factory.js (2054B)
1 /** 2 * @license Apache-2.0 3 * 4 * Copyright (c) 2018 The Stdlib Authors. 5 * 6 * Licensed under the Apache License, Version 2.0 (the "License"); 7 * you may not use this file except in compliance with the License. 8 * You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 */ 18 19 'use strict'; 20 21 // MODULES // 22 23 var isNonNegativeInteger = require( '@stdlib/math/base/assert/is-nonnegative-integer' ); 24 var constantFunction = require( '@stdlib/utils/constant-function' ); 25 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 26 var ibetaDerivative = require( './ibeta_derivative.js' ); 27 28 29 // MAIN // 30 31 /** 32 * Returns a function for evaluating the probability mass function (PMF) for a negative binomial distribution with number of successes until experiment is stopped `r` and success probability `p`. 33 * 34 * @param {PositiveNumber} r - number of successes until experiment is stopped 35 * @param {Probability} p - success probability 36 * @returns {Function} PMF 37 * 38 * @example 39 * var pmf = factory( 10, 0.5 ); 40 * var y = pmf( 3.0 ); 41 * // returns ~0.027 42 * 43 * y = pmf( 5.0 ); 44 * // returns ~0.061 45 */ 46 function factory( r, p ) { 47 if ( 48 isnan( r ) || 49 isnan( p ) || 50 r <= 0.0 || 51 p <= 0.0 || 52 p > 1.0 53 ) { 54 return constantFunction( NaN ); 55 } 56 return pmf; 57 58 /** 59 * Evaluates the probability mass function (PMF) for a negative binomial distribution. 60 * 61 * @private 62 * @param {number} x - input value 63 * @returns {Probability} evaluated PMF 64 * 65 * @example 66 * var y = pmf( 2.0 ); 67 * // returns <number> 68 */ 69 function pmf( x ) { 70 if ( isnan( x ) ) { 71 return NaN; 72 } 73 if ( !isNonNegativeInteger( x ) ) { 74 return 0.0; 75 } 76 return ( p / ( r + x ) ) * ibetaDerivative( p, r, x + 1.0 ); 77 } 78 } 79 80 81 // EXPORTS // 82 83 module.exports = factory;