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 isNonNegativeInteger = require( '@stdlib/math/base/assert/is-nonnegative-integer' );
  var constantFunction = require( '@stdlib/utils/constant-function' );
  var factorialln = require( '@stdlib/math/base/special/factorialln' );
  var degenerate = require( '@stdlib/stats/base/dists/degenerate/pmf' ).factory;
  var isnan = require( '@stdlib/math/base/assert/is-nan' );
  var exp = require( '@stdlib/math/base/special/exp' );
  var ln = require( '@stdlib/math/base/special/ln' );
  var PINF = require( '@stdlib/constants/float64/pinf' );
  
  
  // MAIN //
  
  /**
  * Returns a function for evaluating the probability mass function (PMF) for a Poisson distribution with mean parameter `lambda`.
  *
  * @param {NonNegativeNumber} lambda - mean parameter
  * @returns {Function} PMF
  *
  * @example
  * var pmf = factory( 1.0 );
  * var y = pmf( 3.0 );
  * // returns ~0.061
  *
  * y = pmf( 1.0 );
  * // returns ~0.368
  */
  function factory( lambda ) {
  	if ( isnan( lambda ) || lambda < 0.0 ) {
  		return constantFunction( NaN );
  	}
  	if ( lambda === 0.0 ) {
  		return degenerate( 0.0 );
  	}
  	return pmf;
  
  	/**
  	* Evaluates the probability mass function (PMF) for a Poisson distribution.
  	*
  	* @private
  	* @param {number} x - input value
  	* @returns {Probability} evaluated PMF
  	*
  	* @example
  	* var y = pmf( 2.0 );
  	* // returns <number>
  	*/
  	function pmf( x ) {
  		var lnl;
  		if ( isnan( x ) ) {
  			return NaN;
  		}
  		if ( isNonNegativeInteger( x ) && x !== PINF ) {
  			lnl = (x * ln( lambda )) - lambda - factorialln( x );
  			return exp( lnl );
  		}
  		return 0.0;
  	}
  }
  
  
  // EXPORTS //
  
  module.exports = factory;
  