factory.js (2145B)
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 sqrt = require( '@stdlib/math/base/special/sqrt' ); 26 27 28 // MAIN // 29 30 /** 31 * Returns a function for evaluating the quantile function for a triangular distribution with lower limit `a`, upper limit `b` and mode `c`. 32 * 33 * @param {number} a - lower limit 34 * @param {number} b - upper limit 35 * @param {number} c - mode 36 * @returns {Function} quantile function 37 * 38 * @example 39 * var quantile = factory( 2.0, 4.0, 2.5 ); 40 * var y = quantile( 0.4 ); 41 * // returns ~2.658 42 * 43 * y = quantile( 0.8 ); 44 * // returns ~3.225 45 */ 46 function factory( a, b, c ) { 47 var pInflection; 48 var fact1; 49 var fact2; 50 51 if ( 52 isnan( a ) || 53 isnan( b ) || 54 isnan( c ) || 55 a > c || 56 c > b 57 ) { 58 return constantFunction( NaN ); 59 } 60 61 pInflection = ( c - a ) / ( b - a ); 62 fact1 = ( b - a ) * ( c - a); 63 fact2 = ( b - a ) * ( b - c ); 64 return quantile; 65 66 /** 67 * Evaluates the quantile function for a triangular distribution. 68 * 69 * @private 70 * @param {Probability} p - input value 71 * @returns {number} evaluated quantile function 72 * 73 * @example 74 * var y = quantile( 0.3 ); 75 * // returns <number> 76 */ 77 function quantile( p ) { 78 if ( isnan( p ) || p < 0.0 || p > 1.0 ) { 79 return NaN; 80 } 81 if ( p < pInflection ) { 82 return a + sqrt( fact1 * p ); 83 } 84 if ( p > pInflection ) { 85 return b - sqrt( fact2 * ( 1.0 - p ) ); 86 } 87 // Case: p = pInflection 88 return c; 89 } 90 } 91 92 93 // EXPORTS // 94 95 module.exports = factory;