From e867a7a8c9ab10549d76218ce7090a09856c4e02 Mon Sep 17 00:00:00 2001
From: Jiri Pittner <jiri@pittnerovi.com>
Date: Thu, 20 Nov 2025 16:24:17 +0100
Subject: [PATCH] tensor: generalized reinterpretation as matrix

---
 tensor.cc | 2 +-
 tensor.h  | 7 ++++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/tensor.cc b/tensor.cc
index 6f76662..a8202ab 100644
--- a/tensor.cc
+++ b/tensor.cc
@@ -1731,7 +1731,7 @@ for(int i=0; i<r; ++i)
 	{
 	Tensor<T> uu=unwind_index(I);
 	ushape=uu.shape; 
-	um=uu.matrix();
+	um=uu.matrix(1);
 	}
 	int mini=um.nrows(); if(um.ncols()<mini) mini=um.ncols(); //compact SVD, expect descendingly sorted values
 	NRMat<T> u(um.nrows(),mini),vt(mini,um.ncols());
diff --git a/tensor.h b/tensor.h
index fcbfe67..e7152f6 100644
--- a/tensor.h
+++ b/tensor.h
@@ -248,7 +248,12 @@ public:
 	explicit Tensor(const NRVec<T> &x);
 	explicit Tensor(const NRMat<T> &x, bool flat=false);
 	explicit Tensor(const NRSMat<T> &x);
-	NRMat<T> matrix() const {return NRMat<T>(data,data.size()/groupsizes[0],groupsizes[0],0);}; //reinterpret as matrix with column index being the tensor's leftmost index group (typically the unwound single index) //@@@generalize to column index being n leftmost tensor index groups
+	NRMat<T> matrix(int n=1) const //reinterpret as matrix with column index being the tensor's n leftmost index groups (typically the unwound single index)
+		{
+		if(n<0||n>shape.size()) laerror("wrong number n in tensor:: matrix()");
+		LA_largeindex ncol = (n==shape.size()) ? data.size() : cumsizes[n];
+		return NRMat<T>(data,data.size()/ncol,ncol,0);
+		};
 
 	bool is_named() const {if(names.size()==0) return false; if(names.size()!=myrank) laerror("bad number of index names"); return true;};
 	bool is_uniquely_named() const; //no repeated names