feat: add blas/ext/base/dcopy-within

lib/node_modules/@stdlib/blas/ext/base/dcopy-within/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.
+
+-->
+
+# dcopyWithin
+
+> Perform an in-place copy of elements within a double-precision floating-point strided array.
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var dcopyWithin = require( '@stdlib/blas/ext/base/dcopy-within' );
+```
+
+#### dcopyWithin( N, target, start, end, x, strideX, workspace, strideW )
+
+Performs an in-place copy of elements within a double-precision floating-point strided array.
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+var x = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+var w = new Float64Array( x.length );
+
+dcopyWithin( x.length, 3, 1, 4, x, 1, w, 1 );
+// x => <Float64Array>[ 1.0, 2.0, 3.0, 2.0, 3.0, 4.0 ]
+```
+
+The function has the following parameters:
+
+-   **N**: number of indexed elements.
+-   **target**: target index.
+-   **start**: source start index (inclusive).
+-   **end**: source end index (exclusive).
+-   **x**: input [`Float64Array`][@stdlib/array/float64].
+-   **strideX**: stride length for `x`.
+-   **workspace**: workspace [`Float64Array`][@stdlib/array/float64]. Must have at least `N` indexed elements.
+-   **strideW**: stride length for `workspace`.
+
+The `N` and stride parameters determine which elements in the strided array are accessed at runtime. For example, to copy every other element:
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+var x = new Float64Array( [ 1.0, 0.0, 3.0, 0.0, 5.0, 0.0, 7.0, 0.0 ] );
+var w = new Float64Array( 3 );
+
+dcopyWithin( 3, 0, 1, 6, x, 2, w, 1 );
+// x => <Float64Array>[ 3.0, 0.0, 5.0, 0.0, 5.0, 0.0, 7.0, 0.0 ]
+```
+
+Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+// Initial array...
+var x0 = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 ] );
+
+// Create an offset view...
+var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+
+// Create a workspace array...
+var w = new Float64Array( 6 );
+
+// Copy within the view...
+dcopyWithin( 6, 0, 3, 6, x1, 1, w, 1 );
+// x0 => <Float64Array>[ 1.0, 5.0, 6.0, 7.0, 5.0, 6.0, 7.0, 8.0 ]
+```
+
+<!-- lint disable maximum-heading-length -->
+
+#### dcopyWithin.ndarray( N, target, start, end, x, strideX, offsetX, workspace, strideW, offsetW )
+
+Performs an in-place copy of elements within a double-precision floating-point strided array using alternative indexing semantics.
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+var x = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+var w = new Float64Array( x.length );
+
+dcopyWithin.ndarray( x.length, 3, 1, 4, x, 1, 0, w, 1, 0 );
+// x => <Float64Array>[ 1.0, 2.0, 3.0, 2.0, 3.0, 4.0 ]
+```
+
+The function has the following additional parameters:
+
+-   **offsetX**: starting index for `x`.
+-   **offsetW**: starting index for `workspace`.
+
+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 copy elements starting from the third element:
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+var x = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 ] );
+var w = new Float64Array( 4 );
+
+dcopyWithin.ndarray( 4, 2, 0, 2, x, 1, 2, w, 1, 0 );
+// x => <Float64Array>[ 1.0, 2.0, 3.0, 4.0, 3.0, 4.0, 7.0, 8.0 ]
+```
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   If `N <= 0`, both functions return the strided array unchanged.
+-   If `target >= N`, both functions return the strided array unchanged.
+-   If `start >= end`, both functions return the strided array unchanged.
+-   If the `start` and `target` index ranges do not overlap, the `workspace` array is unused and thus ignored.
+-   Both functions **mutate** the provided input strided array.
