implement DoubleEndedIterator for 1d lanes

This is especially useful when one wants to iterate over rows or
columns of a 2d array in reverse.

Co-authored-by: Ulrik Sverdrup <bluss@users.noreply.github.com>

src/iterators/mod.rs

@@ -732,6 +732,15 @@ where
     }
 }
 
+impl<'a, A> DoubleEndedIterator for LanesIter<'a, A, Ix1>
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.iter.next_back().map(|ptr| unsafe {
+            ArrayView::new_(ptr, Ix1(self.inner_len), Ix1(self.inner_stride as Ix))
+        })
+    }
+}
+
 // NOTE: LanesIterMut is a mutable iterator and must not expose aliasing
 // pointers. Due to this we use an empty slice for the raw data (it's unused
 // anyway).
@@ -772,6 +781,15 @@ where
     }
 }
 
+impl<'a, A> DoubleEndedIterator for LanesIterMut<'a, A, Ix1>
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.iter.next_back().map(|ptr| unsafe {
+            ArrayViewMut::new_(ptr, Ix1(self.inner_len), Ix1(self.inner_stride as Ix))
+        })
+    }
+}
+
 #[derive(Debug)]
 pub struct AxisIterCore<A, D> {
     /// Index along the axis of the value of `.next()`, relative to the start

tests/iterators.rs

@@ -39,6 +39,22 @@ fn double_ended() {
     assert_equal(aview1(&[1, 2, 3]).into_iter().rev(), [1, 2, 3].iter().rev());
 }
 
+#[test]
+fn double_ended_rows() {
+    let a = ArcArray::from_iter(0..8).into_shape_clone((4, 2)).unwrap();
+    let mut row_it = a.rows().into_iter();
+    assert_equal(row_it.next_back().unwrap(), &[6, 7]);
+    assert_equal(row_it.next().unwrap(), &[0, 1]);
+    assert_equal(row_it.next_back().unwrap(), &[4, 5]);
+    assert_equal(row_it.next_back().unwrap(), &[2, 3]);
+    assert!(row_it.next().is_none());
+    assert!(row_it.next_back().is_none());
+
+    for (row, check) in a.rows().into_iter().rev().zip(&[[6, 7], [4, 5], [2, 3], [0, 1]]) {
+        assert_equal(row, check);
+    }
+}
+
 #[test]
 fn iter_size_hint() {
     // Check that the size hint is correctly computed