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* @license Apache-2.0
*
* Copyright (c) 2026 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
'use strict';
// MODULES //
var isLayout = require( '@stdlib/blas/base/assert/is-layout' );
var format = require( '@stdlib/string/format' );
var base = require( './base.js' );
// MAIN //
/**
* Solves a tridiagonal system of the form `A * X = B` using the `L * D * L^T` factorization of `A` computed by `dpttrf`.
*
* @param {string} order - storage layout of `B`
* @param {NonNegativeInteger} N - order of the tridiagonal matrix `A`
* @param {NonNegativeInteger} nrhs - number of right-hand sides (i.e., the number of columns of the matrix `B`)
* @param {Float64Array} D - the `N` diagonal elements of the diagonal matrix `D` from the factorization of `A`
* @param {Float64Array} E - the `N-1` subdiagonal elements of the unit bidiagonal factor `L` from the factorization of `A`
* @param {Float64Array} B - input matrix (overwritten by the solution matrix `X`)
* @param {NonNegativeInteger} LDB - leading dimension of `B`
* @throws {TypeError} first argument must be a valid order
* @throws {RangeError} seventh argument must be greater than or equal to nrhs
* @returns {Float64Array} solution matrix `X` (stored in `B`)
*
* @example
* var Float64Array = require( '@stdlib/array/float64' );
*
* var D = new Float64Array( [ 4.0, 4.75, 5.157894736842105 ] );
* var E = new Float64Array( [ 0.25, 0.42105263157894735 ] );
* var B = new Float64Array( [ 1.0, 2.0, 3.0 ] );
*
* dptts2( 'column-major', 3, 1, D, E, B, 3 );
* // B => <Float64Array>[ ~0.2041, ~0.1837, ~0.4388 ]
*/
function dptts2( order, N, nrhs, D, E, B, LDB ) {
var sb1;
var sb2;
if ( !isLayout( order ) ) {
throw new TypeError( format( 'invalid argument. First argument must be a valid order. Value: `%s`.', order ) );
}
if ( order === 'column-major' ) {
sb1 = 1;
sb2 = LDB;
} else { // order === 'row-major'
if ( LDB < nrhs ) {
throw new RangeError( format( 'invalid argument. Seventh argument must be greater than or equal to %d. Value: `%d`.', nrhs, LDB ) );
}
sb1 = LDB;
sb2 = 1;
}
return base( N, nrhs, D, 1, 0, E, 1, 0, B, sb1, sb2, 0 );
}
// EXPORTS //
module.exports = dptts2;
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