logpmf.js (1970B)
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 ln = require( '@stdlib/math/base/special/ln' ); 26 var NINF = require( '@stdlib/constants/float64/ninf' ); 27 28 29 // MAIN // 30 31 /** 32 * Evaluates the natural logarithm of the probability mass function (PMF) for a discrete uniform distribution with minimum support `a` and maximum support `b` at a value `x`. 33 * 34 * @param {number} x - input value 35 * @param {integer} a - minimum support 36 * @param {integer} b - maximum support 37 * @returns {number} evaluated logPMF 38 * 39 * @example 40 * var y = logpmf( 2.0, 0, 4 ); 41 * // returns ~-1.609 42 * 43 * @example 44 * var y = logpmf( 5.0, 0, 4 ); 45 * // returns -Infinity 46 * 47 * @example 48 * var y = logpmf( 2, 0, 8 ); 49 * // returns ~-2.197 50 * 51 * @example 52 * var y = logpmf( NaN, 0.0, 1.0 ); 53 * // returns NaN 54 * 55 * @example 56 * var y = logpmf( 0.0, NaN, 1.0 ); 57 * // returns NaN 58 * 59 * @example 60 * var y = logpmf( 0.0, 0.0, NaN ); 61 * // returns NaN 62 * 63 * @example 64 * var y = logpmf( 2.0, 3.0, 1.0 ); 65 * // returns NaN 66 */ 67 function logpmf( x, a, b ) { 68 if ( 69 isnan( x ) || 70 isnan( a ) || 71 isnan( b ) || 72 !isInteger( a ) || 73 !isInteger( b ) || 74 a > b 75 ) { 76 return NaN; 77 } 78 if ( x < a || x > b || !isInteger( x ) ) { 79 return NINF; 80 } 81 return -ln( b - a + 1.0 ); 82 } 83 84 85 // EXPORTS // 86 87 module.exports = logpmf;