@stdlib/stats/base/dists/geometric/logpmf/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 isNonNegativeInteger = require( '@stdlib/math/base/assert/is-nonnegative-integer' );
var isnan = require( '@stdlib/math/base/assert/is-nan' );
var ln = require( '@stdlib/math/base/special/ln' );
var NINF = require( '@stdlib/constants/float64/ninf' );


// MAIN //

/**
* Evaluates the logarithm of the probability mass function (PMF) for a geometric distribution with success probability `p` at a value `x`.
*
* @param {number} x - input value
* @param {Probability} p - success probability
* @returns {NonPositiveNumber} logarithm of PMF
*
* @example
* var y = logpmf( 4.0, 0.3 );
* // returns ~-2.631
*
* @example
* var y = logpmf( 2.0, 0.7 );
* // returns ~-2.765
*
* @example
* var y = logpmf( -1.0, 0.5 );
* // returns -Infinity
*
* @example
* var y = logpmf( 0.0, NaN );
* // returns NaN
*
* @example
* var y = logpmf( NaN, 0.5 );
* // returns NaN
*
* @example
* // Invalid success probability:
* var y = logpmf( 2.0, 1.5 );
* // returns NaN
*/
function logpmf( x, p ) {
	var q;
	if ( isnan( x ) || isnan( p ) ) {
		return NaN;
	}
	if ( p < 0.0 || p > 1.0 ) {
		return NaN;
	}
	if ( isNonNegativeInteger( x ) ) {
		q = 1.0 - p;
		return ln( p ) + (x * ln( q ));
	}
	return NINF;
}


// EXPORTS //

module.exports = logpmf;