57 lines
1.4 KiB
C
57 lines
1.4 KiB
C
/**
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* @license Apache-2.0
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*
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* Copyright (c) 2020 The Stdlib Authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "stdlib/blas/base/dnrm2.h"
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#include <math.h>
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/**
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* Computes the L2-norm of a double-precision floating-point vector.
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*
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* @param N number of values over which to compute the norm
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* @param X input array
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* @param stride stride length
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* @return output value
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*/
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double c_dnrm2( const int N, const double *X, const int stride ) {
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double scale;
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double ssq;
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double ax;
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int i;
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if ( N <= 0 || stride <= 0 ) {
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return 0.0;
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}
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if ( N == 1 ) {
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return fabs( X[ 0 ] );
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}
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scale = 0.0;
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ssq = 1.0;
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for ( i = 0; i < N*stride; i += stride ) {
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if ( X[ i ] != 0.0 ) {
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ax = fabs( X[ i ] );
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if ( scale < ax ) {
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ssq = 1.0 + ( ssq * pow( scale/ax, 2 ) );
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scale = ax;
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} else {
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ssq += pow( ax/scale, 2 );
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}
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}
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}
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return scale * sqrt( ssq );
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}
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