factory.js (1608B)
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 constantFunction = require( '@stdlib/utils/constant-function' ); 24 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 25 var exp = require( '@stdlib/math/base/special/exp' ); 26 27 28 // MAIN // 29 30 /** 31 * Returns a function for evaluating the moment-generating function (MGF) of a Poisson distribution with mean parameter `lambda`. 32 * 33 * @param {PositiveNumber} lambda - mean parameter 34 * @returns {Function} MGF 35 * 36 * @example 37 * var mgf = factory( 2.0 ); 38 * var y = mgf( 0.1 ); 39 * // returns ~1.234 40 */ 41 function factory( lambda ) { 42 if ( isnan( lambda ) || lambda <= 0.0 ) { 43 return constantFunction( NaN ); 44 } 45 return mgf; 46 47 /** 48 * Evaluates the moment-generating function (MGF) for a Poisson distribution. 49 * 50 * @private 51 * @param {number} t - input value 52 * @returns {number} evaluated MGF 53 * 54 * @example 55 * var y = mgf( 0.5 ); 56 * // returns <number> 57 */ 58 function mgf( t ) { 59 return exp( lambda * ( exp(t) - 1.0 ) ); 60 } 61 } 62 63 64 // EXPORTS // 65 66 module.exports = factory;