Add methods .cell_view() and .into_cell_view()

src/argument_traits.rs

@@ -1,6 +1,7 @@
 use std::cell::Cell;
 use std::mem::MaybeUninit;
 
+use crate::math_cell::MathCell;
 
 /// A producer element that can be assigned to once
 pub trait AssignElem<T> {
@@ -22,6 +23,13 @@ impl<'a, T> AssignElem<T> for &'a Cell<T> {
     }
 }
 
+/// Assignable element, simply `self.set(input)`.
+impl<'a, T> AssignElem<T> for &'a MathCell<T> {
+    fn assign_elem(self, input: T) {
+        self.set(input);
+    }
+}
+
 /// Assignable element, the item in the MaybeUninit is overwritten (prior value, if any, is not
 /// read or dropped).
 impl<'a, T> AssignElem<T> for &'a mut MaybeUninit<T> {

src/impl_methods.rs

@@ -20,6 +20,7 @@ use crate::dimension::{
     abs_index, axes_of, do_slice, merge_axes, size_of_shape_checked, stride_offset, Axes,
 };
 use crate::error::{self, ErrorKind, ShapeError};
+use crate::math_cell::MathCell;
 use crate::itertools::zip;
 use crate::zip::Zip;
 
@@ -151,6 +152,20 @@ where
         unsafe { ArrayViewMut::new(self.ptr, self.dim.clone(), self.strides.clone()) }
     }
 
+    /// Return a shared view of the array with elements as if they were embedded in cells.
+    ///
+    /// The cell view requires a mutable borrow of the array. Once borrowed the
+    /// cell view itself can be copied and accessed without exclusivity.
+    ///
+    /// The view acts "as if" the elements are temporarily in cells, and elements
+    /// can be changed through shared references using the regular cell methods.
+    pub fn cell_view(&mut self) -> ArrayView<'_, MathCell<A>, D>
+    where
+        S: DataMut,
+    {
+        self.view_mut().into_cell_view()
+    }
+
     /// Return an uniquely owned copy of the array.
     ///
     /// If the input array is contiguous and its strides are positive, then the

src/impl_views/conversions.rs

@@ -13,6 +13,7 @@ use crate::imp_prelude::*;
 use crate::{Baseiter, ElementsBase, ElementsBaseMut, Iter, IterMut};
 
 use crate::iter::{self, AxisIter, AxisIterMut};
+use crate::math_cell::MathCell;
 use crate::IndexLonger;
 
 /// Methods for read-only array views.
@@ -117,6 +118,21 @@ where
     pub fn into_slice(self) -> Option<&'a mut [A]> {
         self.try_into_slice().ok()
     }
+
+    /// Return a shared view of the array with elements as if they were embedded in cells.
+    ///
+    /// The cell view itself can be copied and accessed without exclusivity.
+    ///
+    /// The view acts "as if" the elements are temporarily in cells, and elements
+    /// can be changed through shared references using the regular cell methods.
+    pub fn into_cell_view(self) -> ArrayView<'a, MathCell<A>, D> {
+        // safety: valid because
+        // A and MathCell<A> have the same representation
+        // &'a mut T is interchangeable with &'a Cell<T> -- see method Cell::from_mut in std
+        unsafe {
+            self.into_raw_view_mut().cast::<MathCell<A>>().deref_into_view()
+        }
+    }
 }
 
 /// Private array view methods

src/lib.rs

@@ -139,6 +139,7 @@ pub use crate::linalg_traits::LinalgScalar;
 
 pub use crate::stacking::{concatenate, stack, stack_new_axis};
 
+pub use crate::math_cell::MathCell;
 pub use crate::impl_views::IndexLonger;
 pub use crate::shape_builder::{Shape, ShapeBuilder, StrideShape};
 
@@ -180,6 +181,7 @@ mod layout;
 mod linalg_traits;
 mod linspace;
 mod logspace;
+mod math_cell;
 mod numeric_util;
 mod partial;
 mod shape_builder;

src/math_cell.rs

@@ -0,0 +1,102 @@
+
+use std::cell::Cell;
+use std::cmp::Ordering;
+use std::fmt;
+
+use std::ops::{Deref, DerefMut};
+
+/// A transparent wrapper of [`Cell<T>`](std::cell::Cell) which is identical in every way, except
+/// it will implement arithmetic operators as well.
+///
+/// The purpose of `MathCell` is to be used from [.cell_view()](crate::ArrayBase::cell_view).
+/// The `MathCell` derefs to `Cell`, so all the cell's methods are available.
+#[repr(transparent)]
+#[derive(Default)]
+pub struct MathCell<T>(Cell<T>);
+
+impl<T> MathCell<T> {
+    /// Create a new cell with the given value
+    #[inline(always)]
+    pub const fn new(value: T) -> Self { MathCell(Cell::new(value)) }
+
+    /// Return the inner value
+    pub fn into_inner(self) -> T { Cell::into_inner(self.0) }
+
+    /// Swap value with another cell
+    pub fn swap(&self, other: &Self) {
+        Cell::swap(&self.0, &other.0)
+    }
+}
+
+impl<T> Deref for MathCell<T> {
+    type Target = Cell<T>;
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target { &self.0 }
+}
+
+impl<T> DerefMut for MathCell<T> {
+    #[inline(always)]
+    fn deref_mut(&mut self) -> &mut Self::Target { &mut self.0 }
+}
+
+impl<T> Clone for MathCell<T>
+    where T: Copy
+{
+    fn clone(&self) -> Self {
+        MathCell::new(self.get())
+    }
+}
+
+impl<T> PartialEq for MathCell<T>
+    where T: Copy + PartialEq
+{
+    fn eq(&self, rhs: &Self) -> bool {
+        self.get() == rhs.get()
+    }
+}
+
+impl<T> Eq for MathCell<T>
+    where T: Copy + Eq
+{ }
+
+impl<T> PartialOrd for MathCell<T>
+    where T: Copy + PartialOrd
+{
+    fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
+        self.get().partial_cmp(&rhs.get())
+    }
+
+    fn lt(&self, rhs: &Self) -> bool { self.get().lt(&rhs.get()) }
+    fn le(&self, rhs: &Self) -> bool { self.get().le(&rhs.get()) }
+    fn gt(&self, rhs: &Self) -> bool { self.get().gt(&rhs.get()) }
+    fn ge(&self, rhs: &Self) -> bool { self.get().ge(&rhs.get()) }
+}
+
+impl<T> Ord for MathCell<T>
+    where T: Copy + Ord
+{
+    fn cmp(&self, rhs: &Self) -> Ordering {
+        self.get().cmp(&rhs.get())
+    }
+}
+
+impl<T> fmt::Debug for MathCell<T>
+    where T: Copy + fmt::Debug
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.get().fmt(f)
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::MathCell;
+
+    #[test]
+    fn test_basic() {
+        let c = &MathCell::new(0);
+        c.set(1);
+        assert_eq!(c.get(), 1);
+    }
+}

tests/views.rs

@@ -0,0 +1,16 @@
+use ndarray::prelude::*;
+use ndarray::Zip;
+
+#[test]
+fn cell_view() {
+    let mut a = Array::from_shape_fn((10, 5), |(i, j)| (i * j) as f32);
+    let answer = &a + 1.;
+
+    {
+        let cv1 = a.cell_view();
+        let cv2 = cv1;
+
+        Zip::from(cv1).and(cv2).apply(|a, b| a.set(b.get() + 1.));
+    }
+    assert_eq!(a, answer);
+}