feat: add blas/ext/base/swhere

PR-URL: #11582
Co-authored-by: Athan Reines <kgryte@gmail.com>
Reviewed-by: Athan Reines <kgryte@gmail.com> 
Closes: stdlib-js/metr-issue-tracker#423

Author: headlessNode

lib/node_modules/@stdlib/blas/ext/base/swhere/README.md

@@ -0,0 +1,382 @@
+<!--
+
+@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.
+
+-->
+
+# swhere
+
+> Take elements from one of two single-precision floating-point strided arrays depending on a condition.
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var swhere = require( '@stdlib/blas/ext/base/swhere' );
+```
+
+#### swhere( N, condition, strideC, x, strideX, y, strideY, out, strideOut )
+
+Takes elements from one of two single-precision floating-point strided arrays depending on a condition.
+
+```javascript
+var BooleanArray = require( '@stdlib/array/bool' );
+var Float32Array = require( '@stdlib/array/float32' );
+
+var condition = new BooleanArray( [ true, false, true ] );
+var x = new Float32Array( [ 1.0, 2.0, 3.0 ] );
+var y = new Float32Array( [ 4.0, 5.0, 6.0 ] );
+var out = new Float32Array( [ 0.0, 0.0, 0.0 ] );
+
+swhere( 3, condition, 1, x, 1, y, 1, out, 1 );
+// out => <Float32Array>[ 1.0, 5.0, 3.0 ]
+```
+
+The function has the following parameters:
+
+-   **N**: number of indexed elements.
+-   **condition**: condition [`BooleanArray`][@stdlib/array/bool].
+-   **strideC**: stride length for `condition`.
+-   **x**: first input [`Float32Array`][@stdlib/array/float32].
+-   **strideX**: stride length for `x`.
+-   **y**: second input [`Float32Array`][@stdlib/array/float32].
+-   **strideY**: stride length for `y`.
+-   **out**: output [`Float32Array`][@stdlib/array/float32].
+-   **strideOut**: stride length for `out`.
+
+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 BooleanArray = require( '@stdlib/array/bool' );
+var Float32Array = require( '@stdlib/array/float32' );
+
+var condition = new BooleanArray( [ true, false, false, true, true, false ] );
+var x = new Float32Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+var y = new Float32Array( [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
+var out = new Float32Array( [ 0.0, 0.0, 0.0 ] );
+
+swhere( 3, condition, 2, x, 2, y, 2, out, 1 );
+// out => <Float32Array>[ 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 BooleanArray = require( '@stdlib/array/bool' );
+var Float32Array = require( '@stdlib/array/float32' );
+
+// Initial arrays...
+var condition0 = new BooleanArray( [ false, true, false, true, false, true ] );
+var x0 = new Float32Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+var y0 = new Float32Array( [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
+var out0 = new Float32Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );
+
+// Create offset views...
+var condition1 = new BooleanArray( condition0.buffer, condition0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+var x1 = new Float32Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+var y1 = new Float32Array( y0.buffer, y0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+var out1 = new Float32Array( out0.buffer, out0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+
+swhere( 3, condition1, 2, x1, 2, y1, 2, out1, 1 );
+// out0 => <Float32Array>[ 0.0, 2.0, 4.0, 6.0, 0.0, 0.0 ]
+```
+
+<!-- lint disable maximum-heading-length -->
+
+#### swhere.ndarray( N, condition, strideC, offsetC, x, strideX, offsetX, y, strideY, offsetY, out, strideOut, offsetOut )
+
+<!-- lint enable maximum-heading-length -->
+
+Takes elements from one of two single-precision floating-point strided arrays depending on a condition using alternative indexing semantics.
+
+```javascript
+var BooleanArray = require( '@stdlib/array/bool' );
+var Float32Array = require( '@stdlib/array/float32' );
+
+var condition = new BooleanArray( [ true, false, true ] );
+var x = new Float32Array( [ 1.0, 2.0, 3.0 ] );
+var y = new Float32Array( [ 4.0, 5.0, 6.0 ] );
+var out = new Float32Array( [ 0.0, 0.0, 0.0 ] );
+
+swhere.ndarray( 3, condition, 1, 0, x, 1, 0, y, 1, 0, out, 1, 0 );
+// out => <Float32Array>[ 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`.
+-   **offsetOut**: starting index for `out`.
+
+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 BooleanArray = require( '@stdlib/array/bool' );
+var Float32Array = require( '@stdlib/array/float32' );
+
+var condition = new BooleanArray( [ false, true, false, false, false, true ] );
+var x = new Float32Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+var y = new Float32Array( [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
+var out = new Float32Array( [ 0.0, 0.0, 0.0 ] );
+
+swhere.ndarray( 3, condition, 2, 1, x, 2, 1, y, 2, 1, out, 1, 0 );
+// out => <Float32Array>[ 2.0, 10.0, 6.0 ]
+```
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   If `N <= 0`, both functions return `out` unchanged.
+-   The `condition` argument must be a [`BooleanArray`][@stdlib/array/bool].
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var bernoulli = require( '@stdlib/random/array/bernoulli' );
+var uniform = require( '@stdlib/random/array/uniform' );
