feat: add stats/incr/nanmape

lib/node_modules/@stdlib/stats/incr/nanmape/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.
+
+-->
+
+# incrnanmape
+
+> Compute the [mean absolute percentage error][mean-absolute-percentage-error] (MAPE) incrementally, ignoring `NaN` values.
+
+<section class="intro">
+
+The [mean absolute percentage error][mean-absolute-percentage-error] is defined as
+
+<!-- <equation class="equation" label="eq:mean_absolute_percentage_error" align="center" raw="\operatorname{MAPE}  = \frac{100}{n} \sum_{i=0}^{n-1} \biggl| \frac{a_i - f_i}{a_i} \biggr|" alt="Equation for the mean absolute percentage error."> -->
+
+```math
+\mathop{\mathrm{MAPE}}  = \frac{100}{n} \sum_{i=0}^{n-1} \biggl| \frac{a_i - f_i}{a_i} \biggr|
+```
+
+<!-- <div class="equation" align="center" data-raw-text="\operatorname{MAPE}  = \frac{100}{n} \sum_{i=0}^{n-1} \biggl| \frac{a_i - f_i}{a_i} \biggr|" data-equation="eq:mean_absolute_percentage_error">
+    <img src="https://cdn.jsdelivr.net/gh/stdlib-js/stdlib@d4867162fd6445b10f93ca01f3f764bc646662d8/lib/node_modules/@stdlib/stats/incr/nanmape/docs/img/equation_mean_absolute_percentage_error.svg" alt="Equation for the mean absolute percentage error.">
+    <br>
+</div> -->
+
+<!-- </equation> -->
+
+where `f_i` is the forecast value and `a_i` is the actual value.
+
+</section>
+
+<!-- /.intro -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var incrnanmape = require( '@stdlib/stats/incr/nanmape' );
+```
+
+#### incrnanmape()
+
+Returns an accumulator `function` which incrementally computes the [mean absolute percentage error][mean-absolute-percentage-error], ignoring `NaN` values.
+
+```javascript
+var accumulator = incrnanmape();
+```
+
+#### accumulator( \[f, a] )
+
+If provided input values `f` and `a`, the accumulator function returns an updated [mean absolute percentage error][mean-absolute-percentage-error]. If not provided input values `f` and `a`, the accumulator function returns the current [mean absolute percentage error][mean-absolute-percentage-error].
+
+```javascript
+var accumulator = incrnanmape();
+
+var m = accumulator( 2.0, 3.0 );
+// returns ~33.33
+
+m = accumulator( 1.0, 4.0 );
+// returns ~54.17
+
+m = accumulator( NaN, 4.0 );
+// returns ~54.17
+
+m = accumulator( 3.0, 5.0 );
+// returns ~49.44
+
+m = accumulator( 3.0, NaN );
+// returns ~49.44
+
+m = accumulator( NaN, NaN );
+// returns ~49.44
+
+m = accumulator();
+// returns ~49.44
+```
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   Input values are **not** type checked. If non-numeric inputs are possible, you are advised to type check and handle accordingly **before** passing the value to the accumulator function.
+
+-   **Warning**: the [mean absolute percentage error][mean-absolute-percentage-error]  has several shortcomings:
+
+    -   The measure is **not** suitable for intermittent demand patterns (i.e., when `a_i` is `0`).
+    -   The [mean absolute percentage error][mean-absolute-percentage-error] is not symmetrical, as the measure cannot exceed 100% for forecasts which are too "low" and has no limit for forecasts which are too "high".
+    -   When used to compare the accuracy of forecast models (e.g., predicting demand), the measure is biased toward forecasts which are too low.
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var randu = require( '@stdlib/random/base/randu' );
+var incrnanmape = require( '@stdlib/stats/incr/nanmape' );
+
+var accumulator;
+var v1;
+var v2;
+var i;
+
+// Initialize an accumulator:
+accumulator = incrnanmape();
+
+// For each simulated datum, update the mean absolute percentage error...
+for ( i = 0; i < 100; i++ ) {
+    v1 = ( randu() < 0.2 ) ? NaN : randu() * 100.0;
+    v2 = ( randu() < 0.2 ) ? NaN : randu() * 100.0;
+    accumulator( v1, v2 );
+}
+console.log( '\nFinal MAPE: %d\n', 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">
+
+[mean-absolute-percentage-error]: https://en.wikipedia.org/wiki/Mean_absolute_percentage_error
+
+</section>
+
+<!-- /.links -->

lib/node_modules/@stdlib/stats/incr/nanmape/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 isnan = require( '@stdlib/math/base/assert/is-nan' );
+var format = require( '@stdlib/string/format' );
+var pkg = require( './../package.json' ).name;
+var incrnanmape = require( './../lib' );
+
+
+// MAIN //
+
+bench( pkg, function benchmark( b ) {
+	var f;
+	var i;
+	b.tic();
+	for ( i = 0; i < b.iterations; i++ ) {
+		f = incrnanmape();
+		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 = incrnanmape();
+
+	b.tic();
+	for ( i = 0; i < b.iterations; i++ ) {
+		v = acc( i + 0.5, i + 0.5 );
+		if ( isnan( v ) ) {
+			b.fail( 'should not return NaN' );
+		}
+	}
+	b.toc();
+	if ( isnan( v ) ) {
+		b.fail( 'should not return NaN' );
+	}
+	b.pass( 'benchmark finished' );
+	b.end();
+});

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

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+
+{{alias}}()
+    Returns an accumulator function which incrementally computes the mean
+    absolute percentage error (MAPE), ignoring `NaN` values.
+
+    If provided input values, the accumulator function returns an updated mean
+    absolute percentage error. If not provided input values, the accumulator
+    function returns the current mean absolute percentage error.
+
+    Returns
+    -------
+    acc: Function
+        Accumulator function.
+
+    Examples
+    --------
+    > var accumulator = {{alias}}();
+    > var m = accumulator()
+    null
+    > m = accumulator( 2.0, 3.0 )
+    ~33.33
+    > m = accumulator( 5.0, 2.0 )
+    ~91.67
+    > m = accumulator( 5.0, NaN )
+    ~91.67
+    > m = accumulator()
+    ~91.67
+
+    See Also
+    --------
+