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
+var zeros = require( '@stdlib/array/zeros' );
+var dcopyWithin = require( '@stdlib/blas/ext/base/dcopy-within' );
+
+var x = discreteUniform( 10, 0, 500, {
+    'dtype': 'float64'
+});
+console.log( x );
+
+var w = zeros( 10, 'float64' );
+
+// Copy the first 3 elements to positions 5, 6, 7:
+dcopyWithin( 10, 5, 0, 3, x, 1, w, 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/dcopy_within.h"
+```
+
+#### stdlib_strided_dcopy_within( N, target, start, end, \*X, strideX, \*W, strideW )
+
+Performs an in-place copy of elements within a double-precision floating-point strided array.
+
+```c
+double x[] = { 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 };
+double w[ 6 ];
+
+stdlib_strided_dcopy_within( 6, 3, 1, 4, x, 1, w, 1 );
+```
+
+The function accepts the following arguments:
+
+-   **N**: `[in] CBLAS_INT` number of indexed elements.
+-   **target**: `[in] CBLAS_INT` target index.
+-   **start**: `[in] CBLAS_INT` source start index (inclusive).
+-   **end**: `[in] CBLAS_INT` source end index (exclusive).
+-   **X**: `[inout] double*` input array.
+-   **strideX**: `[in] CBLAS_INT` stride length for `X`.
+-   **W**: `[out] double*` workspace array. Must have at least `N` indexed elements.
+-   **strideW**: `[in] CBLAS_INT` stride length for `W`.
+
+```c
+void stdlib_strided_dcopy_within( const CBLAS_INT N, const CBLAS_INT target, const CBLAS_INT start, const CBLAS_INT end, double *X, const CBLAS_INT strideX, double *W, const CBLAS_INT strideW );
+```
+
+#### stdlib_strided_dcopy_within_ndarray( N, target, start, end, \*X, strideX, offsetX, \*W, strideW, offsetW )
+
+Performs an in-place copy of elements within a double-precision floating-point strided array using alternative indexing semantics.
+
+```c
+double x[] = { 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 };
+double w[ 6 ];
+
+stdlib_strided_dcopy_within_ndarray( 4, 2, 0, 2, x, 1, 1, w, 1, 0 );
+```
+
+The function accepts the following arguments:
+
+-   **N**: `[in] CBLAS_INT` number of indexed elements.
+-   **target**: `[in] CBLAS_INT` target index.
+-   **start**: `[in] CBLAS_INT` source start index (inclusive).
+-   **end**: `[in] CBLAS_INT` source end index (exclusive).
+-   **X**: `[inout] double*` input array.
+-   **strideX**: `[in] CBLAS_INT` stride length for `X`.
+-   **offsetX**: `[in] CBLAS_INT` starting index for `X`.
+-   **W**: `[out] double*` workspace array. Must have at least `N` indexed elements.
+-   **strideW**: `[in] CBLAS_INT` stride length for `W`.
+-   **offsetW**: `[in] CBLAS_INT` starting index for `W`.
+
+```c
+void stdlib_strided_dcopy_within_ndarray( const CBLAS_INT N, const CBLAS_INT target, const CBLAS_INT start, const CBLAS_INT end, double *X, const CBLAS_INT strideX, const CBLAS_INT offsetX, double *W, const CBLAS_INT strideW, const CBLAS_INT offsetW );
+```
+
+</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/dcopy_within.h"
+#include <stdio.h>
+
+int main( void ) {
+    // Create a strided array:
+    double x[] = { 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 };
+
+    // Create a workspace array:
+    double w[ 8 ];
+
+    // Specify the number of indexed elements:
+    const int N = 8;
+
+    // Specify strides:
+    const int strideX = 1;
+    const int strideW = 1;
+
+    // Copy elements:
+    stdlib_strided_dcopy_within( N, 4, 1, 4, x, strideX, w, strideW );
+
+    // Print the result:
+    for ( int i = 0; i < 8; i++ ) {
+        printf( "x[ %i ] = %lf\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/float64]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/float64
+
+[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
+
+<!-- <related-links> -->
+
+<!-- </related-links> -->
+
+</section>
+
+<!-- /.links -->