Refactor ambiguous braiding checks

Author: lkdvos

src/transfermatrix/transfer.jl

@@ -68,25 +68,21 @@ end
 # the transfer operation of a density matrix with a utility leg in its codomain is ill defined - how should one braid the utility leg?
 # hence the checks - to make sure that this operation is uniquely defined
 function transfer_left(v::MPSTensor{S}, A::MPSTensor{S}, Ab::MPSTensor{S}) where {S}
-    _can_unambiguously_braid(space(v, 2)) ||
-        throw(ArgumentError("transfer is not uniquely defined with utility space $(space(v,2))"))
+    check_ambiguous_braiding(space(v, 2))
     @plansor v[-1 -2; -3] := v[1 2; 4] * A[4 5; -3] * τ[2 3; 5 -2] * conj(Ab[1 3; -1])
 end
 function transfer_right(v::MPSTensor{S}, A::MPSTensor{S}, Ab::MPSTensor{S}) where {S}
-    _can_unambiguously_braid(space(v, 2)) ||
-        throw(ArgumentError("transfer is not uniquely defined with utility space $(space(v,2))"))
+    check_ambiguous_braiding(space(v, 2))
     @plansor v[-1 -2; -3] := A[-1 2; 1] * τ[-2 4; 2 3] * conj(Ab[-3 4; 5]) * v[1 3; 5]
 end
 
 # the transfer operation with a utility leg in both the domain and codomain is also ill defined - only due to the codomain utility space
 function transfer_left(v::MPOTensor{S}, A::MPSTensor{S}, Ab::MPSTensor{S}) where {S}
-    _can_unambiguously_braid(space(v, 2)) ||
-        throw(ArgumentError("transfer is not uniquely defined with utility space $(space(v,2))"))
+    check_ambiguous_braiding(space(v, 2))
     @plansor t[-1 -2; -3 -4] := v[1 2; -3 4] * A[4 5; -4] * τ[2 3; 5 -2] * conj(Ab[1 3; -1])
 end
 function transfer_right(v::MPOTensor{S}, A::MPSTensor{S}, Ab::MPSTensor{S}) where {S}
-    _can_unambiguously_braid(space(v, 2)) ||
-        throw(ArgumentError("transfer is not uniquely defined with utility space $(space(v,2))"))
+    check_ambiguous_braiding(space(v, 2))
     @plansor t[-1 -2; -3 -4] := A[-1 2; 1] * τ[-2 4; 2 3] * conj(Ab[-4 4; 5]) * v[1 3; -3 5]
 end
 

src/utility/utility.jl

@@ -89,17 +89,6 @@ function _embedders(spaces)
     return maps
 end
 
-function _can_unambiguously_braid(sp::VectorSpace)
-    s = sectortype(sp)
-
-    BraidingStyle(s) isa SymmetricBraiding && return true
-
-    # if it's not symmetric, then we are only really garantueed that this is possible when only one irrep occurs - the trivial one
-    for sect in sectors(sp)
-        sect == one(sect) || return false
-    end
-    return true
-end
 
 #=
 map every element in the tensormap to dfun(E)
@@ -143,3 +132,15 @@ end
 function fuser(::Type{T}, V1::S, V2::S) where {T,S<:IndexSpace}
     return isomorphism(T, fuse(V1 ⊗ V2), V1 ⊗ V2)
 end
+
+# Verify that the braiding is unambiguous: either through symmetric braiding or because the
+# sectors are all trivial
+function check_ambiguous_braiding(::Type{Bool}, V::VectorSpace)
+    I = sectortype(V)
+    BraidingStyle(I) isa SymmetricBraiding && return true
+    return all(isone, sectors(V))
+end
+function check_ambiguous_braiding(V::VectorSpace)
+    return check_ambiguous_braiding(Bool, V) ||
+        throw(ArgumentError("cannot unambiguously braid $V"))
+end