Reshape for a OneElement (#365)

* Reshape for a OneElement

* Tests for 0D

src/oneelement.jl

@@ -372,6 +372,16 @@ function broadcasted(::DefaultArrayStyle{N}, ::typeof(\), x::Number, r::OneEleme
     OneElement(x \ r.val, r.ind, axes(r))
 end
 
+# reshape
+
+function Base.reshape(A::OneElement, shape::Tuple{Vararg{Int}})
+    prod(shape) == length(A) || throw(DimensionMismatch("new dimension $shape must be consistent with array size $(length(A))"))
+    # we use the fact that the linear index of the non-zero value is preserved
+    oldlinind = LinearIndices(A)[A.ind...]
+    newcartind = CartesianIndices(shape)[oldlinind]
+    OneElement(A.val, Tuple(newcartind), shape)
+end
+
 # show
 _maybesize(t::Tuple{Base.OneTo{Int}, Vararg{Base.OneTo{Int}}}) = size.(t,1)
 _maybesize(t) = t

test/runtests.jl

@@ -2215,6 +2215,18 @@ end
         @test adjoint(A) == OneElement(3, (1,2), (1,4))
     end
 
+    @testset "reshape" begin
+        for O in (OneElement(2, (2,3), (4,5)), OneElement(2, (2,), (20,)),
+                    OneElement(2, (1,2,2), (2,2,5)))
+            A = Array(O)
+            for shp in ((2,5,2), (5,1,4), (20,), (2,2,5,1,1))
+                @test reshape(O, shp) == reshape(A, shp)
+            end
+        end
+        O = OneElement(2, (), ())
+        @test reshape(O, ()) === O
+    end
+
     @testset "isassigned" begin
         f = OneElement(2, (3,3), (4,4))
         @test !isassigned(f, 0, 0)