Use default link tags in movesites (#137)

Author: VinceNeede

src/abstractmps.jl

@@ -1907,7 +1907,20 @@ function setindex!(
     end
   end
 
-  ψA = MPST(A, sites; leftinds=lind, orthocenter=orthocenter - first(r) + 1, kwargs...)
+  # use the first link index if present, otherwise use the default tag
+  linktags = TagSet[
+    (b = linkind(ψ, i);
+    isnothing(b) ? defaultlinkindtags(i) : tags(b)) for i in firstsite:(lastsite - 1)
+  ]
+
+  ψA = MPST(
+    A,
+    sites;
+    leftinds=lind,
+    orthocenter=orthocenter - first(r) + 1,
+    tags=linktags,
+    kwargs...,
+  )
   #@assert prod(ψA) ≈ A
 
   ψ[firstsite:lastsite] = ψA
@@ -1942,11 +1955,20 @@ by site according to the site indices `sites`.
    in `sites` and `leftinds` will be dangling off of the right side of the MPS/MPO.
 - `orthocenter::Integer = length(sites)`: the desired final orthogonality
    center of the output MPS/MPO.
+- `tags = [defaultlinktags(i) for i in 1:(length(sites) - 1)]`:
+   the tags to use for the link indices. The length of `tags` must be
+   `length(sites) - 1`. The default is to use the default link tags for each
+   site.
 - `cutoff`: the desired truncation error at each link.
 - `maxdim`: the maximum link dimension.
 """
 function (::Type{MPST})(
-  A::ITensor, sites; leftinds=nothing, orthocenter::Integer=length(sites), kwargs...
+  A::ITensor,
+  sites;
+  leftinds=nothing,
+  orthocenter::Integer=length(sites),
+  tags=[defaultlinktags(i) for i in 1:(length(sites) - 1)],
+  kwargs...,
 ) where {MPST<:AbstractMPS}
   N = length(sites)
   for s in sites
@@ -1967,7 +1989,7 @@ function (::Type{MPST})(
     if !isnothing(l)
       Lis = unioninds(Lis, l)
     end
-    L, R = factorize(Ã, Lis; kwargs..., tags="Link,n=$n", ortho="left")
+    L, R = factorize(Ã, Lis; kwargs..., tags=tags[n], ortho="left")
     l = commonind(L, R)
     ψ[n] = L
     Ã = R

test/base/test_mps.jl

@@ -1424,6 +1424,18 @@ end
     end
   end
 
+  @testset "movesites tags" begin
+    N = 4
+    s0 = siteinds("S=1/2", N)
+    ψ0 = random_mps(s0; linkdims=1)
+    for perm in permutations(1:N)
+      s = s0[perm]
+      ns′ = [findfirst(==(i), s0) for i in s]
+      ψ = movesites(ψ0, 1:N .=> ns′; cutoff=1e-15)
+      @test ITensorMPS.hasdefaultlinktags(ψ)
+    end
+  end
+
   @testset "product(::Vector{ITensor}, ::MPS)" begin
     N = 6
     s = siteinds("Qubit", N)