From 883d201e67127d22076fa57a67e4ff001c91c77a Mon Sep 17 00:00:00 2001
From: Jiri Pittner <jiri@pittnerovi.com>
Date: Mon, 6 May 2024 18:30:01 +0200
Subject: [PATCH] tensor: split and merge index groups

---
 tensor.cc | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 tensor.h  |  4 ++-
 2 files changed, 91 insertions(+), 1 deletion(-)

diff --git a/tensor.cc b/tensor.cc
index cb4b3f9..0ca3599 100644
--- a/tensor.cc
+++ b/tensor.cc
@@ -22,6 +22,7 @@
 #include "qsort.h"
 #include "miscfunc.h"
 #include <complex>
+#include "bitvector.h"
 
 
 namespace LA {
@@ -752,6 +753,93 @@ loopover(permutationalgebra_callback);
 }
 
 
+template<typename T>
+void Tensor<T>::split_index_group(int group)
+{
+if(group<0||group >= shape.size()) laerror("illegal index group number");
+if(shape[group].number==1) return; //nothing to split
+if(shape[group].symmetry!=0) laerror("only non-symmetric index group can be splitted");
+
+NRVec<indexgroup> newshape(shape.size()+shape[group].number-1);
+int gg=0;
+for(int g=0; g<shape.size(); ++g)
+	{
+	if(g==group)
+		{
+		for(int i=0; i<shape[group].number; ++i)
+			{
+			newshape[gg] = shape[group]; 
+			newshape[gg].number = 1;
+			gg++;
+			}
+		}
+	else
+		newshape[gg++] = shape[g];
+	}
+
+shape=newshape;
+LA_largeindex newsize = calcsize(); //recalculate auxiliary arrays
+if(data.size()!=newsize) laerror("internal error in split_index_group");
+}
+
+
+template<typename T>
+void Tensor<T>:: merge_adjacent_index_groups(int groupfrom, int groupto)
+{
+if(groupfrom<0||groupfrom>= shape.size()) laerror("illegal index group number");
+if(groupto<0||groupto>= shape.size()) laerror("illegal index group number");
+if(groupfrom==groupto) return;
+if(groupfrom>groupto) {int t=groupfrom; groupfrom=groupto; groupto=t;}
+
+int newnumber=0;
+for(int g=groupfrom; g<=groupto; ++g)
+	{
+	if(shape[g].symmetry!=0) laerror("only non-symmetric index groups can be merged");
+	if(shape[g].offset!=shape[groupfrom].offset) laerror("incompatible offset in merge_adjacent_index_groups");
+	if(shape[g].range!=shape[groupfrom].range) laerror("incompatible range in merge_adjacent_index_groups");
+	newnumber += shape[g].number;
+	}
+
+NRVec<indexgroup> newshape(shape.size()-(groupto-groupfrom+1)+1);
+for(int g=0; g<=groupfrom; ++g) newshape[g]=shape[g];
+newshape[groupfrom].number=newnumber;
+for(int g=groupfrom+1; g<newshape.size(); ++g) newshape[g]=shape[g+groupto-groupfrom];
+
+shape=newshape;
+LA_largeindex newsize = calcsize(); //recalculate auxiliary arrays
+if(data.size()!=newsize) laerror("internal error in merge_adjacent_index_groups");
+}
+
+
+template<typename T>
+Tensor<T> Tensor<T>::merge_index_groups(const NRVec<int> &groups) const
+{
+if(groups.size()<=1) return *this;
+NRPerm<int> p(shape.size());
+int gg=0;
+bitvector selected(shape.size());
+selected.clear();
+for(int g=0; g<groups.size(); ++g) 
+	{
+	if(groups[g]<0||groups[g]>=shape.size()) laerror("illegal group number in merge_index_groups");
+	if(selected[g]) laerror("repeated group number in merge_index_groups");
+	selected.set(g);
+	p[gg++] = 1+groups[g];
+	}
+for(int g=0; g<shape.size(); ++g)
+	if(!selected[g]) 
+		p[gg++] = 1+g;
+if(gg!=shape.size() || !p.is_valid()) laerror("internal error in merge_index_groups");
+
+Tensor<T> r =  permute_index_groups(p);
+r.merge_adjacent_index_groups(0,groups.size()-1);
+return r;
+}
+
+
+
+
+
 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 b34b171..c71f86a 100644
--- a/tensor.h
+++ b/tensor.h
@@ -190,7 +190,9 @@ public:
 	void apply_permutation_algebra(const  Tensor &rhs, const PermutationAlgebra<int,T> &pa, bool inverse=false, T alpha=1, T beta=0); //general (not optimally efficient) symmetrizers, antisymmetrizers etc. acting on the flattened index list:
 //								 this *=beta; for I over this: this(I) += alpha * sum_P c_P rhs(P(I))
 //								 PermutationAlgebra can represent e.g. general_antisymmetrizer in Kucharski-Bartlett notation
-
+	void split_index_group(int group); //formal split of a non-symmetric index group WITHOUT the need for data reorganization
+	void merge_adjacent_index_groups(int groupfrom, int groupto); //formal merge of non-symmetric index groups  WITHOUT the need for data reorganization
+	Tensor merge_index_groups(const NRVec<int> &groups) const;
 
 	//TODO perhaps implement application of a permutation algebra to a product of several tensors
 };