define constructors

Author: ogauthe

src/fusiontensor/base_interface.jl

@@ -81,15 +81,6 @@ end
 
 Base.permutedims(ft::FusionTensor, args...) = fusiontensor_permutedims(ft, args...)
 
-function Base.randn(::Type{T}, fta::FusionTensorAxes) where {T}
-  ft = FusionTensor(T, fta)
-  for m in eachstoredblock(data_matrix(ft))
-    m = randn!(m)
-  end
-  return ft
-end
-Base.randn(fta::FusionTensorAxes) = randn(Float64, fta)
-
 function Base.similar(ft::FusionTensor, T::Type)
   # reuse trees_block_mapping
 
@@ -133,6 +124,3 @@ function Base.view(ft::FusionTensor, f1::SectorFusionTree, f2::SectorFusionTree)
   charge_matrix = @view! data_matrix(ft)[trees_block_mapping(ft)[f1, f2]]
   return reshape(charge_matrix, charge_block_size(ft, f1, f2))
 end
-
-Base.zeros(::Type{T}, fta::FusionTensorAxes) where {T} = FusionTensor(T, fta)
-Base.zeros(fta::FusionTensorAxes) = zeros(Float64, fta)

src/fusiontensor/fusiontensor.jl

@@ -19,6 +19,7 @@ using GradedArrays:
   sectormergesort,
   sectors,
   space_isequal
+using LinearAlgebra: UniformScaling
 using TensorAlgebra: BlockedTuple, trivial_axis, tuplemortar
 using TensorProducts: tensor_product
 using TypeParameterAccessors: type_parameters
@@ -209,6 +210,30 @@ function FusionTensor(
   return FusionTensor(x, tuplemortar((codomain_legs, domain_legs)))
 end
 
+# specific constructors
+Base.zeros(::Type{T}, fta::FusionTensorAxes) where {T} = FusionTensor(T, fta)
+Base.zeros(fta::FusionTensorAxes) = zeros(Float64, fta)
+
+function Base.randn(::Type{T}, fta::FusionTensorAxes) where {T}
+  ft = FusionTensor(T, fta)
+  for m in eachstoredblock(data_matrix(ft))
+    m = randn!(m)
+  end
+  return ft
+end
+Base.randn(fta::FusionTensorAxes) = randn(Float64, fta)
+
+function FusionTensor(
+  ::UniformScaling, codomain_legs::Tuple{Vararg{AbstractGradedUnitRange}}
+)
+  fta = FusionTensorAxes(codomain_legs, dual.(codomain_legs))
+  ft = FusionTensor(Float64, fta)
+  for m in eachstoredblock(data_matrix(ft))
+    m .= LinearAlgebra.I(size(m, 1))
+  end
+  return ft
+end
+
 # ================================  BlockArrays interface  =================================
 
 function BlockArrays.findblock(ft::FusionTensor, f1::SectorFusionTree, f2::SectorFusionTree)

test/test_basics.jl

@@ -1,7 +1,7 @@
 using Test: @test, @test_throws, @testset
 
 using BlockArrays: Block
-using BlockSparseArrays: BlockSparseArray
+using BlockSparseArrays: BlockSparseArray, eachblockstoredindex
 using FusionTensors:
   FusionTensor,
   FusionTensorAxes,
@@ -28,6 +28,7 @@ using GradedArrays:
   space_isequal
 using TensorAlgebra: tuplemortar
 using TensorProducts: tensor_product
+using LinearAlgebra: LinearAlgebra
 
 include("setup.jl")
 
@@ -270,13 +271,29 @@ end
   @test_throws DimensionMismatch ft7 + ft3
   @test_throws DimensionMismatch ft7 - ft3
   @test_throws DimensionMismatch ft7 * ft3
+end
+
+@testset "specific constructors" begin
+  g1 = gradedrange([U1(0) => 1, U1(1) => 2, U1(2) => 3])
+  g2 = gradedrange([U1(0) => 2, U1(1) => 2, U1(3) => 1])
+  g3 = gradedrange([U1(-1) => 1, U1(0) => 2, U1(1) => 1])
+  g4 = gradedrange([U1(-1) => 1, U1(0) => 1, U1(1) => 1])
 
-  fta = FusionTensorAxes((g1,), (g2, g2))
+  fta = FusionTensorAxes((g1,), (g2, g3))
   @test zeros(fta) isa FusionTensor{Float64,3}
   @test zeros(ComplexF64, fta) isa FusionTensor{ComplexF64,3}
-  ft9 = randn(ComplexF64, fta)
-  @test ft9 isa FusionTensor{ComplexF64,3}
-  @test all(data_matrix(ft9)[Block(1, 6)] .!= 0)
+  ft1 = randn(ComplexF64, fta)
+  @test ft1 isa FusionTensor{ComplexF64,3}
+  @test all(data_matrix(ft1)[Block(1, 5)] .!= 0)
+
+  ft2 = FusionTensor(LinearAlgebra.I, (g1, g2))
+  @test ft2 isa FusionTensor{Float64,4}
+  @test axes(ft2) == FusionTensorAxes((g1, g2), dual.((g1, g2)))
+  @test collect(eachblockstoredindex(data_matrix(ft2))) == map(i -> Block(i, i), 1:6)
+  for i in 1:6
+    m = data_matrix(ft2)[Block(i, i)]
+    @test m == LinearAlgebra.I(size(m, 1))
+  end
 end
 
 @testset "missing SectorProduct" begin