From 6acef745915aa6c7ac73fb109ad451c688056328 Mon Sep 17 00:00:00 2001
From: Sam Buercklin <sam@metalenz.com>
Date: Tue, 9 Aug 2022 14:59:08 -0400
Subject: [PATCH 1/2] add array-quantity multiplication/divison rules

---
 Project.toml             |  1 +
 src/UnitfulChainRules.jl |  5 ++-
 src/arraymath.jl         | 66 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 71 insertions(+), 1 deletion(-)
 create mode 100644 src/arraymath.jl

diff --git a/Project.toml b/Project.toml
index aaa7ef2..c6bb3ec 100644
--- a/Project.toml
+++ b/Project.toml
@@ -5,6 +5,7 @@ version = "0.1.2"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
+LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Unitful = "1986cc42-f94f-5a68-af5c-568840ba703d"
 
 [compat]
diff --git a/src/UnitfulChainRules.jl b/src/UnitfulChainRules.jl
index 6a610fe..7419fc2 100644
--- a/src/UnitfulChainRules.jl
+++ b/src/UnitfulChainRules.jl
@@ -2,8 +2,9 @@ module UnitfulChainRules
 
 using Unitful
 using Unitful: Quantity, Units, NoDims, FreeUnits
-using ChainRulesCore: NoTangent, @scalar_rule, @thunk
+using ChainRulesCore
 import ChainRulesCore: rrule, frule, ProjectTo
+using LinearAlgebra
 
 const REALCOMPLEX = Union{Real, Complex}
 
@@ -18,4 +19,6 @@ include("./trig.jl") # sin, cos, tan, etc for degrees
 
 include("./math.jl") # other math 
 
+include("./arraymath.jl") # Simple scalar-array math
+
 end # module
diff --git a/src/arraymath.jl b/src/arraymath.jl
new file mode 100644
index 0000000..4b13325
--- /dev/null
+++ b/src/arraymath.jl
@@ -0,0 +1,66 @@
+const CommutativeMulQuantity = Quantity{T,D,U} where {T<:Union{Real,Complex}, D, U}
+const CommMulVal = Union{Real, Complex, CommutativeMulQuantity}
+
+# Reference: https://github.com/JuliaDiff/ChainRules.jl/blob/148fa8875725a19cf658405609fa1a56671d0cbd/src/rulesets/Base/arraymath.jl
+
+# Defines *, / for the pairs where:
+#   1. The scalar is a commutative/mul quantity and the array is real, complex, or a comm/mul quantity
+#   2. The scalar is a commutative/mul number and the array is a comm/mul quantity
+# We have to be careful defining this so that we always have a Quantity in the signature, otherwise
+#   we overwrite methods from ChainRules.jl
+for (s_type,a_type) in (
+    (:CommutativeMulQuantity, :(<:CommMulVal)), 
+    (:(Union{Real,Complex}), :(<:CommutativeMulQuantity))
+    )
+    @eval function rrule(
+       ::typeof(*), A::$(s_type), B::AbstractArray{$(a_type)}
+    )
+        project_A = ProjectTo(A)
+        project_B = ProjectTo(B)
+        function times_pullback(ȳ)
+            Ȳ = unthunk(ȳ)
+            return (
+                NoTangent(),
+                @thunk(project_A(dot(Ȳ, B)')),
+                InplaceableThunk(
+                    X̄ -> mul!(X̄, conj(A), Ȳ, true, true),
+                    @thunk(project_B(A' * Ȳ)),
+                )
+            )
+        end
+        return A * B, times_pullback
+    end
+
+    @eval function rrule(
+        ::typeof(*), B::AbstractArray{$(a_type)}, A::$(s_type)
+    )
+        project_A = ProjectTo(A)
+        project_B = ProjectTo(B)
+        function times_pullback(ȳ)
+            Ȳ = unthunk(ȳ)
+            return (
+                NoTangent(),
+                InplaceableThunk(
+                    X̄ -> mul!(X̄, conj(A), Ȳ, true, true),
+                    @thunk(project_B(A' * Ȳ)),
+                ),
+                @thunk(project_A(dot(Ȳ, B)')),
+            )
+        end
+        return A * B, times_pullback
+    end
+
+    @eval function rrule(::typeof(/), A::AbstractArray{$(a_type)}, b::$(s_type))
+        Y = A/b
+        function slash_pullback_scalar(ȳ)
+            Ȳ = unthunk(ȳ)
+            Athunk = InplaceableThunk(
+                dA -> dA .+= Ȳ ./ conj(b),
+                @thunk(Ȳ / conj(b)),
+            )
+            bthunk = @thunk(-dot(A,Ȳ) / conj(b^2))
+            return (NoTangent(), Athunk, bthunk)
+        end
+        return Y, slash_pullback_scalar
+    end
+end

From 53f588412404606ada5a5bd024c4afdf033ad446 Mon Sep 17 00:00:00 2001
From: Sam Buercklin <sam@metalenz.com>
Date: Tue, 9 Aug 2022 15:20:16 -0400
Subject: [PATCH 2/2] add basic array-scalar math tests

---
 test/arraymath.jl | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 test/runtests.jl  |  4 ++++
 2 files changed, 50 insertions(+)
 create mode 100644 test/arraymath.jl

diff --git a/test/arraymath.jl b/test/arraymath.jl
new file mode 100644
index 0000000..d3c1e3a
--- /dev/null
+++ b/test/arraymath.jl
@@ -0,0 +1,46 @@
+using Unitful
+using UnitfulChainRules
+
+using Zygote
+
+using Random
+rng = VERSION >= v"1.7" ? Random.Xoshiro(0x0451) : Random.MersenneTwister(0x0451)
+
+@testset "Array-Scalar Multiplication" begin
+    for (a_unit, s_unit) in ((1.0,oneunit(1.0u"m")), (oneunit(1.0u"m"), 1.0))
+        A = randn(rng, 5) * a_unit
+        s = randn(rng) * s_unit
+
+        @testset "A * s ($a_unit, $s_unit)" begin
+            Ω, pb = Zygote.pullback(*, A, s)
+
+            @test Ω ≈ A * s
+            @test all(first(pb(one.(Ω))) .≈ s)
+            @test last(pb(one.(Ω))) ≈ sum(A)
+        end
+
+        @testset "s * A ($s_unit, $a_unit)" begin
+
+            Ω, pb = Zygote.pullback(*, s, A)
+
+            @test Ω ≈ s * A
+            @test all(last(pb(one.(Ω))) .≈ s)
+            @test first(pb(one.(Ω))) ≈ sum(A)
+        end
+    end
+end
+
+@testset "Array-Scalar Division" begin
+    for (a_unit, s_unit) in ((1.0,oneunit(1.0u"m")), (oneunit(1.0u"m"), 1.0))
+        @testset "($a_unit, $s_unit) division" begin
+            A = randn(rng, 5) * a_unit
+            s = randn(rng) * s_unit
+
+            Ω, pb = Zygote.pullback(/, A, s)
+
+            @test Ω ≈ A / s
+            @test all(first(pb(one.(Ω))) .≈ inv(s))
+            @test last(pb(one.(Ω))) ≈ -sum(A)/s^2
+        end
+    end
+end
\ No newline at end of file
diff --git a/test/runtests.jl b/test/runtests.jl
index 2fe38f6..cc277b6 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -16,4 +16,8 @@ end
 
 @safetestset "Math" begin
     include("./math.jl")
+end
+
+@safetestset "Array Math" begin
+    include("./arraymath.jl")
 end
\ No newline at end of file