<!--

@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.

-->

# smskrev

> Reverse a single-precision floating-point strided array in-place according to a mask.

<section class="usage">

## Usage

```javascript
var smskrev = require( '@stdlib/blas/ext/base/smskrev' );
```

#### smskrev( N, x, strideX, mask, strideMask )

Reverses a single-precision floating-point strided array in-place according to a mask.

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );
var mask = new Uint8Array( [ 0, 0, 0, 1, 0, 0, 0, 0 ] );

smskrev( x.length, x, 1, mask, 1 );
// x => <Float32Array>[ -3.0, -1.0, 0.0, -5.0, 4.0, 3.0, 1.0, -2.0 ]
```

The function has the following parameters:

-   **N**: number of indexed elements.
-   **x**: input [`Float32Array`][@stdlib/array/float32].
-   **strideX**: stride length for `x`.
-   **mask**: mask [`Uint8Array`][@stdlib/array/uint8]. If a `mask` array element is `0`, the corresponding element in `x` is considered valid and **included** in the reversal. If a `mask` array element is `1`, the corresponding element in `x` is considered invalid/missing and **excluded** from the reversal.
-   **strideMask**: stride length for `mask`.

The `N` and stride parameters determine which elements in the strided arrays are accessed at runtime. For example, to reverse every other element:

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );
var mask = new Uint8Array( [ 0, 0, 0, 0 ] );

smskrev( 4, x, 2, mask, 1 );
// x => <Float32Array>[ -1.0, 1.0, 4.0, -5.0, 3.0, 0.0, -2.0, -3.0 ]
```

Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

// Initial arrays...
var x0 = new Float32Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );
var m0 = new Uint8Array( [ 0, 0, 0, 0, 0, 0 ] );

// Create offset views...
var x1 = new Float32Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var m1 = new Uint8Array( m0.buffer, m0.BYTES_PER_ELEMENT*1 );

// Reverse every other element...
smskrev( 3, x1, 2, m1, 1 );
// x0 => <Float32Array>[ 1.0, -6.0, 3.0, -4.0, 5.0, -2.0 ]
```

#### smskrev.ndarray( N, x, strideX, offsetX, mask, strideMask, offsetMask )

Reverses a single-precision floating-point strided array in-place according to a mask and using alternative indexing semantics.

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );
var mask = new Uint8Array( [ 0, 0, 0, 1, 0, 0, 0, 0 ] );

smskrev.ndarray( x.length, x, 1, 0, mask, 1, 0 );
// x => <Float32Array>[ -3.0, -1.0, 0.0, -5.0, 4.0, 3.0, 1.0, -2.0 ]
```

The function has the following additional parameters:

-   **offsetX**: starting index for `x`.
-   **offsetMask**: starting index for `mask`.

While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying buffer, the offset parameters support indexing semantics based on starting indices. For example, to access only the last three elements:

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

var x = new Float32Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );
var mask = new Uint8Array( [ 0, 0, 0, 0, 0, 0 ] );

smskrev.ndarray( 3, x, 1, x.length-3, mask, 1, 3 );
// x => <Float32Array>[ 1.0, -2.0, 3.0, -6.0, 5.0, -4.0 ]
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If `N <= 0`, both functions return `x` unchanged.
-   Where possible, one should "reverse" a strided array by negating its stride, which is an `O(1)` operation, in contrast to performing an in-place reversal, which is `O(N)`. However, in certain circumstances, this is not tenable, particularly when interfacing with libraries which assume and/or expect a specific memory layout (e.g., strided array elements arranged in memory in ascending order). In general, when working with strided arrays, only perform an in-place reversal when strictly necessary.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var Uint8Array = require( '@stdlib/array/uint8' );
var smskrev = require( '@stdlib/blas/ext/base/smskrev' );

var x = discreteUniform( 10, -100, 100, {
    'dtype': 'float32'
});
console.log( x );

var mask = new Uint8Array( [ 0, 0, 0, 1, 0, 0, 1, 0, 0, 0 ] );
console.log( mask );

smskrev( x.length, x, 1, mask, 1 );
console.log( x );
```

</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/smskrev.h"
```

#### stdlib_strided_smskrev( N, \*X, strideX, \*Mask, strideMask )

Reverses a single-precision floating-point strided array in-place according to a mask.

```c
#include <stdint.h>

float x[] = { 1.0f, 2.0f, 3.0f, 4.0f };
const uint8_t mask[] = { 0, 0, 0, 0 };

stdlib_strided_smskrev( 4, x, 1, mask, 1 );
```

The function accepts the following arguments:

-   **N**: `[in] CBLAS_INT` number of indexed elements.
-   **X**: `[inout] float*` input array.
-   **strideX**: `[in] CBLAS_INT` stride length for `X`.
-   **Mask**: `[in] uint8_t*` mask array.
-   **strideMask**: `[in] CBLAS_INT` stride length for `Mask`.

```c
void stdlib_strided_smskrev( const CBLAS_INT N, float *X, const CBLAS_INT strideX, const uint8_t *Mask, const CBLAS_INT strideMask );
```

<!-- lint disable maximum-heading-length -->

#### stdlib_strided_smskrev_ndarray( N, \*X, strideX, offsetX, \*Mask, strideMask, offsetMask )

<!-- lint enable maximum-heading-length -->

Reverses a single-precision floating-point strided array in-place according to a mask and using alternative indexing semantics.

```c
#include <stdint.h>

float x[] = { 1.0f, 2.0f, 3.0f, 4.0f };
const uint8_t mask[] = { 0, 0, 0, 0 };

stdlib_strided_smskrev_ndarray( 4, x, 1, 0, mask, 1, 0 );
```

The function accepts the following arguments:

-   **N**: `[in] CBLAS_INT` number of indexed elements.
-   **X**: `[inout] float*` input array.
-   **strideX**: `[in] CBLAS_INT` stride length for `X`.
-   **offsetX**: `[in] CBLAS_INT` starting index for `X`.
-   **Mask**: `[in] uint8_t*` mask array.
-   **strideMask**: `[in] CBLAS_INT` stride length for `Mask`.
-   **offsetMask**: `[in] CBLAS_INT` starting index for `Mask`.

```c
void stdlib_strided_smskrev_ndarray( const CBLAS_INT N, float *X, const CBLAS_INT strideX, const CBLAS_INT offsetX, const uint8_t *Mask, const CBLAS_INT strideMask, const CBLAS_INT offsetMask );
```

</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/smskrev.h"
#include <stdint.h>
#include <stdio.h>

int main( void ) {
    // Create a strided array:
    float x[] = { 1.0f, -2.0f, 3.0f, -4.0f, 5.0f, -6.0f, 7.0f, -8.0f };

    // Create a mask array:
    const uint8_t mask[] = { 0, 0, 0, 1, 0, 0, 0, 0 };

    // Specify the number of elements:
    const int N = 8;

    // Specify strides:
    const int strideX = 1;
    const int strideMask = 1;

    // Reverse the array:
    stdlib_strided_smskrev( N, x, strideX, mask, strideMask );

    // Print the result:
    for ( int i = 0; i < 8; i++ ) {
        printf( "x[ %i ] = %f\n", i, x[ 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/float32]: https://www.npmjs.com/package/@stdlib/array-float32

[@stdlib/array/uint8]: https://www.npmjs.com/package/@stdlib/array-uint8

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

<!-- <related-links> -->

<!-- </related-links> -->

</section>

<!-- /.links -->
