feat: add blas/ext/find-index

lib/node_modules/@stdlib/blas/ext/find-index/README.md

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+<!--
+
+@license Apache-2.0
+
+Copyright (c) 2025 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.
+
+-->
+
+# findIndex
+
+> Return the first index of an element along an [ndarray][@stdlib/ndarray/ctor] dimension which passes a test implemented by a predicate function.
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var findIndex = require( '@stdlib/blas/ext/find-index' );
+```
+
+#### findIndex( x\[, options], clbk\[, thisArg] )
+
+Returns the first index of an element along an [ndarray][@stdlib/ndarray/ctor] dimension which passes a test implemented by a predicate function.
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+
+function isEven( v ) {
+    return v % 2.0 === 0.0;
+}
+
+// Create an input ndarray:
+var x = array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+// returns <ndarray>
+
+// Perform operation:
+var out = findIndex( x, isEven );
+// returns <ndarray>
+
+var idx = out.get();
+// returns 1
+```
+
+The function has the following parameters:
+
+-   **x**: input [ndarray][@stdlib/ndarray/ctor]. Must have at least one dimension.
+-   **options**: function options (_optional_).
+-   **clbk**: callback function.
+-   **thisArg**: callback execution context (_optional_).
+
+The invoked callback is provided three arguments:
+
+-   **value**: current array element.
+-   **idx**: current array element index.
+-   **array**: input ndarray.
+
+To set the callback execution context, provide a `thisArg`.
+
+<!-- eslint-disable no-invalid-this -->
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+
+function isEven( v ) {
+    this.count += 1;
+    return v % 2.0 === 0.0;
+}
+
+var x = array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+
+var ctx = {
+    'count': 0
+};
+var out = findIndex( x, isEven, ctx );
+// returns <ndarray>
+
+var idx = out.get();
+// returns 1
+
+var count = ctx.count;
+// returns 2
+```
+
+The function accepts the following options:
+
+-   **dtype**: output ndarray [data type][@stdlib/ndarray/dtypes]. Must be an integer index or generic [data type][@stdlib/ndarray/dtypes].
+-   **dim**: dimension over which to perform operation. If provided a negative integer, the dimension along which to perform the operation is determined by counting backward from the last dimension (where `-1` refers to the last dimension). Default: `-1`.
+-   **keepdims**: boolean indicating whether the reduced dimensions should be included in the returned [ndarray][@stdlib/ndarray/ctor] as singleton dimensions. Default: `false`.
+
+If no element along an [ndarray][@stdlib/ndarray/ctor] passes a test implemented by the predicate function, the corresponding element in the returned [ndarray][@stdlib/ndarray/ctor] is `-1`.
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+
+function isEven( v ) {
+    return v % 2.0 === 0.0;
+}
+
+// Create an input ndarray:
+var x = array( [ 1.0, 3.0, 5.0, 7.0 ] );
+// returns <ndarray>
+
+// Perform operation:
+var out = findIndex( x, isEven );
+// returns <ndarray>
+
+var idx = out.get();
+// returns -1
+```
+
+By default, the function performs the operation over elements in the last dimension. To perform the operation over a different dimension, provide a `dim` option.
+
+```javascript
+var ndarray2array = require( '@stdlib/ndarray/to-array' );
+var array = require( '@stdlib/ndarray/array' );
+
+function isEven( v ) {
+    return v % 2.0 === 0.0;
+}
+
+var x = array( [ [ -1.0, 2.0 ], [ -3.0, 4.0 ] ] );
+
+var opts = {
+    'dim': 0
+};
+
+var out = findIndex( x, opts, isEven );
+// returns <ndarray>
+
+var idx = ndarray2array( out );
+// returns [ -1, 0 ]
+```
+
+By default, the function excludes reduced dimensions from the output [ndarray][@stdlib/ndarray/ctor]. To include the reduced dimensions as singleton dimensions, set the `keepdims` option to `true`.
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+var ndarray2array = require( '@stdlib/ndarray/to-array' );
+
+function isEven( v ) {
+    return v % 2.0 === 0.0;
+}
+
+var x = array( [ [ -1.0, 2.0 ], [ -3.0, 4.0 ] ] );
+
+var opts = {
+    'dim': 0,
+    'keepdims': true
+};
+
+var out = findIndex( x, opts, isEven );
+// returns <ndarray>
+
+var idx = ndarray2array( out );
+// returns [ [ -1, 0 ] ]
+```
+
+By default, the function returns an [ndarray][@stdlib/ndarray/ctor] having a [data type][@stdlib/ndarray/dtypes] determined by the function's output data type [policy][@stdlib/ndarray/output-dtype-policies]. To override the default behavior, set the `dtype` option.
