From 91d8f4cc468e1e63716e2410c9305ea39bd33212 Mon Sep 17 00:00:00 2001
From: Jiri Pittner <jiri@pittnerovi.com>
Date: Thu, 23 Oct 2025 18:09:21 +0200
Subject: [PATCH] working on fourindex conversions in tensor class

---
 tensor.cc |  3 ---
 tensor.h  | 22 +++++++++++++++++++++-
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/tensor.cc b/tensor.cc
index 4414497..f419ead 100644
--- a/tensor.cc
+++ b/tensor.cc
@@ -1417,9 +1417,6 @@ else
 
 
 
-
-
-
 template class Tensor<double>;
 template class Tensor<std::complex<double> >;
 template std::ostream & operator<<(std::ostream &s, const Tensor<double> &x);
diff --git a/tensor.h b/tensor.h
index 221822f..a746ec5 100644
--- a/tensor.h
+++ b/tensor.h
@@ -46,7 +46,7 @@
 //@@@ will need to store vector of INDEX to the original tensor for the result's flatindex
 //@@@ will not be particularly efficient
 //
-//@@@conversions to/from fourindex, optional negarive range for beta spin handling
+//@@@conversions to/from fourindex, optional negative range for beta spin handling in some cases
 //@@@use the fact that fourindex_dense is inherited from Mat/SMat and construct tensor from the (unsymmetrized) NRMat sharing data, just rewrite then the shape
 //
 //@@@?general permutation of individual indices - check the indices in sym groups remain adjacent, calculate result's shape, loopover the result and permute using unwind_callback
@@ -265,6 +265,26 @@ public:
 };
 
 
+//due to template nesting and specialization limitation, this cannot be class member - the S parameter has to be the outer one
+template<fourindexsymtype S, typename T, typename I> 
+Tensor<T> fourindex2tensor(const fourindex_dense<S,T,I> &f);
+template<fourindexsymtype S, typename T, typename I>          
+void tensor2fourindex(const Tensor<T> &t, fourindex_dense<S,T,I> &f);
+
+//conversions from/to fourindex specialized by symmetry type
+template<typename T, typename I>
+Tensor<T>  fourindex2tensor(const fourindex_dense<nosymmetry,T,I> &f)
+{
+}
+
+template<typename T, typename I>
+void tensor2fourindex(const Tensor<T> &t, fourindex_dense<nosymmetry,T,I> &f)
+{
+}
+
+
+
+
 
 template <typename T>
 std::ostream & operator<<(std::ostream &s, const Tensor<T> &x);