text: /**
  * @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 isnan = require( '@stdlib/math/base/assert/is-nan' );
  var pow = require( '@stdlib/math/base/special/pow' );
  var exp = require( '@stdlib/math/base/special/exp' );
  var PINF = require( '@stdlib/constants/math/float64-pinf' );
  var NINF = require( '@stdlib/constants/math/float64-ninf' );
  
  
  // MAIN //
  
  /**
  * Evaluates the probability density function (PDF) for a Weibull distribution with shape parameter `k` and scale parameter `lambda` at a value `x`.
  *
  * @param {number} x - input value
  * @param {PositiveNumber} k - shape parameter
  * @param {PositiveNumber} lambda - scale parameter
  * @returns {number} evaluated probability density function
  *
  * @example
  * var y = pdf( 2.0, 1.0, 0.5 );
  * // returns ~0.037
  *
  * @example
  * var y = pdf( 0.1, 1.0, 1.0 );
  * // returns ~0.905
  *
  * @example
  * var y = pdf( -1.0, 4.0, 2.0 );
  * // returns 0.0
  *
  * @example
  * var y = pdf( NaN, 0.6, 1.0 );
  * // returns NaN
  *
  * @example
  * var y = pdf( 0.0, NaN, 1.0 );
  * // returns NaN
  *
  * @example
  * var y = pdf( 0.0, 0.0, NaN );
  * // returns NaN
  *
  * @example
  * var y = pdf( 2.0, 0.0, -1.0 );
  * // returns NaN
  */
  function pdf( x, k, lambda ) {
  	var xol;
  	var z;
  	if (
  		isnan( k ) ||
  		isnan( lambda ) ||
  		k <= 0.0 ||
  		lambda <= 0.0
  	) {
  		return NaN;
  	}
  	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 = pdf;
  