From 21b110c0ec7ed965e6f06868b76843cb5d056832 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: Sun, 28 Apr 2024 14:38:39 +0200
Subject: [PATCH] Add `HyperGroup` transformation

---
 src/Tensor.jl          |  1 +
 src/Transformations.jl | 31 +++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/src/Tensor.jl b/src/Tensor.jl
index 03e44ceee..31c4df7f5 100644
--- a/src/Tensor.jl
+++ b/src/Tensor.jl
@@ -89,6 +89,7 @@ Base.replace(t::Tensor, old_new::Pair{Symbol,Symbol}...) = Tensor(parent(t), rep
 
 Base.parent(t::Tensor) = t.data
 parenttype(::Type{Tensor{T,N,A}}) where {T,N,A} = A
+parenttype(::T) where {T<:Tensor} = parenttype(T)
 
 dim(::Tensor, i::Number) = i
 dim(t::Tensor, i::Symbol) = first(findall(==(i), inds(t)))
diff --git a/src/Transformations.jl b/src/Transformations.jl
index 6470be14e..03413de13 100644
--- a/src/Transformations.jl
+++ b/src/Transformations.jl
@@ -41,6 +41,8 @@ end
 
 Convert hyperindices to COPY-tensors, represented by `DeltaArray`s.
 This transformation is always used by default when visualizing a `TensorNetwork` with `plot`.
+
+See also: [`HyperGroup`](@ref).
 """
 struct HyperFlatten <: Transformation end
 
@@ -72,6 +74,35 @@ function transform!(tn::TensorNetwork, ::HyperFlatten)
     end
 end
 
+"""
+    HyperGroup <: Transformation
+
+Convert COPY-tensors, represented by `DeltaArray`s, to hyperindices.
+
+See also: [`HyperFlatten`](@ref).
+"""
+struct HyperGroup <: Transformation end
+
+function transform!(tn::TensorNetwork, ::HyperGroup)
+    targets = Iterators.filter(x -> parenttype(x) isa DeltaArray, tensors(tn))
+    for tensor in targets
+        # remove COPY tensor
+        delete!(tn, target)
+
+        # insert hyperindex
+        hyperindex = uuid4()
+
+        # insert weights vector
+        if !all(isone, delta(parent(tensor)))
+            push!(tn, Tensor(delta(parent(tensor)), [hyperindex]))
+        end
+
+        for flatindex in inds(tensor)
+            replace!(tn, flatindex => hyperindex)
+        end
+    end
+end
+
 """
     DiagonalReduction <: Transformation