@stdlib/math-base-special-factorialln/lib/main.js

Evaluate the natural logarithm of the factorial function.

/**
* @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 isNegativeInteger = require( '@stdlib/math-base-assert-is-negative-integer' );
var gammaln = require( '@stdlib/math-base-special-gammaln' );


// MAIN //

/**
* Evaluates the natural logarithm of the factorial of `x`.
*
* @param {number} x - input value
* @returns {number} natural logarithm of factorial of `x`
*
* @example
* var v = factorialln( 3.0 );
* // returns ~1.792
*
* @example
* var v = factorialln( 2.4 );
* // returns ~1.092
*
* @example
* var v = factorialln( -1.0 );
* // returns NaN
*
* @example
* var v = factorialln( -1.5 );
* // returns ~1.266
*
* @example
* var v = factorialln( NaN );
* // returns NaN
*/
function factorialln( x ) {
	if ( isNegativeInteger( x ) ) {
		return NaN;
	}
	return gammaln( x + 1.0 );
}


// EXPORTS //

module.exports = factorialln;