Implement MatButterfly::nonzero_indices.

src/faust_linear_operator/CPU/faust_MatButterfly.h

@@ -87,7 +87,7 @@ namespace Faust
 			MatGeneric<FPP,Cpu>* get_rows(faust_unsigned_int row_id_start, faust_unsigned_int num_rows) const;
 			MatGeneric<FPP,Cpu>* get_cols(const faust_unsigned_int* col_ids, faust_unsigned_int num_cols) const;
 			MatGeneric<FPP,Cpu>* get_rows(const faust_unsigned_int* row_ids, faust_unsigned_int num_rows) const;
-			std::list<std::pair<int,int>> nonzeros_indices() const;
+			std::list<std::pair<int,int>> nonzeros_indices(const double& tol=0) const;
 			void setZeros();
 			bool containsNaN()const;
 			const FPP& operator()(faust_unsigned_int i, faust_unsigned_int j)const;

src/faust_linear_operator/CPU/faust_MatButterfly.hpp

@@ -345,9 +345,25 @@ namespace Faust
 	}
 
 	template<typename FPP>
-	std::list<std::pair<int,int>> MatButterfly<FPP, Cpu>::nonzeros_indices() const
+	std::list<std::pair<int,int>> MatButterfly<FPP, Cpu>::nonzeros_indices(const double& tol/*=0*/) const
 	{
-		//TODO
+		auto s = this->getNbRow();
+		auto k = s >> (level + 1); // D2 diagonal offset
+		std::list<std::pair<int,int>> indices;
+		const FPP *d1_ptr, *d2_ptr;
+		d1_ptr = D1.diagonal().data();
+		d2_ptr = D2.diagonal().data();
+		for(int i=0; i < s; i++)
+		{
+			if(std::abs(d1_ptr[i]) > tol)
+				indices.push_back(std::make_pair(i, i)); // main diag coeff
+			if(std::abs(d2_ptr[i]) > tol)
+				if((i / k) & 1)
+					indices.push_back(std::make_pair(i, i - k)); // below diag coeff
+				else
+					indices.push_back(std::make_pair(i, i + k)); // above diag coeff
+		}
+		return indices;
 	}
 
 	template<typename FPP>