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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 isnan = require( '@stdlib/math/base/assert/is-nan' );
var isInteger = require( '@stdlib/math/base/assert/is-integer' );
var gamma = require( '@stdlib/math/base/special/gamma' );
var PINF = require( '@stdlib/constants/float64/pinf' );
var FLOAT64_MAX_NTH_FACTORIAL = require( '@stdlib/constants/float64/max-nth-factorial' );
var FACTORIALS = require( './factorials.json' );
// MAIN //
/**
* Evaluates the factorial of `x`.
*
* @param {number} x - input value
* @returns {number} factorial
*
* @example
* var v = factorial( 3.0 );
* // returns 6.0
*
* @example
* var v = factorial( -1.5 );
* // returns ~-3.545
*
* @example
* var v = factorial( -0.5 );
* // returns ~1.772
*
* @example
* var v = factorial( 0.5 );
* // returns ~0.886
*
* @example
* var v = factorial( -10.0 );
* // returns NaN
*
* @example
* var v = factorial( 171.0 );
* // returns Infinity
*
* @example
* var v = factorial( NaN );
* // returns NaN
*/
function factorial( x ) {
if ( isnan( x ) ) {
return NaN;
}
if ( isInteger( x ) ) {
if ( x < 0 ) {
return NaN;
}
if ( x <= FLOAT64_MAX_NTH_FACTORIAL ) {
return FACTORIALS[ x ];
}
return PINF;
}
return gamma( x + 1.0 );
}
// EXPORTS //
module.exports = factorial;
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