From a1d688f64b9dc2f76969628c49ddb7881417d0da Mon Sep 17 00:00:00 2001
From: mtfishman <mfishman@flatironinstitute.org>
Date: Wed, 17 Dec 2025 08:18:12 -0500
Subject: [PATCH] Combine FusionStyles

---
 .github/workflows/IntegrationTest.yml |  1 +
 Project.toml                          |  2 +-
 src/contract/contract.jl              |  5 +----
 src/matricize.jl                      |  7 +++----
 test/test_fusionstyle.jl              | 28 +++++++++++++++++++++++++++
 5 files changed, 34 insertions(+), 9 deletions(-)
 create mode 100644 test/test_fusionstyle.jl

diff --git a/.github/workflows/IntegrationTest.yml b/.github/workflows/IntegrationTest.yml
index 17d2d40..bc64b44 100644
--- a/.github/workflows/IntegrationTest.yml
+++ b/.github/workflows/IntegrationTest.yml
@@ -21,6 +21,7 @@ jobs:
            - 'FusionTensors'
            - 'GradedArrays'
            - 'ITensorBase'
+           - 'ITensorNetworksNext'
            - 'KroneckerArrays'
            - 'NamedDimsArrays'
     uses: "ITensor/ITensorActions/.github/workflows/IntegrationTest.yml@main"
diff --git a/Project.toml b/Project.toml
index 24c36ed..0341623 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "TensorAlgebra"
 uuid = "68bd88dc-f39d-4e12-b2ca-f046b68fcc6a"
 authors = ["ITensor developers <support@itensor.org> and contributors"]
-version = "0.6.5"
+version = "0.6.6"
 
 [deps]
 ArrayLayouts = "4c555306-a7a7-4459-81d9-ec55ddd5c99a"
diff --git a/src/contract/contract.jl b/src/contract/contract.jl
index a77eecc..92b5cdb 100644
--- a/src/contract/contract.jl
+++ b/src/contract/contract.jl
@@ -10,10 +10,7 @@ end
 Matricize() = Matricize(ReshapeFusion())
 
 function default_contract_algorithm(A1::Type{<:AbstractArray}, A2::Type{<:AbstractArray})
-    style1 = FusionStyle(A1)
-    style2 = FusionStyle(A2)
-    style1 == style2 || error("Styles must match.")
-    return Matricize(style1)
+    return Matricize(FusionStyle(FusionStyle(A1), FusionStyle(A2)))
 end
 
 # Required interface if not using
diff --git a/src/matricize.jl b/src/matricize.jl
index 01e9f66..48d476b 100644
--- a/src/matricize.jl
+++ b/src/matricize.jl
@@ -6,6 +6,8 @@ abstract type FusionStyle end
 
 FusionStyle(x) = FusionStyle(typeof(x))
 FusionStyle(T::Type) = throw(MethodError(FusionStyle, (T,)))
+FusionStyle(style1::Style, style2::Style) where {Style <: FusionStyle} = Style()
+FusionStyle(style1::FusionStyle, style2::FusionStyle) = ReshapeFusion()
 
 # =======================================  misc  ========================================
 function trivial_axis(
@@ -72,10 +74,7 @@ function tensor_product_axis(r1::AbstractUnitRange, r2::AbstractUnitRange)
     return tensor_product_axis(style, r1, r2)
 end
 function tensor_product_fusionstyle(r1::AbstractUnitRange, r2::AbstractUnitRange)
-    style1 = FusionStyle(r1)
-    style2 = FusionStyle(r2)
-    style1 == style2 || error("Styles must match.")
-    return style1
+    return FusionStyle(FusionStyle(r1), FusionStyle(r2))
 end
 
 function fused_axis(
diff --git a/test/test_fusionstyle.jl b/test/test_fusionstyle.jl
new file mode 100644
index 0000000..a37a812
--- /dev/null
+++ b/test/test_fusionstyle.jl
@@ -0,0 +1,28 @@
+using TensorAlgebra: TensorAlgebra as TA, FusionStyle, Matricize, ReshapeFusion
+using Test: @test, @testset
+
+module FusionStyleTestUtils
+    using TensorAlgebra: TensorAlgebra as TA
+    struct MyArray{T, N, A <: AbstractArray{T, N}} <: AbstractArray{T, N}
+        parent::A
+    end
+    struct MyArrayFusion <: TA.FusionStyle end
+    TA.FusionStyle(::Type{<:MyArray}) = MyArrayFusion()
+end
+using .FusionStyleTestUtils: MyArray, MyArrayFusion
+
+@testset "FusionStyle" begin
+    a1 = randn(2, 2)
+    a2 = MyArray(randn(2, 2))
+    @test FusionStyle(a1) ≡ ReshapeFusion()
+    @test FusionStyle(a2) ≡ MyArrayFusion()
+    @test FusionStyle(typeof(a1)) ≡ ReshapeFusion()
+    @test FusionStyle(ReshapeFusion(), ReshapeFusion()) ≡ ReshapeFusion()
+    @test FusionStyle(MyArrayFusion(), MyArrayFusion()) ≡ MyArrayFusion()
+    @test FusionStyle(MyArrayFusion(), ReshapeFusion()) ≡ ReshapeFusion()
+    @test FusionStyle(ReshapeFusion(), MyArrayFusion()) ≡ ReshapeFusion()
+    @test TA.default_contract_algorithm(typeof(a1), typeof(a1)) ≡ Matricize(ReshapeFusion())
+    @test TA.default_contract_algorithm(typeof(a1), typeof(a2)) ≡ Matricize(ReshapeFusion())
+    @test TA.default_contract_algorithm(typeof(a2), typeof(a1)) ≡ Matricize(ReshapeFusion())
+    @test TA.default_contract_algorithm(typeof(a2), typeof(a2)) ≡ Matricize(MyArrayFusion())
+end