@stdlib/stats/base/dists/geometric/entropy/lib/main.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.
*/

'use strict';

// MODULES //

var isnan = require( '@stdlib/math/base/assert/is-nan' );
var ln = require( '@stdlib/math/base/special/ln' );


// MAIN //

/**
* Returns the entropy of a geometric distribution.
*
* @param {Probability} p - success probability
* @returns {PositiveNumber} entropy
*
* @example
* var v = entropy( 0.1 );
* // returns ~3.251
*
* @example
* var v = entropy( 0.5 );
* // returns ~1.386
*
* @example
* var v = entropy( 0.9 );
* // returns ~0.361
*
* @example
* var v = entropy( 1.1 );
* // returns NaN
*
* @example
* var v = entropy( NaN );
* // returns NaN
*/
function entropy( p ) {
	var q;
	if (
		isnan( p ) ||
		p <= 0.0 ||
		p >= 1.0
	) {
		return NaN;
	}
	q = 1.0 - p;
	return ( -( p*ln(p) ) - ( q*ln(q) ) ) / p;
}


// EXPORTS //

module.exports = entropy;