time-to-botec

Benchmark sampling in different programming languages
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pdf.js (2162B)

      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 isnan = require( '@stdlib/math/base/assert/is-nan' );
     24 var pow = require( '@stdlib/math/base/special/pow' );
     25 var exp = require( '@stdlib/math/base/special/exp' );
     26 var PINF = require( '@stdlib/constants/float64/pinf' );
     27 var NINF = require( '@stdlib/constants/float64/ninf' );
     28 
     29 
     30 // MAIN //
     31 
     32 /**
     33 * Evaluates the probability density function (PDF) for a Weibull distribution with shape parameter `k` and scale parameter `lambda` at a value `x`.
     34 *
     35 * @param {number} x - input value
     36 * @param {PositiveNumber} k - shape parameter
     37 * @param {PositiveNumber} lambda - scale parameter
     38 * @returns {number} evaluated probability density function
     39 *
     40 * @example
     41 * var y = pdf( 2.0, 1.0, 0.5 );
     42 * // returns ~0.037
     43 *
     44 * @example
     45 * var y = pdf( 0.1, 1.0, 1.0 );
     46 * // returns ~0.905
     47 *
     48 * @example
     49 * var y = pdf( -1.0, 4.0, 2.0 );
     50 * // returns 0.0
     51 *
     52 * @example
     53 * var y = pdf( NaN, 0.6, 1.0 );
     54 * // returns NaN
     55 *
     56 * @example
     57 * var y = pdf( 0.0, NaN, 1.0 );
     58 * // returns NaN
     59 *
     60 * @example
     61 * var y = pdf( 0.0, 0.0, NaN );
     62 * // returns NaN
     63 *
     64 * @example
     65 * var y = pdf( 2.0, 0.0, -1.0 );
     66 * // returns NaN
     67 */
     68 function pdf( x, k, lambda ) {
     69 	var xol;
     70 	var z;
     71 	if (
     72 		isnan( k ) ||
     73 		isnan( lambda ) ||
     74 		k <= 0.0 ||
     75 		lambda <= 0.0
     76 	) {
     77 		return NaN;
     78 	}
     79 	if ( x < 0.0 ) {
     80 		return 0.0;
     81 	}
     82 	if ( x === PINF || x === NINF ) {
     83 		return 0.0;
     84 	}
     85 	if ( x === 0.0 ) {
     86 		return ( k === 1.0 ) ? k/lambda : 0.0;
     87 	}
     88 	xol = x / lambda;
     89 	z = pow( xol, k - 1.0 );
     90 	return ( k / lambda ) * z * exp( -pow( xol, k ) );
     91 }
     92 
     93 
     94 // EXPORTS //
     95 
     96 module.exports = pdf;