Add dual and isdual axis-duality generics

Project.toml

@@ -1,6 +1,6 @@
 name = "TensorAlgebra"
 uuid = "68bd88dc-f39d-4e12-b2ca-f046b68fcc6a"
-version = "0.17.7"
+version = "0.17.8"
 authors = ["ITensor developers <support@itensor.org> and contributors"]
 
 [workspace]

ext/TensorAlgebraTensorKitExt.jl

@@ -37,6 +37,10 @@ TensorAlgebra.scalar(t::AbstractTensorMap) = TensorKit.scalar(t)
 TensorAlgebra.data(t::TensorMap) = TensorAlgebra.data(t.data)
 TensorAlgebra.data(t::DiagonalTensorMap) = TensorAlgebra.data(t.data)
 
+# Duality queries on a single index space route to TensorKit's own space duality.
+TensorAlgebra.isdual(V::ElementarySpace) = TensorKit.isdual(V)
+TensorAlgebra.dual(V::ElementarySpace) = TensorKit.dual(V)
+
 # The trivial length-1 axis of a space is its unit space (`oneunit`), the trivial-sector
 # one-dimensional space; the length-`n` form is the direct sum of `n` unit spaces.
 TensorAlgebra.trivialrange(V::ElementarySpace) = oneunit(V)

src/TensorAlgebra.jl

@@ -1,7 +1,7 @@
 module TensorAlgebra
 
-export contract, contract!, eig_full, eig_trunc, eig_vals, eigh_full, eigh_trunc,
-    eigh_vals, gram_eigh_full, gram_eigh_full_with_pinv, invsqrth_safe, left_null,
+export contract, contract!, dual, eig_full, eig_trunc, eig_vals, eigh_full, eigh_trunc,
+    eigh_vals, gram_eigh_full, gram_eigh_full_with_pinv, invsqrth_safe, isdual, left_null,
     left_orth, left_polar, lq_compact, lq_full, project_hermitian, qr_compact,
     qr_full, right_null, right_orth, right_polar, sqrth_invsqrth_safe, sqrth_safe,
     svd_compact, svd_full, svd_trunc, svd_vals
@@ -24,6 +24,7 @@ include("matricize.jl")
 include("concatenate.jl")
 include("directsum.jl")
 include("diagonal.jl")
+include("dual.jl")
 include("to_range.jl")
 include("contract/contractalgorithm.jl")
 include("contract/contract.jl")

src/dual.jl

@@ -0,0 +1,21 @@
+"""
+    isdual(a) -> Bool
+
+Returns `true` or `false` depending on if the axis `a` is dual. Falls back to `false`
+for `AbstractUnitRange`.
+
+See also [`dual`](@ref).
+"""
+function isdual end
+isdual(::AbstractUnitRange) = false
+
+"""
+    dual(a)
+
+Returns the dual of the axis `a`. Falls back to returning `a` unchanged for
+`AbstractUnitRange`.
+
+See also [`isdual`](@ref).
+"""
+function dual end
+dual(a::AbstractUnitRange) = a

test/test_dual.jl

@@ -0,0 +1,14 @@
+using TensorAlgebra: TensorAlgebra, dual, isdual
+using Test: @test, @test_throws, @testset
+
+@testset "dual/isdual fallbacks on ranges" begin
+    # An ordinary range has no arrow: never dual, and its own dual.
+    for r in (Base.OneTo(4), 2:5)
+        @test isdual(r) == false
+        @test dual(r) === r
+    end
+    # No universal fallback: a type with no duality concept errors instead of
+    # silently returning a default.
+    @test_throws MethodError isdual(3)
+    @test_throws MethodError dual(3)
+end

test/test_exports.jl

@@ -6,6 +6,7 @@ using Test: @test, @testset
         :TensorAlgebra,
         :contract,
         :contract!,
+        :dual,
         :eig_full,
         :eig_trunc,
         :eig_vals,
@@ -15,6 +16,7 @@ using Test: @test, @testset
         :gram_eigh_full,
         :gram_eigh_full_with_pinv,
         :invsqrth_safe,
+        :isdual,
         :left_null,
         :left_orth,
         :left_polar,

test/test_tensorkitext.jl

@@ -330,3 +330,10 @@ using Test: @test, @test_throws, @testset
             LinearAlgebra.tr(t)
     end
 end
+
+@testset "dual/isdual on a TensorKit space" begin
+    V = Rep[U₁](0 => 2, 1 => 1)
+    @test TensorAlgebra.isdual(V) == false
+    @test TensorAlgebra.isdual(dual(V)) == true
+    @test TensorAlgebra.dual(V) == dual(V)
+end