Implement broadcast_axis

src/ArrayInterface.jl

@@ -792,6 +792,7 @@ include("dimensions.jl")
 include("axes.jl")
 include("size.jl")
 include("stridelayout.jl")
+include("broadcast.jl")
 
 
 abstract type AbstractArray2{T,N} <: AbstractArray{T,N} end

src/broadcast.jl

@@ -0,0 +1,61 @@
+
+"""
+    BroadcastAxis
+
+An abstract trait that is used to determine how axes are combined when calling `broadcast_axis`.
+"""
+abstract type BroadcastAxis end
+
+struct BroadcastAxisDefault <: BroadcastAxis end
+
+BroadcastAxis(x) = BroadcastAxis(typeof(x))
+BroadcastAxis(::Type{T}) where {T} = BroadcastAxisDefault()
+
+"""
+    broadcast_axis(x, y)
+
+Broadcast axis `x` and `y` into a common space. The resulting axis should be equal in length
+to both `x` and `y` unless one has a length of `1`, in which case the longest axis will be
+equal to the output.
+
+```julia
+julia> ArrayInterface.broadcast_axis(1:10, 1:10)
+
+julia> ArrayInterface.broadcast_axis(1:10, 1)
+1:10
+
+```
+"""
+broadcast_axis(x, y) = broadcast_axis(BroadcastAxis(x), x, y)
+# stagger default broadcasting in case y has something other than default
+broadcast_axis(::BroadcastAxisDefault, x, y) = _broadcast_axis(BroadcastAxis(y), x, y)
+function _broadcast_axis(::BroadcastAxisDefault, x, y)
+    return One():_combine_length(static_length(x), static_length(y))
+end
+_broadcast_axis(s::BroadcastAxis, x, y) = broadcast_axis(s, x, y)
+
+# we can use a similar trick as we do with `indices` where unequal sizes error and we just
+# keep the static value. However, axes can be unequal if one of them is `1` so we have to
+# fall back to dynamic values in those cases
+_combine_length(x::StaticInt{X}, y::StaticInt{Y}) where {X,Y} = static(_combine_length(X, Y))
+_combine_length(x::StaticInt{X}, ::Int) where {X} = x
+_combine_length(x::StaticInt{1}, y::Int) = y
+_combine_length(x::StaticInt{1}, y::StaticInt{1}) = y
+_combine_length(x::Int, y::StaticInt{Y}) where {Y} = y
+_combine_length(x::Int, y::StaticInt{1}) = x
+@inline function _combine_length(x::Int, y::Int)
+    if x === y
+        return x
+    elseif y === 1
+        return x
+    elseif x === 1
+        return y
+    else
+        _dimerr(x, y)
+    end
+end
+
+function _dimerr(@nospecialize(x), @nospecialize(y))
+    throw(DimensionMismatch("axes could not be broadcast to a common size; " *
+                            "got axes with lengths $(x) and $(y)"))
+end

test/broadcast.jl

@@ -0,0 +1,30 @@
+
+s5 = static(1):static(5)
+s4 = static(1):static(4)
+s1 = static(1):static(1)
+d5 = static(1):5
+d4 = static(1):static(4)
+d1 = static(1):static(1)
+
+struct DummyBroadcast <: ArrayInterface.BroadcastAxis end
+
+struct DummyAxis end
+
+ArrayInterface.BroadcastAxis(::Type{DummyAxis}) = DummyBroadcast()
+
+ArrayInterface.broadcast_axis(::DummyBroadcast, x, y) = y
+
+@inferred(ArrayInterface.broadcast_axis(s1, s1)) === s1
+@inferred(ArrayInterface.broadcast_axis(s5, s5)) === s5
+@inferred(ArrayInterface.broadcast_axis(s5, s1)) === s5
+@inferred(ArrayInterface.broadcast_axis(s1, s5)) === s5
+@inferred(ArrayInterface.broadcast_axis(s5, d5)) === s5
+@inferred(ArrayInterface.broadcast_axis(d5, s5)) === s5
+@inferred(ArrayInterface.broadcast_axis(d5, d1)) === d5
+@inferred(ArrayInterface.broadcast_axis(d1, d5)) === d5
+@inferred(ArrayInterface.broadcast_axis(s1, d5)) === d5
+@inferred(ArrayInterface.broadcast_axis(d5, s1)) === d5
+@inferred(ArrayInterface.broadcast_axis(s5, DummyAxis())) === s5
+
+@test_throws DimensionMismatch ArrayInterface.broadcast_axis(s5, s4)
+

test/runtests.jl

@@ -716,3 +716,8 @@ end
 include("indexing.jl")
 include("dimensions.jl")
 
+@testset "broadcast" begin
+    include("broadcast.jl")
+end
+
+