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feat: add lapack/base/dlarf1f
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/**
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* @license Apache-2.0
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*
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* Copyright (c) 2025 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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'use strict';
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/* eslint-disable max-len */
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// MODULES //
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var iladlc = require( '@stdlib/lapack/base/iladlc' ).ndarray;
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var iladlr = require( '@stdlib/lapack/base/iladlr' ).ndarray;
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var dgemv = require( '@stdlib/blas/base/dgemv' ).ndarray;
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var dger = require( '@stdlib/blas/base/dger' ).ndarray;
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var daxpy = require( '@stdlib/blas/base/daxpy' ).ndarray;
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var dscal = require( '@stdlib/blas/base/dscal' ).ndarray;
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// MAIN //
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/**
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* Applies a real elementary reflector `H = I - tau * v * v ^ T` to a real M by N matrix `C`.
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*
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* ## Notes
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*
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* - `work` should have `N` indexed elements if side = `left` and `M` indexed elements if side = `right`.
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* - `V` should have `1 + (M-1) * abs(strideV)` indexed elements if side = `left` and `1 + (N-1) * abs(strideV)` indexed elements if side = `right`.
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* - `C` is overwritten by `H * C` if side = `left` and `C * H` if side = `right`.
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*
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* @private
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* @param {string} side - use `left` to form `H * C` and `right` to from `C * H`
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* @param {NonNegativeInteger} M - number of rows in `C`
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* @param {NonNegativeInteger} N - number of columns in `C`
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* @param {Float64Array} V - the vector `v` in the representation of `H`
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* @param {integer} strideV - stride length for `V`
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* @param {NonNegativeInteger} offsetV - starting index for `V`
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* @param {number} tau - the value of `tau` in representation of `H`
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* @param {Float64Array} C - input matrix
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* @param {integer} strideC1 - stride of the first dimension of `C`
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* @param {integer} strideC2 - stride of the second dimension of `C`
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* @param {NonNegativeInteger} offsetC - starting index for `C`
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* @param {Float64Array} work - workspace array
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* @param {integer} strideWork - stride length for `work`
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* @param {NonNegativeInteger} offsetWork - starting index for `work`
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* @returns {Float64Array} `C * H` or `H * C`
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*
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* @example
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* var Float64Array = require( '@stdlib/array/float64' );
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*
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* var C = new Float64Array( [ 1, 5, 9, 2, 6, 10, 3, 7, 11, 4, 8, 12 ] );
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* var V = new Float64Array( [ 0.5, 0.5, 0.5, 0.5 ] );
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* var work = new Float64Array( 3 );
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*
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* var out = dlarf1f( 'left', 4, 3, V, 1, 0, 1.0, C, 3, 1, 0, work, 1, 0 );
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* // returns <Float64Array>[ -4.5, -10.5, -16.5, -0.75, -1.75, -2.75, 0.25, -0.75, -1.75, 1.25, 0.25, -0.75 ]
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*/
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function dlarf1f( side, M, N, V, strideV, offsetV, tau, C, strideC1, strideC2, offsetC, work, strideWork, offsetWork ) { // eslint-disable-line max-params
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var lastv;
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var lastc;
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var i;
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lastv = 1;
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lastc = 0;
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if ( tau !== 0.0 ) {
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if ( side === 'left' ) {
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lastv = M;
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} else {
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lastv = N;
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}
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if ( strideV > 0 ) {
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i = offsetV + ( ( lastv - 1 ) * strideV );
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} else {
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i = offsetV;
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}
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while ( lastv > 0 && V[ i ] === 0.0 ) {
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lastv -= 1;
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i -= strideV;
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}
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if ( side === 'left' ) {
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lastc = iladlc( lastv, N, C, strideC1, strideC2, offsetC ) + 1; // to account for the difference between zero-based and one-based indexing
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} else {
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lastc = iladlr( M, lastv, C, strideC1, strideC2, offsetC ) + 1; // to account for the difference between zero-based and one-based indexing
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}
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// lastc is zero if a matrix is filled with zeros
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}
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if ( lastc === 0 ) {
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// Returns C unchanged if tau is zero or all elements in C are zero
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return C;
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}
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if ( side === 'left' ) {
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if ( lastv === 0 ) {
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dscal( lastc, 1.0 - tau, C, strideC2, offsetC ); // scale the first row
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} else {
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dgemv( 'transpose', lastv-1, lastc, 1.0, C, strideC1, strideC2, offsetC + strideC1, V, strideV, offsetV + strideV, 0.0, work, strideWork, offsetWork ); // C( 1, 0 ) is accessed here
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daxpy( lastc, 1.0, C, strideC2, offsetC, work, strideWork, offsetWork ); // operates on the first row of C
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daxpy( lastc, -tau, work, strideWork, offsetWork, C, strideC2, offsetC ); // operates on the first row of C
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dger( lastv-1, lastc, -tau, V, strideV, offsetV + strideV, work, strideWork, offsetWork, C, strideC1, strideC2, offsetC + strideC1 ); // C( 1, 0 ) is accessed here
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}
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} else if ( lastv === 0 ) {
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dscal( lastc, 1.0 - tau, C, strideC1, offsetC ); // scale the first column
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} else {
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dgemv( 'no-transpose', lastc, lastv-1, 1.0, C, strideC1, strideC2, offsetC + strideC2, V, strideV, offsetV + strideV, 0.0, work, strideWork, offsetWork ); // C( 0, 1 ) is accessed here
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daxpy( lastc, 1.0, C, strideC1, offsetC, work, strideWork, offsetWork ); // operates on the first column of C
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daxpy( lastc, -tau, work, strideWork, offsetWork, C, strideC1, offsetC ); // operates on the first column of C
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dger( lastc, lastv-1, -tau, work, strideWork, offsetWork, V, strideV, offsetV + strideV, C, strideC1, strideC2, offsetC + strideC2 ); // C( 0, 1 ) is accessed here
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}
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return C;
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}
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// EXPORTS //
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module.exports = dlarf1f;

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