feat: add stats/incr/nanmpe

lib/node_modules/@stdlib/stats/incr/nanmpe/README.md

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+<!--
+
+@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.
+
+-->
+
+# incrnanmpe
+
+> Compute the [mean percentage error][mean-percentage-error] (MPE) incrementally, ignoring `NaN` values.
+
+<section class="intro">
+
+The [mean percentage error][mean-percentage-error] is defined as
+
+<!-- <equation class="equation" label="eq:mean_percentage_error" align="center" raw="\operatorname{MPE}  = \frac{100}{n} \sum_{i=0}^{n-1} \frac{a_i - f_i}{a_i}" alt="Equation for the mean percentage error."> -->
+
+```math
+\mathop{\mathrm{MPE}}  = \frac{100}{n} \sum_{i=0}^{n-1} \frac{a_i - f_i}{a_i}
+```
+
+<!-- <div class="equation" align="center" data-raw-text="\operatorname{MPE}  = \frac{100}{n} \sum_{i=0}^{n-1} \frac{a_i - f_i}{a_i}" data-equation="eq:mean_percentage_error">
+    <img src="https://cdn.jsdelivr.net/gh/stdlib-js/stdlib@2acedf866c9a4f1353af22f95780535612c5ee06/lib/node_modules/@stdlib/stats/incr/nanmpe/docs/img/equation_mean_percentage_error.svg" alt="Equation for the mean 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 incrnanmpe = require( '@stdlib/stats/incr/nanmpe' );
+```
+
+#### incrnanmpe()
+
+Returns an accumulator `function` which incrementally computes the [mean percentage error][mean-percentage-error], ignoring `NaN` values.
+
+```javascript
+var accumulator = incrnanmpe();
+```
+
+#### accumulator( \[f, a] )
+
+If provided input values `f` and `a`, the accumulator function returns an updated [mean percentage error][mean-percentage-error]. If not provided input values `f` and `a`, the accumulator function returns the current [mean percentage error][mean-percentage-error].
+
+```javascript
+var accumulator = incrnanmpe();
+
+var m = accumulator( 2.0, 3.0 );
+// returns ~33.33
+
+m = accumulator( 2.0, NaN );
+// returns ~33.33
+
+m = accumulator( 1.0, 4.0 );
+// returns ~54.17
+
+m = accumulator( 3.0, 5.0 );
+// 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.
+-   Be careful when interpreting the [mean percentage error][mean-percentage-error] as errors can cancel. This stated, that errors can cancel makes the [mean percentage error][mean-percentage-error] suitable for measuring the bias in forecasts.
+-   **Warning**: the [mean percentage error][mean-percentage-error] is **not** suitable for intermittent demand patterns (i.e., when `a_i` is `0`). Interpretation is most straightforward when actual and forecast values are positive valued (e.g., number of widgets sold).
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var bernoulli = require( '@stdlib/random/base/bernoulli' );
+var uniform = require( '@stdlib/random/base/uniform' );
+var incrnanmpe = require( '@stdlib/stats/incr/nanmpe' );
+
+// Initialize an accumulator:
+var accumulator = incrnanmpe();
+
+// For each simulated datum, update the mean percentage error...
+var v1;
+var v2;
+var i;
+for ( i = 0; i < 100; i++ ) {
+    v1 = ( bernoulli( 0.8 ) < 1 ) ? NaN : uniform( -50.0, 50.0 );
+    v2 = ( bernoulli( 0.8 ) < 1 ) ? NaN : uniform( -50.0, 50.0 );
+    accumulator( v1, v2 );
+}
+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">
+
+[mean-percentage-error]: https://en.wikipedia.org/wiki/Mean_percentage_error
+
+</section>
+
+<!-- /.links -->

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

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+/**
+* @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.
+*/
+
+'use strict';
+
+// MODULES //
+
+var bench = require( '@stdlib/bench' );
+var randu = require( '@stdlib/random/base/randu' );
+var pkg = require( './../package.json' ).name;
+var incrnanmpe = require( './../lib' );
+
+
+// MAIN //
+
+bench( pkg, function benchmark( b ) {
+	var f;
+	var i;
+	b.tic();
+	for ( i = 0; i < b.iterations; i++ ) {
+		f = incrnanmpe();
+		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( pkg+'::accumulator', function benchmark( b ) {
+	var acc;
+	var v;
+	var i;
+
+	acc = incrnanmpe();
+
+	b.tic();
+	for ( i = 0; i < b.iterations; i++ ) {
+		v = acc( randu()+0.5, randu()+0.5 );
+		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/nanmpe/docs/repl.txt

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+
+{{alias}}()
+    Returns an accumulator function which incrementally computes the mean
+    percentage error (MPE), ignoring `NaN` values.
+
+    If provided input values, the accumulator function returns an updated mean
+    percentage error. If not provided input values, the accumulator function
+    returns the current mean 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( 2.0, NaN )
+    ~33.33
+    > m = accumulator( 5.0, 2.0 )
+    ~-58.33
+    > m = accumulator()
+    ~-58.33
+
+    See Also
+    --------

lib/node_modules/@stdlib/stats/incr/nanmpe/docs/types/index.d.ts

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+/*
+* @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.
+*/
+
+// TypeScript Version: 4.1
+
+/// <reference types="@stdlib/types"/>
+
+/**
+* If provided input values, the accumulator function returns an updated mean percentage error. If not provided input values, the accumulator function returns the current mean percentage error.
+*
+* @param f - input value
+* @param a - input value
+* @returns mean percentage error or null
+*/
+type accumulator = ( f?: number, a?: number ) => number | null;
+
+/**
+* Returns an accumulator function which incrementally computes the mean percentage error, ignoring `NaN` values.
+*
+* @returns accumulator function
+*
+* @example
+* var accumulator = incrnanmpe();
+*
+* var m = accumulator();
+* // returns null
+*
+* m = accumulator( 2.0, 3.0 );
+* // returns ~33.33
+*
+* m = accumulator( 2.0, NaN );
+* // returns ~33.33
+*
+* m = accumulator( 5.0, 2.0 );
+* // returns ~-58.33
+*
+* m = accumulator();
+* // returns ~-58.33
+*/
+declare function incrnanmpe(): accumulator;
+
+
+// EXPORTS //
+
+export = incrnanmpe;