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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 isnan = require( '@stdlib/math/base/assert/is-nan' );
var log1p = require( '@stdlib/math/base/special/log1p' );
var pow = require( '@stdlib/math/base/special/pow' );
var ln = require( '@stdlib/math/base/special/ln' );
var LNPI = require( '@stdlib/constants/float64/ln-pi' );
// MAIN //
/**
* Returns a function for evaluating the natural logarithm of the probability density function (logPDF) for a Cauchy distribution with location parameter `x0` and scale parameter `gamma`.
*
* @param {number} x0 - location parameter
* @param {PositiveNumber} gamma - scale parameter
* @returns {Function} logPDF
*
* @example
* var logpdf = factory( 4.0, 2.0 );
*
* var y = logpdf( 10.0 );
* // returns ~-4.14
*
* y = logpdf( 3.0 );
* // returns ~-2.061
*/
function factory( x0, gamma ) {
if (
isnan( gamma ) ||
isnan( x0 ) ||
gamma <= 0.0
) {
return constantFunction( NaN );
}
return logpdf;
/**
* Evaluates the natural logarithm of the probability density function (logPDF) for a Cauchy distribution.
*
* @private
* @param {number} x - input value
* @returns {number} evaluated logPDF
*
* @example
* var y = logpdf( 2.3 );
* // returns <number>
*/
function logpdf( x ) {
if ( isnan( x ) ) {
return NaN;
}
return -( LNPI + ln( gamma ) + log1p( pow( (x-x0)/gamma, 2.0 ) ) );
}
}
// EXPORTS //
module.exports = factory;
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