function gamma( x ) {
	var w;
	var y;
	var v;

	w = 1.0 / x;
	w = 1.0 + ( w * polyval( w ) );
	y = exp( x );

	// Check `x` to avoid `pow()` overflow...
	if ( x > MAX_STIRLING ) {
		v = pow( x, ( 0.5*x ) - 0.25 );
		y = v * (v/y);
	} else {
		y = pow( x, x-0.5 ) / y;
	}
	return SQRT_TWO_PI * y * w;
}
Evaluates the gamma function using Stirling's formula. The polynomial is valid for \\(33 \leq x \leq 172\\).

@private
@param {number} x - input value
@returns {number} function value