factory.js (2216B)
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 factorialln = require( '@stdlib/math/base/special/factorialln' ); 26 var degenerate = require( './../../../../../base/dists/degenerate/pmf' ).factory; 27 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 28 var exp = require( '@stdlib/math/base/special/exp' ); 29 var ln = require( '@stdlib/math/base/special/ln' ); 30 var PINF = require( '@stdlib/constants/float64/pinf' ); 31 32 33 // MAIN // 34 35 /** 36 * Returns a function for evaluating the probability mass function (PMF) for a Poisson distribution with mean parameter `lambda`. 37 * 38 * @param {NonNegativeNumber} lambda - mean parameter 39 * @returns {Function} PMF 40 * 41 * @example 42 * var pmf = factory( 1.0 ); 43 * var y = pmf( 3.0 ); 44 * // returns ~0.061 45 * 46 * y = pmf( 1.0 ); 47 * // returns ~0.368 48 */ 49 function factory( lambda ) { 50 if ( isnan( lambda ) || lambda < 0.0 ) { 51 return constantFunction( NaN ); 52 } 53 if ( lambda === 0.0 ) { 54 return degenerate( 0.0 ); 55 } 56 return pmf; 57 58 /** 59 * Evaluates the probability mass function (PMF) for a Poisson 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 var lnl; 71 if ( isnan( x ) ) { 72 return NaN; 73 } 74 if ( isNonNegativeInteger( x ) && x !== PINF ) { 75 lnl = (x * ln( lambda )) - lambda - factorialln( x ); 76 return exp( lnl ); 77 } 78 return 0.0; 79 } 80 } 81 82 83 // EXPORTS // 84 85 module.exports = factory;