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/**
* @license Apache-2.0
*
* Copyright (c) 2018 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 incrmvariance = require( '@stdlib/stats/incr/mvariance' );
var isnan = require( '@stdlib/math/base/assert/is-nan' );
 
 
// MAIN //
 
/**
* Returns an accumulator function which incrementally computes a moving unbiased sample variance, ignoring `NaN` values.
*
* @param {PositiveInteger} W - window size
* @param {number} [mean] - mean value
* @throws {TypeError} first argument must be a positive integer
* @throws {TypeError} second argument must be a number
* @returns {Function} accumulator function
*
* @example
* var accumulator = incrnanmvariance( 3 );
*
* var s2 = accumulator();
* // returns null
*
* s2 = accumulator( 2.0 );
* // returns 0.0
*
* s2 = accumulator( -5.0 );
* // returns 24.5
*
* s2 = accumulator( NaN );
* // returns 24.5
*
* s2 = accumulator( 3.0 );
* // returns 19.0
*
* s2 = accumulator( 5.0 );
* // returns 28.0
*
* s2 = accumulator();
* // returns 28.0
*
* @example
* var accumulator = incrnanmvariance( 3, -2.0 );
*/
function incrnanmvariance( W, mean ) {
	var acc;
 
	if ( arguments.length > 1 ) {
		acc = incrmvariance( W, mean );
	} else {
		acc = incrmvariance( W );
	}
	return accumulator;
 
	/**
	* If provided a value, returns an updated unbiased sample variance. If not provided a value or provided a `NaN` value, returns the current unbiased sample variance.
	*
	* @private
	* @param {number} [x] - input value
	* @returns {(number|null)} unbiased sample variance or null
	*/
	function accumulator( x ) {
		if ( arguments.length === 0 || isnan( x ) ) {
			return acc();
		}
		return acc( x );
	}
}
 
 
// EXPORTS //
 
module.exports = incrnanmvariance;