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* @license Apache-2.0
*
* Copyright (c) 2026 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 isnan = require( '@stdlib/math/base/assert/is-nan' );
var ln = require( '@stdlib/math/base/special/ln' );
var log1p = require( '@stdlib/math/base/special/log1p' );
var NINF = require( '@stdlib/constants/float64/ninf' );
// MAIN //
/**
* Evaluates the logarithm of the probability density function (PDF) for a Bradford distribution with shape parameter `c` at a value `x`.
*
* @param {number} x - input value
* @param {PositiveNumber} c - shape parameter
* @returns {number} evaluated logPDF
*
* @example
* var y = logpdf( 0.1, 0.1 );
* // returns ~0.038
*
* @example
* var y = logpdf( 0.5, 5.0 );
* // returns ~-0.227
*
* @example
* var y = logpdf( 1.0, 10.0 );
* // returns ~-0.970
*
* @example
* var y = logpdf( 2.0, 0.5 );
* // returns -Infinity
*
* @example
* var y = logpdf( -1.0, 0.5 );
* // returns -Infinity
*
* @example
* var y = logpdf( 0.5, 0.0 );
* // returns NaN
*
* @example
* var y = logpdf( 0.5, -5.0 );
* // returns NaN
*
* @example
* var y = logpdf( NaN, 1.0 );
* // returns NaN
*
* @example
* var y = logpdf( 1.0, NaN );
* // returns NaN
*/
function logpdf( x, c ) {
var k;
if ( isnan( c ) || isnan( x ) || c <= 0.0 ) {
return NaN;
}
if ( x < 0.0 || x > 1.0 ) {
return NINF;
}
k = log1p( c );
return ln( c ) - ln( 1.0 + ( c*x ) ) - ln( k );
}
// EXPORTS //
module.exports = logpdf;
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