+var BooleanArray = require( '@stdlib/array/bool' );
+var Float32Array = require( '@stdlib/array/float32' );
+var swhere = require( '@stdlib/blas/ext/base/swhere' );
+
+var cbuf = bernoulli( 20, 0.5, {
+    'dtype': 'uint8'
+});
+var condition = new BooleanArray( cbuf.buffer );
+console.log( condition );
+
+var x = uniform( 20, 0.0, 100.0, {
+    'dtype': 'float32'
+});
+console.log( x );
+
+var y = uniform( 20, -100.0, 0.0, {
+    'dtype': 'float32'
+});
+console.log( y );
+
+var out = new Float32Array( condition.length );
+console.log( out );
+
+swhere( condition.length, condition, 1, x, 1, y, 1, out, 1 );
+console.log( out );
+```
+
+</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
+#include "stdlib/blas/ext/base/swhere.h"
+```
+
+<!-- lint disable maximum-heading-length -->
+
+#### stdlib_strided_swhere( N, \*Condition, strideC, \*X, strideX, \*Y, strideY, \*Out, strideOut )
+
+<!-- lint enable maximum-heading-length -->
+
+Takes elements from one of two single-precision floating-point strided arrays depending on a condition.
+
+```c
+#include <stdbool.h>
+
+const bool condition[] = { true, false, true };
+const float x[] = { 1.0f, 2.0f, 3.0f };
+const float y[] = { 4.0f, 5.0f, 6.0f };
+float out[] = { 0.0f, 0.0f, 0.0f };
+
+stdlib_strided_swhere( 3, condition, 1, x, 1, y, 1, out, 1 );
+```
+
+The function accepts the following arguments:
+
+-   **N**: `[in] CBLAS_INT` number of indexed elements.
+-   **Condition**: `[in] bool*` condition array.
+-   **strideC**: `[in] CBLAS_INT` stride length for `Condition`.
+-   **X**: `[in] float*` first input array.
+-   **strideX**: `[in] CBLAS_INT` stride length for `X`.
+-   **Y**: `[in] float*` second input array.
+-   **strideY**: `[in] CBLAS_INT` stride length for `Y`.
+-   **Out**: `[out] float*` output array.
+-   **strideOut**: `[in] CBLAS_INT` stride length for `Out`.
+
+```c
+void stdlib_strided_swhere( const CBLAS_INT N, const bool *Condition, const CBLAS_INT strideC, const float *X, const CBLAS_INT strideX, const float *Y, const CBLAS_INT strideY, float *Out, const CBLAS_INT strideOut );
+```
+
+<!-- lint disable maximum-heading-length -->
+
+#### stdlib_strided_swhere_ndarray( N, \*Condition, strideC, offsetC, \*X, strideX, offsetX, \*Y, strideY, offsetY, \*Out, strideOut, offsetOut )
+
+<!-- lint enable maximum-heading-length -->
+
+Takes elements from one of two single-precision floating-point strided arrays depending on a condition using alternative indexing semantics.
+
+```c
+#include <stdbool.h>
+
+const bool condition[] = { true, false, true };
+const float x[] = { 1.0f, 2.0f, 3.0f };
+const float y[] = { 4.0f, 5.0f, 6.0f };
+float out[] = { 0.0f, 0.0f, 0.0f };
+
+stdlib_strided_swhere_ndarray( 3, condition, 1, 0, x, 1, 0, y, 1, 0, out, 1, 0 );
+```
+
+The function accepts the following arguments:
+
+-   **N**: `[in] CBLAS_INT` number of indexed elements.
+-   **Condition**: `[in] bool*` condition array.
+-   **strideC**: `[in] CBLAS_INT` stride length for `Condition`.
+-   **offsetC**: `[in] CBLAS_INT` starting index for `Condition`.
+-   **X**: `[in] float*` first input array.
+-   **strideX**: `[in] CBLAS_INT` stride length for `X`.
+-   **offsetX**: `[in] CBLAS_INT` starting index for `X`.
+-   **Y**: `[in] float*` second input array.
+-   **strideY**: `[in] CBLAS_INT` stride length for `Y`.
+-   **offsetY**: `[in] CBLAS_INT` starting index for `Y`.
+-   **Out**: `[out] float*` output array.
+-   **strideOut**: `[in] CBLAS_INT` stride length for `Out`.
+-   **offsetOut**: `[in] CBLAS_INT` starting index for `Out`.
+
+```c
+void stdlib_strided_swhere_ndarray( const CBLAS_INT N, const bool *Condition, const CBLAS_INT strideC, const CBLAS_INT offsetC, const float *X, const CBLAS_INT strideX, const CBLAS_INT offsetX, const float *Y, const CBLAS_INT strideY, const CBLAS_INT offsetY, float *Out, const CBLAS_INT strideOut, const CBLAS_INT offsetOut );
+```
+
+</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
+#include "stdlib/blas/ext/base/swhere.h"
+#include <stdio.h>
+#include <stdbool.h>
+
+int main( void ) {
+    // Create strided arrays:
+    const bool condition[] = { true, false, true, false, true };
+    const float x[] = { 1.0f, 2.0f, 3.0f, 4.0f, 5.0f };
+    const float y[] = { 6.0f, 7.0f, 8.0f, 9.0f, 10.0f };
+    float out[] = { 0.0f, 0.0f, 0.0f, 0.0f, 0.0f };
+
+    // Specify the number of indexed elements:
+    const int N = 5;
+
+    // Specify stride lengths:
+    const int strideC = 1;
+    const int strideX = 1;
+    const int strideY = 1;
+    const int strideOut = 1;
+
+    // Select from `x` or `y` based on the condition array:
+    stdlib_strided_swhere( N, condition, strideC, x, strideX, y, strideY, out, strideOut );
+
+    // Print the result:
+    for ( int i = 0; i < N; i++ ) {
+        printf( "out[ %i ] = %f\n", i, out[ i ] );
+    }
+}
+```
+
+</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">
+
+[@stdlib/array/bool]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/bool
+
+[@stdlib/array/float32]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/float32
+
+[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
+
+</section>
+
+<!-- /.links -->