/// <summary>
/// Sample from the distribution tail (defined as having x >= __R).
/// </summary>
/// <returns></returns>
private double SampleTail()
{
double x, y;
do
{
// Note. we use NextDoubleNonZero() because Log(0) returns NaN and will also tend to be a very slow execution path (when it occurs, which is rarely).
x = -Math.Log(_rng.NextDoubleNonZero()) / __R;
y = -Math.Log(_rng.NextDoubleNonZero());
}while (y + y < x * x);
return(__R + x);
}
