Add accumulate_axis_inplace method

src/impl_methods.rs

@@ -2138,4 +2138,60 @@ where
             }
         })
     }
+
+    /// Iterates over pairs of consecutive elements along the axis.
+    ///
+    /// The first argument to the closure is an element, and the second
+    /// argument is the next element along the axis. Iteration is guaranteed to
+    /// proceed in order along the specified axis, but in all other respects
+    /// the iteration order is unspecified.
+    ///
+    /// # Example
+    ///
+    /// For example, this can be used to compute the cumulative sum along an
+    /// axis:
+    ///
+    /// ```
+    /// use ndarray::{array, Axis};
+    ///
+    /// let mut arr = array![
+    ///     [[1, 2], [3, 4], [5, 6]],
+    ///     [[7, 8], [9, 10], [11, 12]],
+    /// ];
+    /// arr.accumulate_axis_inplace(Axis(1), |&prev, curr| *curr += prev);
+    /// assert_eq!(
+    ///     arr,
+    ///     array![
+    ///         [[1, 2], [4, 6], [9, 12]],
+    ///         [[7, 8], [16, 18], [27, 30]],
+    ///     ],
+    /// );
+    /// ```
+    pub fn accumulate_axis_inplace<F>(&mut self, axis: Axis, mut f: F)
+    where
+        F: FnMut(&A, &mut A),
+        S: DataMut,
+    {
+        if self.len_of(axis) <= 1 {
+            return;
+        }
+        let mut prev = self.raw_view();
+        prev.slice_axis_inplace(axis, Slice::from(..-1));
+        let mut curr = self.raw_view_mut();
+        curr.slice_axis_inplace(axis, Slice::from(1..));
+        // This implementation relies on `Zip` iterating along `axis` in order.
+        Zip::from(prev).and(curr).apply(|prev, curr| unsafe {
+            // These pointer dereferences and borrows are safe because:
+            //
+            // 1. They're pointers to elements in the array.
+            //
+            // 2. `S: DataMut` guarantees that elements are safe to borrow
+            //    mutably and that they don't alias.
+            //
+            // 3. The lifetimes of the borrows last only for the duration
+            //    of the call to `f`, so aliasing across calls to `f`
+            //    cannot occur.
+            f(&*prev, &mut *curr)
+        });
+    }
 }

tests/array.rs

@@ -2004,6 +2004,47 @@ fn test_map_axis() {
     assert_eq!(c, answer2);
 }
 
+#[test]
+fn test_accumulate_axis_inplace_noop() {
+    let mut a = Array2::<u8>::zeros((0, 3));
+    a.accumulate_axis_inplace(Axis(0), |&prev, curr| *curr += prev);
+    assert_eq!(a, Array2::zeros((0, 3)));
+
+    let mut a = Array2::<u8>::zeros((3, 1));
+    a.accumulate_axis_inplace(Axis(1), |&prev, curr| *curr += prev);
+    assert_eq!(a, Array2::zeros((3, 1)));
+}
+
+#[test]
+fn test_accumulate_axis_inplace_nonstandard_layout() {
+    let a = arr2(&[[1, 2, 3],
+                   [4, 5, 6],
+                   [7, 8, 9],
+                   [10,11,12]]);
+
+    let mut a_t = a.clone().reversed_axes();
+    a_t.accumulate_axis_inplace(Axis(0), |&prev, curr| *curr += prev);
+    assert_eq!(a_t, aview2(&[[1, 4, 7, 10],
+                             [3, 9, 15, 21],
+                             [6, 15, 24, 33]]));
+
+    let mut a0 = a.clone();
+    a0.invert_axis(Axis(0));
+    a0.accumulate_axis_inplace(Axis(0), |&prev, curr| *curr += prev);
+    assert_eq!(a0, aview2(&[[10, 11, 12],
+                            [17, 19, 21],
+                            [21, 24, 27],
+                            [22, 26, 30]]));
+
+    let mut a1 = a.clone();
+    a1.invert_axis(Axis(1));
+    a1.accumulate_axis_inplace(Axis(1), |&prev, curr| *curr += prev);
+    assert_eq!(a1, aview2(&[[3, 5, 6],
+                            [6, 11, 15],
+                            [9, 17, 24],
+                            [12, 23, 33]]));
+}
+
 #[test]
 fn test_to_vec() {
     let mut a = arr2(&[[1, 2, 3],