fixed return value of calculate_svd (closes #220) and updated its documentation

ChangeLog.md

@@ -4,6 +4,7 @@ Potentially breaking changes are marked with an exclamation point '!' at the beg
 
 * 2019-03-?? v4.0.0
  * Added the HTTensor class for Hierarchical Tensor Decompositions with balanced trees.
+ * ! Changed the truncated SVD, TensorNetwork::round_edge and TT:round with epsilon to limit their respective relative errors to epsilon.
  * ! Removed support for the replacement allocator.
  * ! Changed some Makefile configs for more flexibility. See the new config.mk.default file and update your own config.
 

include/xerus/tensor.h

@@ -945,7 +945,8 @@ namespace xerus {
 	 * @param _input input Tensor of which the SVD shall be calculated.
 	 * @param _splitPos index position at defining the matrification for which the SVD is calculated.
 	 * @param _maxRank maximal Rank to be kept by SVD
-	 * @param _eps all singular values smaller than _eps are cut off
+	 * @param _eps small singular values are removed as long as the introduced error is < @a _eps (_after_ @a _maxRank was applied)
+	 * @return the frobenious norm of the error introduced by the rank reduction
 	 */
 	value_t calculate_svd(Tensor& _U, Tensor& _S, Tensor& _Vt, Tensor _input, const size_t _splitPos, const size_t _maxRank, const value_t _eps);
 	

src/xerus/tensor.cpp

@@ -1491,8 +1491,11 @@ namespace xerus {
 		_U.resize_mode(_U.degree()-1, rank);
 		_Vt.resize_mode(0, rank);
 
-        if (rank < full_rank) { return std::abs(_input.factor)*tmpS[rank]; }
-        else { return 0; }
+        if (rank < full_rank) { 
+			return blasWrapper::two_norm(tmpS.get()+rank, full_rank-rank);
+		} else {
+			return 0;
+		}
 	}