From ac9e3daf4ba67afd19af28a352228e6685b3ee2a Mon Sep 17 00:00:00 2001
From: Benjamin Kay <benjamin@benkay.net>
Date: Wed, 3 May 2023 12:15:03 -0500
Subject: [PATCH] Make mapv_into_any() work for ArcArray, resolves #1280

---
 src/impl_methods.rs | 42 ++++++++++++++++++++++++++++++++----------
 tests/array.rs      | 30 ++++++++++++++++++++++++++++--
 2 files changed, 60 insertions(+), 12 deletions(-)

diff --git a/src/impl_methods.rs b/src/impl_methods.rs
index 4a00ea000..6d450ca03 100644
--- a/src/impl_methods.rs
+++ b/src/impl_methods.rs
@@ -17,6 +17,7 @@ use std::mem::{size_of, ManuallyDrop};
 use crate::imp_prelude::*;
 
 use crate::argument_traits::AssignElem;
+use crate::data_traits::RawDataSubst;
 use crate::dimension;
 use crate::dimension::broadcast::co_broadcast;
 use crate::dimension::reshape_dim;
@@ -2814,15 +2815,29 @@ where
     /// map is performed as in [`mapv`].
     ///
     /// Elements are visited in arbitrary order.
-    ///
+    /// 
+    /// Note that the compiler will need some hint about the return type, which
+    /// is generic over [`DataOwned`], and can thus be an [`Array`] or
+    /// [`ArcArray`]. Example:
+    /// 
+    /// ```rust
+    /// # use ndarray::{array, Array};
+    /// let a = array![[1., 2., 3.]];
+    /// let a_plus_one: Array<_, _> = a.mapv_into_any(|a| a + 1.);
+    /// ```
+    /// 
     /// [`mapv_into`]: ArrayBase::mapv_into
     /// [`mapv`]: ArrayBase::mapv
-    pub fn mapv_into_any<B, F>(self, mut f: F) -> Array<B, D>
+    pub fn mapv_into_any<B, F, T>(self, mut f: F) -> ArrayBase<T, D>
     where
-        S: DataMut,
+        S: DataMut<Elem = A>,
         F: FnMut(A) -> B,
         A: Clone + 'static,
         B: 'static,
+        T: DataOwned<Elem = B> + RawDataSubst<A> + 'static, // lets us introspect on the types of array representations containing different data elements
+        <T as RawDataSubst<A>>::Output: RawData, // required by mapv_into()
+        ArrayBase<<T as RawDataSubst<A>>::Output, D>: From<ArrayBase<S, D>>, // required by into() to convert from the DataMut array representation of S to the DataOwned array representation of T
+        ArrayBase<T, D>: From<Array<B, D>>, // required by mapv()
     {
         if core::any::TypeId::of::<A>() == core::any::TypeId::of::<B>() {
             // A and B are the same type.
@@ -2832,16 +2847,23 @@ where
                 // Safe because A and B are the same type.
                 unsafe { unlimited_transmute::<B, A>(b) }
             };
-            // Delegate to mapv_into() using the wrapped closure.
-            // Convert output to a uniquely owned array of type Array<A, D>.
-            let output = self.mapv_into(f).into_owned();
-            // Change the return type from Array<A, D> to Array<B, D>.
-            // Again, safe because A and B are the same type.
-            unsafe { unlimited_transmute::<Array<A, D>, Array<B, D>>(output) }
+            // Delegate to mapv_into() to map from element type A to type A.
+            let output = self.mapv_into(f);
+            // Convert from S's data storage to T's data storage.
+            // Suppose `T is `OwnedRepr<B>`.
+            // Then `<T as RawDataSubst<A>>::Output` is `OwnedRepr<A>`.
+            let output: ArrayBase<<T as RawDataSubst<A>>::Output, D> = output.into();
+            // Since A == B and T stores elements of type B, it should be true
+            // that <T as RawDataSubst<A>>::Output == T.
+            // Verify that this is indeed the case.
+            assert!(core::any::TypeId::of::<<T as RawDataSubst<A>>::Output>() == core::any::TypeId::of::<T>());
+            // Now we can safely transmute the element type from A to the
+            // identical type B, keeping the same data storage.
+            unsafe { unlimited_transmute::<ArrayBase<<T as RawDataSubst<A>>::Output, D>, ArrayBase<T,D>>(output) }
         } else {
             // A and B are not the same type.
             // Fallback to mapv().
-            self.mapv(f)
+            self.mapv(f).into()
         }
     }
 
diff --git a/tests/array.rs b/tests/array.rs
index 696904dab..7d4e5512e 100644
--- a/tests/array.rs
+++ b/tests/array.rs
@@ -1054,7 +1054,8 @@ fn mapv_into_any_same_type()
 {
     let a: Array<f64, _> = array![[1., 2., 3.], [4., 5., 6.]];
     let a_plus_one: Array<f64, _> = array![[2., 3., 4.], [5., 6., 7.]];
-    assert_eq!(a.mapv_into_any(|a| a + 1.), a_plus_one);
+    let b: Array<_, _> = a.mapv_into_any(|a| a + 1.);
+    assert_eq!(b, a_plus_one);
 }
 
 #[test]
@@ -1062,7 +1063,32 @@ fn mapv_into_any_diff_types()
 {
     let a: Array<f64, _> = array![[1., 2., 3.], [4., 5., 6.]];
     let a_even: Array<bool, _> = array![[false, true, false], [true, false, true]];
-    assert_eq!(a.mapv_into_any(|a| a.round() as i32 % 2 == 0), a_even);
+    let b: Array<_, _> = a.mapv_into_any(|a| a.round() as i32 % 2 == 0);
+    assert_eq!(b, a_even);
+}
+
+#[test]
+fn mapv_into_any_arcarray_same_type() {
+    let a: ArcArray<f64, _> = array![[1., 2., 3.], [4., 5., 6.]].into_shared();
+    let a_plus_one: Array<f64, _> = array![[2., 3., 4.], [5., 6., 7.]];
+    let b: ArcArray<_, _> = a.mapv_into_any(|a| a + 1.);
+    assert_eq!(b, a_plus_one);
+}
+
+#[test]
+fn mapv_into_any_arcarray_diff_types() {
+    let a: ArcArray<f64, _> = array![[1., 2., 3.], [4., 5., 6.]].into_shared();
+    let a_even: Array<bool, _> = array![[false, true, false], [true, false, true]];
+    let b: ArcArray<_, _> = a.mapv_into_any(|a| a.round() as i32 % 2 == 0);
+    assert_eq!(b, a_even);
+}
+
+#[test]
+fn mapv_into_any_diff_outer_types() {
+    let a: Array<f64, _> = array![[1., 2., 3.], [4., 5., 6.]];
+    let a_plus_one: Array<f64, _> = array![[2., 3., 4.], [5., 6., 7.]];
+    let b: ArcArray<_, _> = a.mapv_into_any(|a| a + 1.);
+    assert_eq!(b, a_plus_one);
 }
 
 #[test]