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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 exp = require( '@stdlib/math/base/special/exp' );
var log1p = require( '@stdlib/math/base/special/log1p' );
var isnan = require( '@stdlib/math/base/assert/is-nan' );
// MAIN //
/**
* Computes the inverse of a one-parameter Box-Cox transformation.
*
* @param {number} y - input value
* @param {number} lambda - power parameter
* @returns {number} inverse of the Box-Cox transformation
*
* @example
* var v = boxcoxinv( 1.0, 2.5 );
* // returns ~1.6505
*
* @example
* var v = boxcoxinv( 4.0, 2.5 );
* // returns ~2.6095
*
* @example
* var v = boxcoxinv( 10.0, 2.5 );
* // returns ~3.6812
*
* @example
* var v = boxcoxinv( 2.0, 0.0 );
* // returns ~7.3891
*
* @example
* var v = boxcoxinv( -1.0, 2.5 );
* // returns NaN
*
* @example
* var v = boxcoxinv( 0.0, -1.0 );
* // returns 1.0
*
* @example
* var v = boxcoxinv( 1.0, NaN );
* // returns NaN
*
* @example
* var v = boxcoxinv( NaN, 3.1 );
* // returns NaN
*/
function boxcoxinv( y, lambda ) {
if ( isnan( y ) || isnan( lambda ) ) {
return NaN;
}
if ( lambda === 0.0 ) {
return exp( y );
}
return exp( log1p( lambda*y ) / lambda );
}
// EXPORTS //
module.exports = boxcoxinv;
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