function logpdf( x, sigma ) {
	var s2i;
	var s2;
	if (
		isnan( x ) ||
		isnan( sigma ) ||
		sigma < 0.0
	) {
		return NaN;
	}
	if ( sigma === 0.0 ) {
		return ( x === 0.0 ) ? PINF : NINF;
	}
	if ( x < 0.0 || x === PINF ) {
		return NINF;
	}
	s2 = pow( sigma, 2.0 );
	s2i = 1.0 / s2;
	return ln( s2i * x ) - ( pow( x, 2.0 ) / ( 2.0 * s2 ) );
}
Evaluates the logarithm of the probability density function (PDF) for a Rayleigh distribution with scale parameter `sigma` at a value `x`.

@param {number} x - input value
@param {NonNegativeNumber} sigma - scale parameter
@returns {number} evaluated logPDF

@example
var y = logpdf( 0.3, 1.0 );
// returns ~-1.249

@example
var y = logpdf( 2.0, 0.8 );
// returns ~-1.986

@example
var y = logpdf( -1.0, 0.5 );
// returns -Infinity

@example
var y = logpdf( 0.0, NaN );
// returns NaN

@example
var y = logpdf( NaN, 2.0 );
// returns NaN

@example
// Negative scale parameter:
var y = logpdf( 2.0, -1.0 );
// returns NaN