+
+```javascript
+var ndarray2array = require( '@stdlib/ndarray/to-array' );
+var dtype = require( '@stdlib/ndarray/dtype' );
+var array = require( '@stdlib/ndarray/array' );
+
+function isEven( v ) {
+    return v % 2.0 === 0.0;
+}
+
+var x = array( [ 1.0, 2.0, 3.0, 4.0 ] );
+
+var opts = {
+    'dtype': 'generic'
+};
+
+var idx = findIndex( x, opts, isEven );
+// returns <ndarray>
+
+var dt = dtype( idx );
+// returns 'generic'
+```
+
+#### findIndex.assign( x, out\[, options], clbk\[, thisArg] )
+
+Returns the first index of an element along an [ndarray][@stdlib/ndarray/ctor] dimension which passes a test implemented by a predicate function and assigns results to a provided output [ndarray][@stdlib/ndarray/ctor].
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+var zeros = require( '@stdlib/ndarray/zeros' );
+
+function isEven( v ) {
+    return v % 2.0 === 0.0;
+}
+
+var x = array( [ 1.0, 2.0, 3.0, 4.0 ] );
+var y = zeros( [], {
+    'dtype': 'int32'
+});
+
+var out = findIndex.assign( x, y, isEven );
+// returns <ndarray>
+
+var idx = out.get();
+// returns 1
+
+var bool = ( out === y );
+// returns true
+```
+
+The method has the following parameters:
+
+-   **x**: input [ndarray][@stdlib/ndarray/ctor]. Must have at least one dimension.
+-   **out**: output [ndarray][@stdlib/ndarray/ctor].
+-   **options**: function options (_optional_).
+-   **clbk**: callback function.
+-   **thisArg**: callback execution context (_optional_).
+
+The method accepts the following options:
+
+-   **dim**: dimension over which to perform operation. If provided a negative integer, the dimension along which to perform the operation is determined by counting backward from the last dimension (where `-1` refers to the last dimension). Default: `-1`.
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   A provided callback function should return a boolean.
+-   Setting the `keepdims` option to `true` can be useful when wanting to ensure that the output [ndarray][@stdlib/ndarray/ctor] is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with ndarrays having the same shape as the input [ndarray][@stdlib/ndarray/ctor].
+-   The output data type [policy][@stdlib/ndarray/output-dtype-policies] only applies to the main function and specifies that, by default, the function must return an [ndarray][@stdlib/ndarray/ctor] having an integer index or "generic" [data type][@stdlib/ndarray/dtypes]. For the `assign` method, the output [ndarray][@stdlib/ndarray/ctor] is allowed to have any supported output [data type][@stdlib/ndarray/dtypes].
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
+var ndarray2array = require( '@stdlib/ndarray/to-array' );
+var ndarray = require( '@stdlib/ndarray/ctor' );
+var findIndex = require( '@stdlib/blas/ext/find-index' );
+
+// Define a callback function:
+function isEven( v ) {
+    return v % 2.0 === 0.0;
+}
+
+// Generate an array of random numbers:
+var xbuf = discreteUniform( 10, 0, 20, {
+    'dtype': 'generic'
+});
+
+// Wrap in an ndarray:
+var x = new ndarray( 'generic', xbuf, [ 5, 2 ], [ 2, 1 ], 0, 'row-major' );
+console.log( ndarray2array( x ) );
+
+var opts = {
+    'dim': 0
+};
+
+// Perform operation:
+var idx = findIndex( x, opts, isEven );
+
+// Print the results:
+console.log( ndarray2array( idx ) );
+```
+
+</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">
+
+[@stdlib/ndarray/ctor]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/ctor
+
+[@stdlib/ndarray/dtypes]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/dtypes
+
+[@stdlib/ndarray/output-dtype-policies]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/output-dtype-policies
+
+[@stdlib/ndarray/base/broadcast-shapes]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/base/broadcast-shapes
+
+</section>
+
+<!-- /.links -->