feat: add implementation of stats/incr/nanmvariance

lib/node_modules/@stdlib/stats/incr/nanmvariance/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.
+
+-->
+
+# incrnanmvariance
+
+> Compute a moving [unbiased sample variance][sample-variance] incrementally, ignoring `NaN` values.
+
+<section class="intro">
+
+For a window of size `W`, the [unbiased sample variance][sample-variance] is defined as
+
+<!-- <equation class="equation" label="eq:unbiased_sample_variance" align="center" raw="s^2 = \frac{1}{W-1} \sum_{i=0}^{W-1} ( x_i - \bar{x} )^2" alt="Equation for the unbiased sample variance."> -->
+
+```math
+s^2 = \frac{1}{W-1} \sum_{i=0}^{W-1} ( x_i - \bar{x} )^2
+```
+
+<!-- <div class="equation" align="center" data-raw-text="s^2 = \frac{1}{W-1} \sum_{i=0}^{W-1} ( x_i - \bar{x} )^2" data-equation="eq:unbiased_sample_variance">
+    <img src="./docs/img/equation_unbiased_sample_variance.svg" alt="Equation for the unbiased sample variance.">
+    <br>
+</div> -->
+
+<!-- </equation> -->
+
+</section>
+
+<!-- /.intro -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var incrnanmvariance = require( '@stdlib/stats/incr/nanmvariance' );
+```
+
+#### incrnanmvariance( window )
+
+Returns an accumulator function which incrementally computes a moving [unbiased sample variance][sample-variance], ignoring `NaN` values. The `window` parameter defines the number of values over which to compute the moving [unbiased sample variance][sample-variance].
+
+```javascript
+var accumulator = incrnanmvariance( 3 );
+```
+
+#### accumulator( \[x] )
+
+If provided an input value `x`, the accumulator function returns an updated [unbiased sample variance][sample-variance]. If not provided an input value `x`, the accumulator function returns the current [unbiased sample variance][sample-variance].
+
+```javascript
+var accumulator = incrnanmvariance( 3 );
+
+var s2 = accumulator();
+// returns null
+
+s2 = accumulator( 2.0 );
+// returns 0.0
+
+s2 = accumulator( NaN );
+// returns 0.0
+
+s2 = accumulator( -5.0 );
+// returns 24.5
+
+s2 = accumulator( 3.0 );
+// returns 19.0
+
+s2 = accumulator( NaN );
+// returns 19.0
+
+s2 = accumulator();
+// returns 19.0
+```
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   `NaN` values are ignored. If all values in the window are `NaN`, the accumulator returns the last valid variance or `null` if no valid values have been provided.
+-   Input values are **not** type checked beyond `NaN` detection. If other non-numeric inputs are possible, you are advised to type check and handle accordingly **before** passing the value to the accumulator function.
+-   As `W` values are needed to fill the window buffer, the first `W-1` returned values are calculated from smaller sample sizes. Until the window is full, each returned value is calculated from all provided non-`NaN` values.
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var uniform = require( '@stdlib/random/base/uniform' );
+var bernoulli = require( '@stdlib/random/base/bernoulli' );
+var incrnanmvariance = require( '@stdlib/stats/incr/nanmvariance' );
+
+// Initialize an accumulator:
+var accumulator = incrnanmvariance( 5 );
+
+// For each simulated datum, update the moving variance...
+var i;
+for ( i = 0; i < 100; i++ ) {
+    accumulator( ( bernoulli( 0.8 ) < 1 ) ? NaN : uniform( -50.0, 50.0 ) );
+}
+console.log( accumulator() );
+```
+
+</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">
+
+[sample-variance]: https://en.wikipedia.org/wiki/Variance
+
+</section>
+
+<!-- /.links -->

lib/node_modules/@stdlib/stats/incr/nanmvariance/benchmark/benchmark.js

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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.
+*/
+
+'use strict';
+
+// MODULES //
+
+var bench = require( '@stdlib/bench' );
+var format = require( '@stdlib/string/format' );
+var pkg = require( './../package.json' ).name;
+var incrnanmvariance = require( './../lib' );
+
+
+// MAIN //
+
+bench( pkg, function benchmark( b ) {
+	var f;
+	var i;
+	b.tic();
+	for ( i = 0; i < b.iterations; i++ ) {
+		f = incrnanmvariance( (i%5)+1 );
+		if ( typeof f !== 'function' ) {
+			b.fail( 'should return a function' );
+		}
+	}
+	b.toc();
+	if ( typeof f !== 'function' ) {
+		b.fail( 'should return a function' );
+	}
+	b.pass( 'benchmark finished' );
+	b.end();
+});
+
+bench( format( '%s::accumulator', pkg ), function benchmark( b ) {
+	var acc;
+	var v;
+	var i;
+
+	acc = incrnanmvariance( 5 );
+
+	b.tic();
+	for ( i = 0; i < b.iterations; i++ ) {
+		v = acc( (i%5) ? i : NaN );
+		if ( v !== v ) {
+			b.fail( 'should not return NaN' );
+		}
+	}
+	b.toc();
+	if ( v !== v ) {
+		b.fail( 'should not return NaN' );
+	}
+	b.pass( 'benchmark finished' );
+	b.end();
+});

lib/node_modules/@stdlib/stats/incr/nanmvariance/docs/repl.txt

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+
+{{alias}}( W )
+    Returns an accumulator function which incrementally computes a moving
+    variance, ignoring `NaN` values.
+
+    The `W` parameter defines the number of values over which to compute the
+    moving unbiased sample variance.
+
+    If provided a value, the accumulator function returns an updated moving
+    variance. If not provided a value, the accumulator function returns the
+    current moving unbiased sample variance.
+
+    As `W` values are needed to fill the window buffer, the first `W-1` returned
+    values are calculated from smaller sample sizes. Until the window is full,
+    each returned value is calculated from all provided values.
+
+    Parameters
+    ----------
+    W: integer
+        Window size.
+
+    Returns
+    -------
+    acc: Function
+        Accumulator function.
+
+    Examples
+    --------
+    > var accumulator = {{alias}}( 3 );
+    > var s2 = accumulator()
+    null
+    > s2 = accumulator( 2.0 )
+    0.0
+    > s2 = accumulator( NaN )
+    0.0
+    > s2 = accumulator( -5.0 )
+    24.5
+    > s2 = accumulator( 3.0 )
+    19.0
+    > s2 = accumulator()
+    19.0
+
+    See Also
+    --------
+