feat: add ndarray/flatten-from-by

PR-URL: #9040
Closes: stdlib-js/metr-issue-tracker#115
Co-authored-by: Athan Reines <kgryte@gmail.com>
Reviewed-by: Athan Reines <kgryte@gmail.com>

Author: headlessNode

lib/node_modules/@stdlib/ndarray/flatten-from-by/README.md

@@ -0,0 +1,226 @@
+<!--
+
+@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.
+
+-->
+
+# flattenFromBy
+
+> Flatten an [ndarray][@stdlib/ndarray/ctor] according to a callback function starting from a specified dimension.
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var flattenFromBy = require( '@stdlib/ndarray/flatten-from-by' );
+```
+
+#### flattenFromBy( x, dim\[, options], fcn\[, thisArg] )
+
+Flattens an [ndarray][@stdlib/ndarray/ctor] according to a callback function starting from a specified dimension.
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+
+function scale( value ) {
+    return value * 2.0;
+}
+
+var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
+// returns <ndarray>
+
+var y = flattenFromBy( x, 1, scale );
+// returns <ndarray>[ [ 2.0, 4.0 ], [ 6.0, 8.0 ], [ 10.0, 12.0 ] ]
+```
+
+The function accepts the following arguments:
+
+-   **x**: input [ndarray][@stdlib/ndarray/ctor]. Must have one or more dimensions.
+-   **dim**: dimension to start flattening from. If provided an integer less than zero, the dimension index is resolved relative to the last dimension, with the last dimension corresponding to the value `-1`.
+-   **options**: function options (_optional_).
+-   **fcn**: callback function.
+-   **thisArg**: callback execution context (_optional_).
+
+The function accepts the following options:
+
+-   **order**: order in which input [ndarray][@stdlib/ndarray/ctor] elements should be flattened. Must be one of the following:
+
+    -   `'row-major'`: flatten elements in lexicographic order. For example, given a two-dimensional input [ndarray][@stdlib/ndarray/ctor] (i.e., a matrix), flattening in lexicographic order means flattening the input [ndarray][@stdlib/ndarray/ctor] row-by-row.
+    -   `'column-major'`: flatten elements in colexicographic order. For example, given a two-dimensional input [ndarray][@stdlib/ndarray/ctor] (i.e., a matrix), flattening in colexicographic order means flattening the input [ndarray][@stdlib/ndarray/ctor] column-by-column.
+    -   `'any'`: flatten according to the physical layout of the input [ndarray][@stdlib/ndarray/ctor] data in memory, regardless of the stated [order][@stdlib/ndarray/orders] of the input [ndarray][@stdlib/ndarray/ctor].
+    -   `'same'`: flatten according to the stated [order][@stdlib/ndarray/orders] of the input [ndarray][@stdlib/ndarray/ctor].
+
+    Default: `'row-major'`.
+
+-   **dtype**: output ndarray [data type][@stdlib/ndarray/dtypes]. By default, the function returns an [ndarray][@stdlib/ndarray/ctor] having the same [data type][@stdlib/ndarray/dtypes] as a provided input [ndarray][@stdlib/ndarray/ctor].
+
+By default, the input [ndarray][@stdlib/ndarray/ctor] is flattened in lexicographic order. To flatten elements in a different order, specify the `order` option.
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+
+function scale( value ) {
+    return value * 2.0;
+}
+
+var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
+// returns <ndarray>
+
+var opts = {
+    'order': 'column-major'
+};
+
+var y = flattenFromBy( x, 0, opts, scale );
+// returns <ndarray>[ 2.0, 6.0, 10.0, 4.0, 8.0, 12.0 ]
+```
+
+By default, the output ndarray [data type][@stdlib/ndarray/dtypes] is inferred from the input [ndarray][@stdlib/ndarray/ctor]. To return an ndarray with a different [data type][@stdlib/ndarray/dtypes], specify the `dtype` option.
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+var dtype = require( '@stdlib/ndarray/dtype' );
+
+function scale( value ) {
+    return value * 2.0;
+}
+
+var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
+// returns <ndarray>
+
+var opts = {
+    'dtype': 'float32'
+};
+
+var y = flattenFromBy( x, 0, opts, scale );
+// returns <ndarray>[ 2.0, 4.0, 6.0, 8.0, 10.0, 12.0 ]
+
+var dt = String( dtype( y ) );
+// returns 'float32'
+```
+
+To set the callback function execution context, provide a `thisArg`.
+
+<!-- eslint-disable no-invalid-this, max-len -->
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+
+function scale( value ) {
+    this.count += 1;
+    return value * 2.0;
+}
+
+var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
+// returns <ndarray>
+
+var ctx = {
+    'count': 0
+};
+
+var y = flattenFromBy( x, 1, scale, ctx );
+// returns <ndarray>[ [ 2.0, 4.0 ], [ 6.0, 8.0 ], [ 10.0, 12.0 ] ]
+
+var count = ctx.count;
+// returns 6
+```
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   The function **always** returns a copy of input [ndarray][@stdlib/ndarray/ctor] data, even when an input [ndarray][@stdlib/ndarray/ctor] already has the desired number of dimensions.
+
+-   The callback function is provided the following arguments:
+
+    -   **value**: current array element.
+    -   **indices**: current array element indices.
+    -   **arr**: the input [ndarray][@stdlib/ndarray/ctor].
+
+-   The order in which array elements are traversed and passed to a provided callback function is **not** guaranteed to match the order of array elements in an [ndarray][@stdlib/ndarray/ctor] view. Accordingly, a provided callback should avoid making assumptions regarding the order of provided elements.
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
+var array = require( '@stdlib/ndarray/array' );
+var ndarray2array = require( '@stdlib/ndarray/to-array' );
+var flattenFromBy = require( '@stdlib/ndarray/flatten-from-by' );
+
+function scale( value ) {
+    return value * 2.0;
+}
+
+var xbuf = discreteUniform( 12, -100, 100, {
+    'dtype': 'generic'
+});
+
+var x = array( xbuf, {
+    'shape': [ 2, 2, 3 ],
+    'dtype': 'generic'
+});
+console.log( ndarray2array( x ) );
+
+var y = flattenFromBy( x, 1, scale );
+console.log( ndarray2array( y ) );
+```
+
+</section>
+
+<!-- /.examples -->
+
+<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
+
+<section class="related">
+
+</section>
+
+<!-- /.related -->
+
+<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/orders]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/orders
+
+<!-- <related-links> -->
+
+<!-- </related-links> -->
+
+</section>
+
+<!-- /.links -->