From 59957102f927e90ac8806d0f4cfc4bc3e07b1130 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sergio=20S=C3=A1nchez=20Ram=C3=ADrez?=
 <sergio.sanchez.ramirez+git@bsc.es>
Date: Fri, 2 Aug 2024 17:50:20 +0200
Subject: [PATCH] Fix `replace!` when old an new indices intersect

---
 src/TensorNetwork.jl | 38 ++++++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/src/TensorNetwork.jl b/src/TensorNetwork.jl
index 4f3027b66..3389ba402 100644
--- a/src/TensorNetwork.jl
+++ b/src/TensorNetwork.jl
@@ -393,13 +393,39 @@ function Base.replace!(tn::TensorNetwork, pair::Pair{<:Tensor,<:Tensor})
 end
 
 function Base.replace!(tn::TensorNetwork, old_new::Pair{Symbol,Symbol}...)
-    first.(old_new) ⊆ keys(tn.indexmap) ||
-        throw(ArgumentError("set of old indices must be a subset of current indices"))
-    isdisjoint(last.(old_new), keys(tn.indexmap)) ||
-        throw(ArgumentError("set of new indices must be disjoint to current indices"))
-    for pair in old_new
-        replace!(tn, pair)
+    from, to = first.(old_new), last.(old_new)
+    allinds = inds(tn)
+
+    # condition: from ⊆ allinds
+    from ⊆ allinds || throw(ArgumentError("set of old indices must be a subset of current indices"))
+
+    # condition: from \ to ∩ allinds = ∅
+    isdisjoint(setdiff(to, from), allinds) || throw(
+        ArgumentError(
+            "new indices must be either a element of the old indices or not an element of the TensorNetwork's indices",
+        ),
+    )
+
+    from′ = setdiff(from, to)
+    to′ = setdiff(to, from)
+
+    # no overlap so easy replacement
+    for (f, t) in zip(from′, to′)
+        replace!(tn, f => t)
+    end
+
+    # overlap between old and new indices => need a temporary name `replace!`
+    overlap = from ∩ to
+    if !isempty(overlap)
+        tmp = Dict([i => gensym(i) for i in overlap])
+
+        # replace old indices with temporary names
+        replace!(tn, pairs(tmp)...)
+
+        # replace temporary names with new indices
+        replace!(tn, [tmp[i] => i for i in Iterators.filter(∈(overlap), to)]...)
     end
+
     return tn
 end