From 94801500d352995e0b80448df6b1b25de060dd63 Mon Sep 17 00:00:00 2001
From: Martin Cornejo <martin.cornejo@tum.de>
Date: Mon, 5 Jun 2023 14:44:14 +0200
Subject: [PATCH 1/3] Add `directsum`

---
 Project.toml      |  3 ++-
 src/TensorCore.jl | 31 +++++++++++++++++++++++++++++++
 test/runtests.jl  | 30 ++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/Project.toml b/Project.toml
index 6f2a06e..beecbcc 100644
--- a/Project.toml
+++ b/Project.toml
@@ -10,7 +10,8 @@ LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 julia = "1"
 
 [extras]
+SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
-test = ["Test"]
+test = ["Test", "SparseArrays"]
diff --git a/src/TensorCore.jl b/src/TensorCore.jl
index e5ac156..e1e6500 100644
--- a/src/TensorCore.jl
+++ b/src/TensorCore.jl
@@ -5,6 +5,7 @@ using LinearAlgebra
 export ⊙, hadamard, hadamard!
 export ⊗, tensor, tensor!
 export ⊡, boxdot, boxdot!
+export ⊕, directsum
 
 """
     hadamard(a, b)
@@ -282,6 +283,36 @@ else
 
 end
 
+"""
+    directsum(A, B)
+    A ⊕ B
+
+    The direct sum of matrices `A` of size m × n and `B` of size p × q constructs a block matrix of size (m + p)×(n + q), 
+    with `A` and `B` as diagonal elements and zero matrices for the off-diagonal blocks.
+
+    `A ⊕ B = [A 0; 0 B]`
+
+    # Examples
+    ```jldoctest; setup=:(using TensorCore)
+    julia> A = [1 3 2; 2 3 1]; B = [1 6; 0 1];
+
+    julia> A ⊕ B
+    4×5 Matrix{Int64}:
+    1  3  2  0  0
+    2  3  1  0  0
+    0  0  0  1  6
+    0  0  0  0  1
+    ```
+"""
+function directsum(A::AbstractArray, B::AbstractArray)
+    Z1 = zeros(Bool, size(A, 1), size(B, 2)) # upper right
+    Z2 = zeros(Bool, size(B, 1), size(A, 2)) # lower left
+
+    return [A Z1; Z2 B]
+end
+
+const ⊕ = directsum
+
 """
     TensorCore._adjoint(A)
 
diff --git a/test/runtests.jl b/test/runtests.jl
index 9279090..754d689 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -1,5 +1,6 @@
 using TensorCore
 using LinearAlgebra
+using SparseArrays
 using Test
 
 @testset "Ambiguities" begin
@@ -279,6 +280,35 @@ end
     @test boxdot!(similar(c,1), c', d) == [dot(c, d)]
 end
 
+@testset "directsum" begin
+    A = rand(3, 2)
+    B = rand(2, 4)
+    b = rand(2)
+
+    # size
+    @test size(A ⊕ B) == (5, 6)
+    @test size(A ⊕ B') == (7, 4)
+    @test size(A ⊕ b) == (5, 3)
+    @test size(A ⊕ b') == (4, 4)
+
+    # eltype
+    eltypes = [(ComplexF64, Float64), (Float64, Float32), (Float32, Int), (Int, Bool)]
+    for (Ta, Tb) in eltypes
+        A = rand(Ta, 2, 2)
+        B = rand(Tb, 2, 2)
+        C = A ⊕ B
+        @test eltype(C) == Ta
+    end
+
+    # sparse
+    A = sprand(4, 4, 0.5)
+    B = sprand(2, 2, 0.5)
+    B´ = Array(B)
+
+    @test A ⊕ B isa SparseMatrixCSC
+    @test A ⊕ B´ isa SparseMatrixCSC
+end
+
 @testset "_adjoint" begin
     A = [1 2+im; 3 4im]
     E3 = cat(A, -A, dims=3)

From d91ce1cf9ffcfaa3babaa274cead8026d8788958 Mon Sep 17 00:00:00 2001
From: Martin Cornejo <martin.cornejo@tum.de>
Date: Mon, 5 Jun 2023 14:53:46 +0200
Subject: [PATCH 2/3] Update README

---
 README.md | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index effda00..0d412b9 100644
--- a/README.md
+++ b/README.md
@@ -5,7 +5,8 @@
 [![Codecov](https://codecov.io/gh/JuliaMath/TensorCore.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaMath/TensorCore.jl)
 
 This package is intended as a lightweight foundation for tensor operations across the Julia ecosystem.
-Currently it exports three operations:
+Currently it exports four operations:
+* `directsum` of matrices, with unicode operator `⊕`,   
 * `hadamard` elementwise multiplication, with unicode operator `⊙`,
 * `tensor` product preserves all dimensions, operator `⊗`, and
 * `boxdot` contracts neighbouring dimensions, named after the unicode `⊡`.
@@ -15,6 +16,13 @@ julia> using TensorCore
 
 julia> A = [1 2 3; 4 5 6]; B = [7 8 9; 0 10 20];
 
+julia> A ⊕ B  # directsum(A, B)
+4×6 Matrix{Int64}:
+ 1  2  3  0   0   0
+ 4  5  6  0   0   0
+ 0  0  0  7   8   9
+ 0  0  0  0  10  20
+
 julia> A ⊙ B  # hadamard(A, B)
 2×3 Matrix{Int64}:
  7  16   27

From c14dcec28afbb429a8ae002f1ad40a885ba3619a Mon Sep 17 00:00:00 2001
From: Martin Cornejo <martin.cornejo@tum.de>
Date: Mon, 5 Jun 2023 15:00:01 +0200
Subject: [PATCH 3/3] Update docs

---
 docs/src/index.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/docs/src/index.md b/docs/src/index.md
index d6f2eb3..eb6485f 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -1,7 +1,7 @@
 # TensorCore.jl
 
 This package is intended as a lightweight foundation for tensor operations across the Julia ecosystem.
-Currently it exports three operations, `hadamard`, `tensor` and `boxdot`, and corresponding unicode operators `⊙`, `⊗` and `⊡`.
+Currently it exports four operations, `hadamard`, `tensor`, `boxdot` and `directsum`, and corresponding unicode operators `⊙`, `⊗`, `⊡` and `⊕`.
 
 ## API
 
@@ -15,4 +15,5 @@ tensor
 tensor!
 boxdot
 boxdot!
+directsum
 ```