factory.js (2066B)
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 degenerate = require( './../../../../../base/dists/degenerate/cdf' ).factory; 25 var gammainc = require( '@stdlib/math/base/special/gammainc' ); 26 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 27 var floor = require( '@stdlib/math/base/special/floor' ); 28 var PINF = require( '@stdlib/constants/float64/pinf' ); 29 30 31 // MAIN // 32 33 /** 34 * Returns a function for evaluating the cumulative distribution function (CDF) for a Poisson distribution with mean parameter `lambda`. 35 * 36 * @param {NonNegativeNumber} lambda - mean parameter 37 * @returns {Function} CDF 38 * 39 * @example 40 * var cdf = factory( 5.0 ); 41 * var y = cdf( 3.0 ); 42 * // returns ~0.265 43 * 44 * y = cdf( 8.0 ); 45 * // returns ~0.932 46 */ 47 function factory( lambda ) { 48 if ( isnan( lambda ) || lambda < 0.0 ) { 49 return constantFunction( NaN ); 50 } 51 if ( lambda === 0.0 ) { 52 return degenerate( 0.0 ); 53 } 54 return cdf; 55 56 /** 57 * Evaluates the cumulative distribution function (CDF) for a Poisson distribution. 58 * 59 * @private 60 * @param {number} x - input value 61 * @returns {Probability} evaluated CDF 62 * 63 * @example 64 * var y = cdf( 2.0 ); 65 * // returns <number> 66 */ 67 function cdf( x ) { 68 if ( isnan( x ) ) { 69 return NaN; 70 } 71 if ( x < 0.0 ) { 72 return 0.0; 73 } 74 if ( x === PINF ) { 75 return 1.0; 76 } 77 return gammainc( lambda, floor( x ) + 1.0, true, true ); 78 } 79 } 80 81 82 // EXPORTS // 83 84 module.exports = factory;