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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.
*
*
* ## Notice
*
* The original C++ code and copyright notice are from the [Boost library]{@link https://www.boost.org/doc/libs/1_83_0/boost/math/special_functions/log1p.hpp}. The implementation has been modified for JavaScript.
*
* ```text
* (C) Copyright John Maddock 2005-2006.
*
* Use, modification and distribution are subject to the
* Boost Software License, Version 1.0. (See accompanying file
* LICENSE or copy at http://www.boost.org/LICENSE_1_0.txt)
* ```
*/
'use strict';
// MODULES //
var absf = require( '@stdlib/math/base/special/absf' );
var lnf = require( '@stdlib/math/base/special/lnf' );
var FLOAT_EPS = require( '@stdlib/constants/float32/eps' );
var sumSeries = require( '@stdlib/math/base/tools/sum-series' );
var f32 = require( '@stdlib/number/float64/base/to-float32' );
var log1pfSeries = require( './log1pf_series.js' );
// VARIABLES //
var ONE = f32( 1.0 );
var TWO = f32( 2.0 );
// MAIN //
/**
* Evaluates \\( \operatorname{log1pmxf}(x) = \lnf(1+x) - x \\) for a single-precision floating point number.
*
* @param {number} x - input value
* @returns {number} function value
*
* @example
* var v = log1pmxf( 1.1 );
* // returns ~-0.358
*
* @example
* var v = log1pmxf( -1.1 );
* // returns NaN
*/
function log1pmxf( x ) {
var opts;
var ax;
x = f32( x );
if ( x <= -ONE ) {
return NaN;
}
ax = absf( x );
// Large |x|: safe direct computation
if ( ax > f32( 0.95 ) ) {
return f32( lnf( f32( ONE + x ) ) - x );
}
// Very small x: use first Taylor term
if ( ax < FLOAT_EPS ) {
return f32( -x * x / TWO );
}
opts = {
'initialValue': f32( -x )
};
return f32( sumSeries( log1pfSeries( x ), opts ) );
}
// EXPORTS //
module.exports = log1pmxf;
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