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 factorialln = require( '@stdlib/math/base/special/factorialln' );
  var isnan = require( '@stdlib/math/base/assert/is-nan' );
  var ln = require( '@stdlib/math/base/special/ln' );
  var NINF = require( '@stdlib/constants/math/float64-ninf' );
  var PINF = require( '@stdlib/constants/math/float64-pinf' );
  
  
  // MAIN //
  
  /**
  * Evaluates the natural logarithm of the probability mass function (PMF) for a Poisson distribution with mean parameter `lambda` at a value `x`.
  *
  * @param {number} x - input value
  * @param {NonNegativeNumber} lambda - mean parameter
  * @returns {number} evaluated logPMF
  *
  * @example
  * var y = logpmf( 4.0, 3.0 );
  * // returns ~-1.784
  *
  * @example
  * var y = logpmf( 1.0, 3.0 );
  * // returns ~-1.901
  *
  * @example
  * var y = logpmf( -1.0, 2.0 );
  * // returns -Infinity
  *
  * @example
  * var y = logpmf( 0.0, NaN );
  * // returns NaN
  *
  * @example
  * var y = logpmf( NaN, 0.5 );
  * // returns NaN
  *
  * @example
  * // Invalid mean parameter:
  * var y = logpmf( 2.0, -0.5 );
  * // returns NaN
  */
  function logpmf( x, lambda ) {
  	if ( isnan( x ) || isnan( lambda ) || lambda < 0.0 ) {
  		return NaN;
  	}
  	if ( lambda === 0.0 ) {
  		return ( x === 0.0 ) ? 0.0 : NINF;
  	}
  	if ( isNonNegativeInteger( x ) && x !== PINF ) {
  		return ( x * ln( lambda ) ) - lambda - factorialln( x );
  	}
  	return NINF;
  }
  
  
  // EXPORTS //
  
  module.exports = logpmf;
  