $B.len(), )*].into_iter().min().unwrap();
+
+                $(
+                    if $B.len() != size {
+                        for _ in 0..$B.len() - size { $B.next_back(); }
+                    }
+                )*
+
+                match ($($B.next_back(),)*) {
+                    ($(Some($B),)*) => Some(($($B,)*)),
+                    _ => None,
+                }
+            }
+        }
     );
 }
 

tests/zip.rs

@@ -3,6 +3,7 @@ extern crate itertools;
 use itertools::Itertools;
 use itertools::EitherOrBoth::{Both, Left, Right};
 use itertools::free::zip_eq;
+use itertools::multizip;
 
 #[test]
 fn zip_longest_fused() {
@@ -42,6 +43,20 @@ fn test_double_ended_zip_longest() {
     assert_eq!(it.next(), None);
 }
 
+#[test]
+fn test_double_ended_zip() {
+    let xs = [1, 2, 3, 4, 5, 6];
+    let ys = [1, 2, 3, 7];
+    let a = xs.iter().map(|&x| x);
+    let b = ys.iter().map(|&x| x);
+    let mut it = multizip((a, b));
+    assert_eq!(it.next_back(), Some((4, 7)));
+    assert_eq!(it.next_back(), Some((3, 3)));
+    assert_eq!(it.next_back(), Some((2, 2)));
+    assert_eq!(it.next_back(), Some((1, 1)));
+    assert_eq!(it.next_back(), None);
+}
+
 
 #[should_panic]
 #[test]
@@ -63,3 +78,4 @@ fn zip_eq_panic2()
     zip_eq(&a, &b).count();
 }
 
+