function logcdf( x, sigma ) {
	var s2;
	var p;
	if (
		isnan( x ) ||
		isnan( sigma ) ||
		sigma < 0.0
	) {
		return NaN;
	}
	if ( sigma === 0.0 ) {
		return ( x < 0.0 ) ? NINF : 0.0;
	}
	if ( x < 0.0 ) {
		return NINF;
	}
	s2 = pow( sigma, 2.0 );
	p = -pow( x, 2.0 ) / ( 2.0 * s2 );
	return ( p < LNHALF ) ? log1p( -exp( p ) ) : ln( -expm1( p ) );
}
Evaluates the logarithm of the cumulative distribution function (CDF) 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 logCDF

@example
var y = logcdf( 2.0, 3.0 );
// returns ~-1.613

@example
var y = logcdf( 1.0, 2.0 );
// returns ~-2.141

@example
var y = logcdf( -1.0, 4.0 );
// returns -Infinity

@example
var y = logcdf( NaN, 1.0 );
// returns NaN

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

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