From 6cad2ce9c76ae8b7ae76987af2933a50d9a1d08e Mon Sep 17 00:00:00 2001
From: mtfishman <mfishman@flatironinstitute.org>
Date: Fri, 17 May 2024 12:57:54 -0400
Subject: [PATCH] [ITensorMPS] Fix bug in replace_siteinds

---
 src/lib/ITensorMPS/src/mps.jl            |  4 ++--
 src/lib/ITensorMPS/test/base/test_mps.jl | 11 +++++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/lib/ITensorMPS/src/mps.jl b/src/lib/ITensorMPS/src/mps.jl
index 42c15b93ab..ef462a8462 100644
--- a/src/lib/ITensorMPS/src/mps.jl
+++ b/src/lib/ITensorMPS/src/mps.jl
@@ -516,8 +516,8 @@ SiteTypes.siteinds(M::MPS; kwargs...) = siteinds(first, M; kwargs...)
 
 function replace_siteinds!(M::MPS, sites)
   for j in eachindex(M)
-    sj = siteind(only, M, j)
-    replaceind!(M[j], sj, sites[j])
+    sj = only(siteinds(M, j))
+    M[j] = replaceinds(M[j], sj => sites[j])
   end
   return M
 end
diff --git a/src/lib/ITensorMPS/test/base/test_mps.jl b/src/lib/ITensorMPS/test/base/test_mps.jl
index 7d07139ed8..8dce11c1e2 100644
--- a/src/lib/ITensorMPS/test/base/test_mps.jl
+++ b/src/lib/ITensorMPS/test/base/test_mps.jl
@@ -271,6 +271,17 @@ include(joinpath(@__DIR__, "utils", "util.jl"))
     end
   end
 
+  @testset "replace_siteinds" begin
+    s = siteinds("S=1/2", 4)
+    x = MPS(s, j -> isodd(j) ? "↑" : "↓")
+    @test siteinds(x) == s
+    t = sim.(s)
+    y = replace_siteinds(x, t)
+    @test siteinds(y) == t
+    # Regression test for https://github.com/ITensor/ITensors.jl/issues/1439.
+    @test siteinds(x) == s
+  end
+
   @testset "copy and deepcopy" begin
     s = siteinds("S=1/2", 3)
     M1 = random_mps(s; linkdims=3)