feat: add blas/ext/base/gwhere

lib/node_modules/@stdlib/blas/ext/base/gwhere/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.
+
+-->
+
+# gwhere
+
+> Take elements from one of two strided arrays depending on a condition.
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var gwhere = require( '@stdlib/blas/ext/base/gwhere' );
+```
+
+#### gwhere( N, condition, strideC, x, strideX, y, strideY )
+
+Takes elements from one of two strided arrays depending on a condition.
+
+```javascript
+var condition = [ 1, 0, 1 ];
+var x = [ 1.0, 2.0, 3.0 ];
+var y = [ 4.0, 5.0, 6.0 ];
+
+var out = gwhere( 3, condition, 1, x, 1, y, 1 );
+// returns [ 1.0, 5.0, 3.0 ]
+```
+
+The function has the following parameters:
+
+-   **N**: number of indexed elements.
+-   **condition**: condition array.
+-   **strideC**: stride length for `condition`.
+-   **x**: first input [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
+-   **strideX**: stride length for `x`.
+-   **y**: second input [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
+-   **strideY**: stride length for `y`.
+
+The `N` and stride parameters determine which elements in the strided arrays are accessed at runtime. For example, to select from every other element:
+
+```javascript
+var condition = [ 1, 0, 0, 1, 1, 0 ];
+var x = [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ];
+var y = [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ];
+
+var out = gwhere( 3, condition, 2, x, 2, y, 2 );
+// returns [ 1.0, 9.0, 5.0 ]
+```
+
+Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.
+
+<!-- eslint-disable stdlib/capitalized-comments, max-len -->
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+// Initial arrays...
+var condition0 = new Float64Array( [ 0.0, 1.0, 0.0, 1.0, 0.0, 1.0 ] );
+var x0 = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+var y0 = new Float64Array( [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
+
+// Create offset views...
+var condition1 = new Float64Array( condition0.buffer, condition0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+var y1 = new Float64Array( y0.buffer, y0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+
+var out = gwhere( 3, condition1, 2, x1, 2, y1, 2 );
+// returns [ 2.0, 4.0, 6.0 ]
+```
+
+<!-- lint disable maximum-heading-length -->
+
+#### gwhere.ndarray( N, condition, strideC, offsetC, x, strideX, offsetX, y, strideY, offsetY )
+
+<!-- lint enable maximum-heading-length -->
+
+Takes elements from one of two strided arrays depending on a condition using alternative indexing semantics.
+
+```javascript
+var condition = [ 1, 0, 1 ];
+var x = [ 1.0, 2.0, 3.0 ];
+var y = [ 4.0, 5.0, 6.0 ];
+
+var out = gwhere.ndarray( 3, condition, 1, 0, x, 1, 0, y, 1, 0 );
+// returns [ 1.0, 5.0, 3.0 ]
+```
+
+The function has the following additional parameters:
+
+-   **offsetC**: starting index for `condition`.
+-   **offsetX**: starting index for `x`.
+-   **offsetY**: starting index for `y`.
+
+While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying buffer, offset parameters support indexing semantics based on starting indices. For example, to select from every other element starting from the second element:
+
+```javascript
+var condition = [ 0, 1, 0, 0, 0, 1 ];
+var x = [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ];
+var y = [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ];
+
+var out = gwhere.ndarray( 3, condition, 2, 1, x, 2, 1, y, 2, 1 );
+// returns [ 2.0, 10.0, 6.0 ]
+```
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   If `N <= 0`, both functions return an empty array.
+-   Both functions support array-like objects having getter and setter accessors for array element access (e.g., [`@stdlib/array/base/accessor`][@stdlib/array/base/accessor]).
+-   Both functions return a new generic `Array`. To assign results to a specific output array, use [`@stdlib/array/base/where`][@stdlib/array/base/where].
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var bernoulli = require( '@stdlib/random/array/bernoulli' );
+var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
+var gwhere = require( '@stdlib/blas/ext/base/gwhere' );
+
+var condition = bernoulli( 20, 0.5, {
+    'dtype': 'generic'
+});
+var x = discreteUniform( 20, 0, 100, {
+    'dtype': 'generic'
+});
+var y = discreteUniform( 20, -100, 0, {
+    'dtype': 'generic'
+});
+console.log( condition );
+console.log( x );
+console.log( y );
+
+var out = gwhere( condition.length, condition, 1, x, 1, y, 1 );
+console.log( out );
+```
+
+</section>
+
+<!-- /.examples -->
+
+<!-- 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">
+
+[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
+
+[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
+
+[@stdlib/array/base/accessor]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/base/accessor
+
+[@stdlib/array/base/where]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/base/where
+
+</section>
+
+<!-- /.links -->

lib/node_modules/@stdlib/blas/ext/base/gwhere/benchmark/benchmark.js

@@ -0,0 +1,107 @@
+/**
+* @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 bernoulli = require( '@stdlib/random/array/bernoulli' );
+var uniform = require( '@stdlib/random/array/uniform' );
+var isArray = require( '@stdlib/assert/is-array' );
+var pow = require( '@stdlib/math/base/special/pow' );
+var format = require( '@stdlib/string/format' );
+var pkg = require( './../package.json' ).name;
+var gwhere = require( './../lib/main.js' );
+
+
+// VARIABLES //
+
+var options = {
+	'dtype': 'generic'
+};
+
+
+// FUNCTIONS //
+
+/**
+* Creates a benchmark function.
+*
+* @private
+* @param {PositiveInteger} len - array length
+* @returns {Function} benchmark function
+*/
+function createBenchmark( len ) {
+	var condition = bernoulli( len, 0.5, options );
+	var x = uniform( len, -100, 100, options );
+	var y = uniform( len, -100, 100, options );
+	return benchmark;
+
+	/**
+	* Benchmark function.
+	*
+	* @private
+	* @param {Benchmark} b - benchmark instance
+	*/
+	function benchmark( b ) {
+		var out;
+		var i;
+
+		b.tic();
+		for ( i = 0; i < b.iterations; i++ ) {
+			condition[ i%len ] = ( condition[ i%len ] + 1 ) % 2;
+			out = gwhere( len, condition, 1, x, 1, y, 1 );
+			if ( !isArray( out ) ) {
+				b.fail( 'should return an array' );
+			}
+		}
+		b.toc();
+		if ( !isArray( out ) ) {
+			b.fail( 'should return an array' );
+		}
+		b.pass( 'benchmark finished' );
+		b.end();
+	}
+}
+
+
+// MAIN //
+
+/**
+* Main execution sequence.
+*
+* @private
+*/
+function main() {
+	var len;
+	var min;
+	var max;
+	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();