feat: add lapack/base/dlangt

lib/node_modules/@stdlib/lapack/base/dlangt/README.md

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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.
+
+-->
+
+# dlangt
+
+> Return the value of the one-norm, Frobenius norm, infinity-norm, or the largest absolute value of any element of a real tridiagonal matrix.
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var dlangt = require( '@stdlib/lapack/base/dlangt' );
+```
+
+#### dlangt( norm, N, DL, strideDL, D, strideD, DU, strideDU )
+
+Returns the value of the specified norm of a real tridiagonal matrix `A`.
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+var DL = new Float64Array( [ 1.0, 2.0, 3.0 ] );
+var D = new Float64Array( [ 4.0, 5.0, 6.0, 7.0 ] );
+var DU = new Float64Array( [ 1.0, 1.0, 1.0 ] );
+
+var norm = dlangt( 'M', 4, DL, 1, D, 1, DU, 1 );
+// returns 7.0
+```
+
+The function has the following parameters:
+
+-   **norm**: specifies the norm type (`'M'`/`'m'`: max norm, `'1'`/`'O'`/`'o'`: one norm, `'I'`/`'i'`: infinity norm, `'F'`/`'f'`/`'E'`/`'e'`: Frobenius norm).
+-   **N**: order of the matrix `A`. When `N = 0`, the function returns `0`.
+-   **DL**: [`Float64Array`][mdn-float64array] containing the `(N-1)` sub-diagonal elements.
+-   **strideDL**: stride length for `DL`.
+-   **D**: [`Float64Array`][mdn-float64array] containing the `N` diagonal elements.
+-   **strideD**: stride length for `D`.
+-   **DU**: [`Float64Array`][mdn-float64array] containing the `(N-1)` super-diagonal elements.
+-   **strideDU**: stride length for `DU`.
+
+Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.
+
+#### dlangt.ndarray( norm, N, DL, sdl, odl, D, sd, od, DU, sdu, odu )
+
+Returns the value of the specified norm using alternative indexing semantics.
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+var DL = new Float64Array( [ 1.0, 2.0, 3.0 ] );
+var D = new Float64Array( [ 4.0, 5.0, 6.0, 7.0 ] );
+var DU = new Float64Array( [ 1.0, 1.0, 1.0 ] );
+
+var norm = dlangt.ndarray( 'M', 4, DL, 1, 0, D, 1, 0, DU, 1, 0 );
+// returns 7.0
+```
+
+The function has the following additional parameters:
+
+-   **sdl**: stride length for `DL`.
+-   **odl**: starting index for `DL`.
+-   **sd**: stride length for `D`.
+-   **od**: starting index for `D`.
+-   **sdu**: stride length for `DU`.
+-   **odu**: starting index for `DU`.
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   `dlangt()` corresponds to the [LAPACK][LAPACK] function [`dlangt`][lapack-dlangt].
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+var dlangt = require( '@stdlib/lapack/base/dlangt' );
+
+var DL = new Float64Array( [ 3.0, -2.0, 1.0, 4.0 ] );
+var D = new Float64Array( [ -1.0, 5.0, -3.0, 2.0, 7.0 ] );
+var DU = new Float64Array( [ 2.0, -1.0, 4.0, -3.0 ] );
+
+console.log( 'Max norm:', dlangt( 'M', 5, DL, 1, D, 1, DU, 1 ) );
+console.log( 'One norm:', dlangt( '1', 5, DL, 1, D, 1, DU, 1 ) );
+console.log( 'Infinity norm:', dlangt( 'I', 5, DL, 1, D, 1, DU, 1 ) );
+console.log( 'Frobenius norm:', dlangt( 'F', 5, DL, 1, D, 1, DU, 1 ) );
+```
+
+</section>
+
+<!-- /.examples -->
+
+<!-- C interface documentation. -->
+
+* * *
+
+<section class="c">
+
+## C APIs
+
+<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<!-- C usage documentation. -->
+
+<section class="usage">
+
+### Usage
+
+```c
+TODO
+```
+
+#### TODO
+
+TODO.
+
+```c
+TODO
+```
+
+TODO
+
+```c
+TODO
+```
+
+</section>
+
+<!-- /.usage -->
+
+<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="notes">
+
+</section>
+
+<!-- /.notes -->
+
+<!-- C API usage examples. -->
+
+<section class="examples">
+
+### Examples
+
+```c
+TODO
+```
+
+</section>
+
+<!-- /.examples -->
+
+</section>
+
+<!-- /.c -->
+
+<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
+
+<section class="related">
+
+</section>
+
+<!-- /.related -->
+
+<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="links">
+
+[lapack]: https://www.netlib.org/lapack/explore-html/
+
+[lapack-dlangt]: https://netlib.org/lapack/explore-html/d5/d0e/group__langt.html
+
+[mdn-float64array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float64Array
+
+[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
+
+</section>
+
+<!-- /.links -->

lib/node_modules/@stdlib/lapack/base/dlangt/benchmark/benchmark.js

@@ -0,0 +1,109 @@
+/**
+* @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 bench = require( '@stdlib/bench' );
+var uniform = require( '@stdlib/random/array/uniform' );
+var isnan = require( '@stdlib/math/base/assert/is-nan' );
+var pow = require( '@stdlib/math/base/special/pow' );
+var format = require( '@stdlib/string/format' );
+var pkg = require( './../package.json' ).name;
+var dlangt = require( './../lib/dlangt.js' );
+
+
+// VARIABLES //
+
+var options = {
+	'dtype': 'float64'
+};
+
+
+// FUNCTIONS //
+
+/**
+* Creates a benchmark function.
+*
+* @private
+* @param {PositiveInteger} len - matrix order
+* @returns {Function} benchmark function
+*/
+function createBenchmark( len ) {
+	var DL;
+	var DU;
+	var D;
+
+	D = uniform( len, -100.0, 100.0, options );
+	DL = uniform( len - 1, -100.0, 100.0, options );
+	DU = uniform( len - 1, -100.0, 100.0, options );
+	return benchmark;
+
+	/**
+	* Benchmark function.
+	*
+	* @private
+	* @param {Benchmark} b - benchmark instance
+	*/
+	function benchmark( b ) {
+		var v;
+		var i;
+
+		b.tic();
+		for ( i = 0; i < b.iterations; i++ ) {
+			v = dlangt( 'M', len, DL, 1, D, 1, DU, 1 );
+			if ( isnan( v ) ) {
+				b.fail( 'should not return NaN' );
+			}
+		}
+		b.toc();
+		if ( isnan( v ) ) {
+			b.fail( 'should not return NaN' );
+		}
+		b.pass( 'benchmark finished' );
+		b.end();
+	}
+}
+
+
+// MAIN //
+
+/**
+* Main execution sequence.
+*
+* @private
+*/
+function main() {
+	var len;
+	var max;
+	var min;
+	var f;
+	var i;
+
+	min = 1; // 10^min
+	max = 6; // 10^max
+
+	for ( i = min; i <= max; i++ ) {
+		len = pow( 10, i );
+		f = createBenchmark( len );
+		bench( format( '%s:len=%d', pkg, len ), f );
+	}
+}
+
+main();