function cdf( x, sigma ) {
	var s2;
	if (
		isnan( x ) ||
		isnan( sigma ) ||
		sigma < 0.0
	) {
		return NaN;
	}
	if ( sigma === 0.0 ) {
		return ( x < 0.0 ) ? 0.0 : 1.0;
	}
	if ( x < 0.0 ) {
		return 0.0;
	}
	s2 = pow( sigma, 2.0 );
	return 1.0 - exp( -pow( x, 2.0 ) / ( 2.0 * s2 ) );
}
Evaluates 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 {Probability} evaluated CDF

@example
var y = cdf( 2.0, 3.0 );
// returns ~0.199

@example
var y = cdf( 1.0, 2.0 );
// returns ~0.118

@example
var y = cdf( -1.0, 4.0 );
// returns 0.0

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

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

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