@stdlib/stats/base/dists/logistic/logcdf/lib/log1pexp.js

Standard library statistical functions.

/**
* @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.
*
*
* ## Notice
*
* The following copyright and license were part of the original implementation available as part of the [StatsFuns.jl]{@link https://github.com/JuliaStats/StatsFuns.jl/blob/master/src/basicfuns.jl} Julia package. The implementation follows the original, but has been modified for JavaScript.
*
* ```text
* Copyright (c) 2015: Dahua Lin.
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
* ```
*/

'use strict';

// MODULES //

var log1p = require( '@stdlib/math/base/special/log1p' );
var exp = require( '@stdlib/math/base/special/exp' );


// MAIN //

/**
* Compute ln( 1 + exp(x) ) without overflow.
*
* @private
* @param {number} x - input value
* @returns {number} function value
*/
function log1pexp( x ) {
	if ( x <= 18.0 ) {
		return log1p( exp(x) );
	}
	if ( x > 33.3 ) {
		return x;
	}
	// Case: 18.0 < x <= 33.3
	return x + exp( -x );
}


// EXPORTS //

module.exports = log1pexp;