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* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
'use strict';
// MODULES //
var constantFunction = require( '@stdlib/utils/constant-function' );
var pow = require( '@stdlib/math/base/special/pow' );
var exp = require( '@stdlib/math/base/special/exp' );
var isnan = require( '@stdlib/math/base/assert/is-nan' );
var PINF = require( '@stdlib/constants/float64/pinf' );
var NINF = require( '@stdlib/constants/float64/ninf' );
// MAIN //
/**
* Returns a function for evaluating the probability density function (PDF) for a Weibull distribution.
*
* @param {PositiveNumber} k - shape parameter
* @param {PositiveNumber} lambda - scale parameter
* @returns {Function} function to evaluate the probability density function
*
* @example
* var pdf = factory( 7.0, 6.0 );
* var y = pdf( 7.0 );
* // returns ~0.155
*
* y = pdf( 5.0 );
* // returns ~0.296
*/
function factory( k, lambda ) {
if (
isnan( k ) ||
isnan( lambda ) ||
k <= 0.0 ||
lambda <= 0.0
) {
return constantFunction( NaN );
}
return pdf;
/**
* Evaluates the probability density function (PDF) for a Weibull distribution.
*
* @private
* @param {number} x - input value
* @returns {number} evaluated PDF
*
* @example
* var y = pdf( 2.3 );
* // returns <number>
*/
function pdf( x ) {
var xol;
var z;
if ( x < 0.0 ) {
return 0.0;
}
if ( x === PINF || x === NINF ) {
return 0.0;
}
if ( x === 0.0 ) {
return ( k === 1.0 ) ? k / lambda : 0.0;
}
xol = x / lambda;
z = pow( xol, k - 1.0 );
return ( k / lambda ) * z * exp( -pow( xol, k ) );
}
}
// EXPORTS //
module.exports = factory;
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