logcdf.js (2133B)
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 isInteger = require( '@stdlib/math/base/assert/is-integer' ); 24 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 25 var floor = require( '@stdlib/math/base/special/floor' ); 26 var ln = require( '@stdlib/math/base/special/ln' ); 27 var NINF = require( '@stdlib/constants/float64/ninf' ); 28 29 30 // MAIN // 31 32 /** 33 * Evaluates the natural logarithm of the cumulative distribution function (CDF) for a discrete uniform distribution with minimum support `a` and maximum support `b` at a value `x`. 34 * 35 * @param {number} x - input value 36 * @param {integer} a - minimum support 37 * @param {integer} b - maximum support 38 * @returns {number} evaluated logCDF 39 * 40 * @example 41 * var y = logcdf( 9.0, 0, 10 ); 42 * // returns ~-0.095 43 * 44 * @example 45 * var y = logcdf( 0.5, 0, 2 ); 46 * // returns ~-1.099 47 * 48 * @example 49 * var y = logcdf( +Infinity, 2, 4 ); 50 * // returns 0.0 51 * 52 * @example 53 * var y = logcdf( -Infinity, 2, 4 ); 54 * // returns -Infinity 55 * 56 * @example 57 * var y = logcdf( NaN, 0, 1 ); 58 * // returns NaN 59 * 60 * @example 61 * var y = logcdf( 0.0, NaN, 1 ); 62 * // returns NaN 63 * 64 * @example 65 * var y = logcdf( 0.0, 0, NaN ); 66 * // returns NaN 67 * 68 * @example 69 * var y = logcdf( 2.0, 1, 0 ); 70 * // returns NaN 71 */ 72 function logcdf( x, a, b ) { 73 if ( 74 isnan( x ) || 75 isnan( a ) || 76 isnan( b ) || 77 !isInteger( a ) || 78 !isInteger( b ) || 79 a > b 80 ) { 81 return NaN; 82 } 83 if ( x < a ) { 84 return NINF; 85 } 86 if ( x >= b ) { 87 return 0.0; 88 } 89 return ln( floor( x ) - a + 1.0 ) - ln( b - a + 1.0 ); 90 } 91 92 93 // EXPORTS // 94 95 module.exports = logcdf;